Well I guess it's that time of year again - more carb modding! I'll be getting back into this pretty slowly, as right now what free time I have is dedicated to my custom head unit (I'll be making a thread for that when I'm a little further along). For now though, I've got a question. Over the next while I'll be slowly working on hogging some secondaries. I was originally thinking of going for 30. Given that this is already running 24mm primaries and is on a 'medium' ish sized street port, would anyone suggest otherwise?

I know this thread has been going a while, so I understand if some things have been forgotten. I can update that info here if need be. The basics are re-timed mech secondaries, and appropriate air bleed and jet changes. I'll get the secondary main air bleeds tapped and whatnot before this carb goes back in again. This time I'm going back and 'improving' whatever things I didn't do as well as I had wished originally.

Any and all opinions are helpful.

Chris,
Thats how I did it. Went to Pull-a-Part and got a carb/intake for $50.00 and worked on it for 2 years on and off. I was running the Dell at the time. I would just swap the intakes to test every few months until it ran the way I like.

If your cats are hollow, what emissions do you have to pass? I mean, those cats aren't doing anything to help. You could go full RB SP system and probably still pass "emissions" I bet.

Thanks Tim, it's a visual inspection for the cats. Gonna get vintage plates next year and they don't do inspections on those. Then I can get the header and lose some weight in the process.

I was able to get a carb for $20 a couple weeks ago. I really like the understand everything I can when it comes to mechanical stuff. Excited to start diving in soon.

Huh, I wonder why I have been getting spotty notifications from the forum as of recently. Didn't get an email about either of your replies...

Anyway...

Dang, y'all are lucky to find carbs around for 20 bucks. There's only one first gen in the pick-n-pull around here and I think it's been mostly stripped (not sure if it was carbed either). I'd like to pick up another FB carb at some point to be my test bed. Right now I've got my S2 stocker (installed), my modded S3 carb, and two SA carbs, only one of which is in operable condition. Regarding hogging out the secondaries, based on what I've seen around the forum in the past, 30mm seems a pretty good bet. Unless someone suggests otherwise I think that's what I'll plan on. Work will likely begin within the next couple weeks.

Well it's been five months since the last update, so here we go. I've got a couple questions as well which will work their way in.

First here's a short update about the car in general. With my clutch issue now solved (turned out to be a pinched thrust bearing, oops...) I finally have a more consistent feeling/responding car to use for my carburetor test-bed, which by itself if quite a nice improvement. That said, over the last while my radiator has bit the dust (replacement on its way), the exhaust is really getting bad (new setup should ship this week, hopefully), and a few days ago I picked up a screw in the front passenger tire (yet to be fixed). So yeah, lots of fun.

Now onto the carb. For about a month now I've been slowly reworking it, going through everything, zinc plating what I had skipped before, improving some of my custom linkages, etc. Today I finally got it back on the car and after a few banjo leaks (I swear, they always leak the first time...) got the car fired up. Now here's the unfortunate part. Even after meticulously checking all passages, ports, jets, and what have you, I still have the same idle and cold start issues as before. If anyone has some thoughts on this - and I know I've asked this many times before - go ahead and shoot, I'm starting to run out of ideas [more info on the issues are below the setup details].

This thread has been going a while so here's a refresher on the carb setup:

Primary:
- Venturis: 24mm
- Main air bleeds: 70 (stock)
- Slow air bleeds: (stock, can't remember what that is)
- No. 2 air bleeds: 120
- Main fuel jets: 120

Secondary:
- Venturis: 28mm (stock)
- Main air bleeds: 140 (stock)
- Slow air bleeds: (stock, can't remember what that is)
- No. 2 air bleeds: 60 (stock)
- Main fuel jets: 160

Idle info:
- MAS: ~2.5 full turns from seated
- idle speed: ~750, able to be adjusted lower (seemingly no vac leaks)
- AFR @ operating temp: ~12.8

General carb mods:
- trimmed booster arms
- polished boosters
- cleaned up air horn
- mechanical secondaries
- retimed secondary opening
- AP mod
- choke delete
- no smog equipment
- still running vac advance
- and of course 24mm primaries

Other relevant mods:
- '82 engine, ported similar to (slightly beyond) '74 spec
- FB intake manifold; channels cut and ported, runners cleaned up
- Carter P4070 fuel pump and Summit regulator set at 2.75 psi
- DLIDFIS with GM HEI ignitors and MSD high vibration coils, Taylor Spiro-Pro 8mm wires
- BUR7EQP in leading, BR8EQ14 in trailing
- engine compression is around 110 psi on all faces


Here's a rundown on how the car runs.

Cold Starts:
The engine will catch every time, but it will usually quickly die if ambient temps are lower than 50F. This is after having pumped the pedal twice before starting. If I lightly and repeatedly nudge the throttle I can usually keep it running on fuel from the AP near 1k rpm. Otherwise it needs to be above 2k to idle on its own. Anywhere between idle or 2k will usually hold for a second before plummeting and killing the engine - most likely momentarily running on fuel from the AP. This is all while the engine is still cold/cool. Note that adjusting the idle to be richer does improve these issues slightly, but doing so makes the warm idle quite unstable.

Operating Temp Running:
Idle is fairly stable although misfires certainly still occur - noticeably more than with my stock carb on the same setup. Misfires do not seem to be ignition related, at least for the most part. At idle it's usually fine, however if you start raising the idle speed just a hair misfires increase in frequency and the AFR starts going lean (sometimes to 16). This issue continues up until just above 2k rpm where it clears up and only occasionally misfires (no misfires are noticeable under load). Sometimes the engine will almost stall when returning to idle, although it doesn't typically die even though it runs rough for a few seconds. The mixture adjustment screw is in the best position I can find. By that I mean it's a quarter turn between undesirable levels of lean hunting and rich cyclical misfires so there isn't much room to play with. Not sure why this carb's MAS is so touchy, but it certainly is... Might have something to do with my issues.

Hot Starts:
Engine catches nearly instantly with no throttle input or AP priming. After catching it runs just like it would hot, because, well, it's still hot...



So here's where I'm at. It seems like the primary transition circuit just isn't doing its job properly. The carburetor drives fine, rips to 7.5k with ease, although anything under 2k is a little weird. Every time I've taken it apart I've checked the passages and they are completely clear. I even made sure to clean those little pressed-in "jets" that hide in the passage between the long slow and no. 2 bleed. Based on the imprint on the t-body gasket I can see that everything is lining up properly, so it's not that. The only possibility left that I can think of is that somehow the long slow or main air bleeds are messed up, although wouldn't that affect normal driving too? There's not much that can go wrong with the no. 2 air bleeds, or so I would think. Could something with the main body/air horn/ t-body themselves be faulty? Just looking for possible leads.

Something else that I should mention is my floats. Currently the car is on a slight hill so I can't confirm float levels, but they were both at just about mid-way and haven't been touched since the last time the carb was installed. The interesting part though is that to achieve proper fuel level I have the floats set at 13.60mm rather than the factory 16.0mm. While this is perhaps due to the fact that I am using SA floats and needles in an FB carb, I figure it's worth mentioning incase anyone see's an issue with that.

As always, any input is welcome. I've taken a few videos of different running cases and added them this album: Modded Carb Troubles #1. Unfortunately I didn't think of it soon enough to get one while cold starting, but maybe I'll do that in the future. For context, the max rpm reached in the videos was ~2300 rpm. One shows different throttle reactions, another shows how touchy the MAS is, and the last just shows it at idle. Pardon the shaky camera. Oh, and yes, I was manipulating the throttle with the choke, but of course there's no choke so that's not a variable - just mentioning it for those who might spot the fast idle arm moving.



Edit: Just remembered I didn't mention one thing about how it runs. When increasing rpm and then holding steady, there is usually a short time (maybe for a second or two) after the increase where it runs nicely. After this it either may try to stall or simply begin running rough. To me this once again points to running lean as momentarily it runs cleanly on the fuel from the AP, although once it's used up it returns to running lean again. If it's not that, perhaps the main circuit is just taking a really long time to respond but that seems unlikely.

I'd like to get some more fuel jet sizes to play with because I'm wondering if it couldn't go a little richer still. Before today I had been running 116's poorly drilled out to ~120 (bit was closer to 118 but the hole wasn't perfectly straight - and the 116's were far too lean, just to note). When I reassembled the carb I installed some actual 120's which, while they probably have a better shape than my hand cut ones, are likely a tad smaller. I'm thinking about picking up some jets up to 126 or so and seeing how that goes, but that can't happen for a little while yet. Driving around afr's with the drilled jets were upper 13's while WOT would see mid 13's most likely thanks to the factory rich secondaries. I'd like to sort out the primary and idle circuit first, but when I do install the 30mm secondaries I'll be changing the secondary air bleeds at the same time (80 on the mains and hopefully 40 on the no. 2's). For now though I don't plan on touching them since I don't have fuel jets on hand of the approximately right size.

I've been reading through the Nikki carb theory of operation manual and I just had a thought, although it will take some investigation on my part.

Here's the deal, when I stripped my carb I pulled the richer solenoid, yeah? I then proceeded to plug the hole where the solenoid was with steelstik. Now, if I recall correctly I plugged the richer circuit hole on the bottom of the main body, but I didn't mess with the upper passages by the jet and plug because those shouldn't cause issues. Hmm, as I'm typing this it seems less likely that this may be causing issues, but I definitely think it's worth checking. Anyway, if I didn't plug the output part of the circuit on the bottom of the main body (which I'm pretty sure I did, but I'm not 100% certain) there would almost certainly be fuel running through it. Now, it seems unlikely that I would have left that untouched, but it's certainly something I'll add to my 'to investigate' list. The reason this would be possible is that my JB welding of the solenoid port only sealed the outside, not the actual passage that the solenoid pin would have modulated. I am relatively sure that I sealed the output, which is where that passage would have led, but there's a small chance I didn't. A very small chance at that unfortunately so my issue is probably still elsewhere.

A little more progress has been made so far today.

First I tried some new primary long slows. If they did anything it was a negligible improvement, although it's probably just placebo.

Next I drilled (with a drill press this time, hooray) my poorly drilled 120's to 125 and installed those. Definitely an improvement, although it might be a hair rich (125 was my next bit size). I'm thinking some 126's or 124's (which are closer to 124 and 122) would probably be just about perfect. Unfortunately, with the addition of these larger jets my idle mixture had to be further reduced and now cold starts are now certainly worse. Just a little while ago after reaching operating temp I let the car sit for about 10 minutes and then tried to restart it. Off the bat it would not idle and at a rough running 1.5k I saw 15-16 afr (and this is 'warm' so... not particularly great).

Now I'm just trying to rack my brain for what else could be amiss. I did double check some photos and the richer port on the bottom of the main body is indeed plugged. One thing that came to mind is the SA phenolic inserts. Jeff, I know you said you've used them in the past with success in order to block off the passages that run to the altitude compensator, so it doesn't seem like it could be an issue. The only reason I'm wondering about it is because it seems like the transition circuit isn't getting the fuel it should be. Since the path from the fuel jet to the idle port is: fuel jet > main air bleed cavity > long slow cavity > long slow > no.2 air bleed cavity: I'm simply trying to sort out whether the insert could be partially obstructing the passage between the main and long slow air bleeds. Now, I'm pretty sure this isn't the case as I recall air being able to pass through there while cleaning things out, however if you or someone else could perhaps chime in with a little info there I'd appreciate it. [currently proofreading and yeah, this seems pretty unlikely. Not to mention slightly contradictory to what I say below somewhere]

Something else I've been thinking about it trying the factory no. 2 air bleed again, just to see what it does. Realistically it should make things worse, but eh, might be worth a shot for the informative factor. Hey Jeff, you've mentioned before that you fine tune the no. 2 air bleeds. What exactly is your process for doing that? What sort of things are you looking for in order to decide which size to try? I'm still a bit thrown by how little the MAS is turned from bottomed out (now only about 2 full turns, perhaps less) and how touchy it is. To me it seems like if it were less touchy - like it is from factory - the issue with cold starts wouldn't be so pronounced or perhaps even present. Problem is I don't know how to go about tracking that down, or if it's even an attainable goal. Currently my limiting factor in terms of experimentation is my lack of jets, hence if anyone has some pointers or theories here it could make things a lot more straightforward in terms of where to start.

Continuing with the idea of the no. 2 air bleeds, it seems to me (based on limited evidence however) that the mixture reaching the MAS is too rich and therefore small changes impact the idle mixture greatly. After all (and someone correct me here if I'm wrong), the MAS doesn't directly adjust the mixture, but rather adjusts the amount of a mixture (determined by the main fuel jet, long slow, and no. 2 air bleed) that is mixed with the air passing by the primary butterflies. That's where my thought of it being too rich, and therefore too sensitive, and ultimately therefore only adjustable to a specific temp range originates from. Anyone have an opinion on this?

To wrap this up there's something else I noticed while testing today. The rear rotor exhaust runner is nearly 100F hotter than the front. This was measured at 1.5k with an overall afr of approximately 13.4. I've also noticed my engine seems a little warm at idle, although this may be due to my leaky radiator being unable to hold pressure properly (replacement should arrive this coming Tuesday). Engine temps were around 192F as measured on the top of the block. Elsewhere temps were cooler, so perhaps there's a little air still trapped in there as well. Anyway, if anyone has a thought on the header temps differences do share. I'm thinking it's probably a combination of a slightly unbalanced carb tune, the fact that the rear rotor tends to run a little warmer, and my cooling situation.

So yeah, that's about it for now. Thanks all for the assistance thus far - heck, this thread is getting pretty darn long.

Mmmmmk, there's been some new developments. Nothing groundbreaking unfortunately, but developments nonetheless.

I tried the stock 170 no. 2's and that was an instant no-go. Couldn't get the darn thing to idle at all until I backed out the mix screw to 4.5 at which point it would idle for perhaps ten seconds. Well those sure got pulled out fast. Next up I took my 120's I'd been using an drilled them to 125. Evidently part of my theory was correct regarding the mixture and increasing the no. 2 air bleed size, because with the 125's I got it to idle a bit better than with the 120's and with the MAS at approximately 2.5 full turns out compared to the previous t < 2. It also took less adjustment between cold idle and hot idle which is exactly what I was looking for. Tomorrow I plan on trying 130's and up, although I don't think I'll go larger than 135's for now without some more investigation; I don't want to be bandaiding some other issue if it's present.

Hey Jeff, pardon all the directed questions, but how did you originally decide on the 120 no. 2 air bleed baseline? How far have you ever deviated from that? I'm still trying to sort out if something else is out of whack with the carb or if it just prefers a leaner mixture. Last question for now would be about how many turns out do your carbs generally require of the mixture screw? Does it vary greatly?

Hopefully I'm not email-bombing anyone here. In addition to using this thread as a "who's done what where and why?" this is also a sort of progress log of mine. Hopefully these frequent updates aren't cluttering up folks' inboxes.

I had a 118 drill bit. It was all I had back then. And a bit of beginner;s luck. As luck would have it, my very first, very poorly cut 24.9mm venturis happened to flow so little CFM that 118 was perfect for the fuel jets. But the idle wasn't very fun until I drilled out a set of old brass 60 air bleeds from a Hitachi carb I had laying around. This worked perfectly and put the mixture screw within a usable range. Imagine that... one drill bit is all I had and it was perfect for what I needed it to be.

Note that if that old clunky 24.9mm had flowed even half as nicely as any of my modern 24.7mm to 25.0mm jobs, I'd have been in trouble. These days, the CFM has gone up enough that the carbs now require 124 to 126 jetting. However the no. 2 air bleeds (I call them my idle air bleeds because it makes it easier to visualize) are still happy at 118 but I've found the best range is 116 to 124 with most of them centering on 118-120, but it depends on the carb's unique personality. Every carb is different. I can tell pretty quickly which direction to go based on how many turns the mixture screw requires. The book calls for like 3 turns or so out from gently seated, on a warmed up engine.

I don't worry about inbox stuff because I haven't subscribed to this thread. :)

Ah, very lucky indeed.

Getting the mix screw to a useable range is essentially my goal right now. ~3 is what I'm aiming for since yeah, that is about what the book calls for on a stock carb. I figure I'll keep bumping up the no. 2 air bleeds until I get there, although I'm a little concerned that having to go as large as 130 or 135 means there's some other issue I'm covering up. Maybe I just have a really special carb...

Sometimes I wonder if I should have tried a little larger than 24mm, especially since you often mention that you do many 24.5-25.0mm primaries. Perhaps I'll try larger in the future; after I sort out my current issues.


Haha, good to know.

You are on the right path. Start small, then go larger after other issues are sorted. Like I said, my first cut was 24.9mm and it was a total piece of crap. Get the inlet shape accurate with a sharp upper edge. Get the narrowest "choke" point as sharp and centered on the hole as possible. Get the outlet shape expanding and straight as you can over the entire length down to the bottom. Also add an anti-reversion bevel to the baseplate because it helps with ported engines that tend to close their ports later (and send up a stronger reversion pulse as a result). You don't want fuel squirting up and out of the carb when you rev it.

So, I'm late to the table on this thread, and I'm trying to get some clarification on the jetting. I've hogged what I believe is an early 80s Nikki to 24mm primaries, leaving the secondaries stock. The boosters have been shaved (the support arm) and polished. I'll do the Stirling AP mod and get rid of all the extraneous stuff (stripping a Nikki). In all of the discussions over the years, the jetting has been all over the place with different folks opinions. I understand that each set up is different. I'm going for an NA approach, no boost. Here's what I think has been perhaps the average of approaches to the jets:
Primary Main Air Bleed - stock 70
Secondary Main Air Bleed - 80 (assume I can solder fill the stock 140 and drill to 80, at least at the ends since drill won't be long enough for one pass)
Primary short slow bleed #2 - use secondary short slow 60
Primary long slow air bleed & jet #1 - leave stock 46 jet/70 bleed
Secondary short slow bleed #2 - fill primary short slow 170 and drill to 0.037mm (about 0.015")
Secondary long slow bleed #1 with emulsion tube - this is where I'm confused. I solder filled the upper 2 holes on the emulsion tube, but am not clear if I'm supposed to solder fill from the very top of the jet as well, leaving only the bottom of the emulsion tube open to relieve vacuum pressure.
Primary Main Fuel jet - this is all over the place - stock is 91 or 92. The different options have been 118, 124, 140, or 145. Assume it has to be larger than stock with the larger venturi.
Secondary Main Fuel jet - again some variation. stock is 160. Choices are 140, 145, 150, and 160.
Idle air bleed & jet - stock is 40 jet/130 bleed - suggestion is 120 I guess on the bleed
AP bango bolt - drill to 118 mm (0.0465")

Again, I understand that there are trade-offs and each is somewhat unique, but advice is welcomed. Thanks all. I'm excited to see how it works on the car.

This lines up pretty well with how mine runs. I believe I have a similar engine and carb setup. I usually give it a pump or two to get up to 2k before I put load on it. I'm wondering if there's something related to the pull from the engine vs the size of the venturis (mine are hogged pretty big), like maybe the air velocity is a little low idle-2k for the primary circuits to provide responsive behavior? Just a guess based on feel

I'm on my phone right now so for the time being I'll give a couple suggestions and then I'll get in a fuller reply when I can get on the computer, if someone doesn't beat me that is.

Ok then. Ericdash, first thing I would highly suggest you do mechanical secondaries (you didn't mention if this was in your plans). Actually, you really need to do mech secondaries if you've hogged the ventruis. There simply isn't enough vacuum to pull open the vac secondaries fully and so you might actually lose some power.

As far as jetting goes, I wouldn't touch the AP bolt or nozzles, yet. I haven't needed to touch them so far and I probably won't so long as I don't need to. Definitely don't drill them all the way out to 120 as a start; if you do change them go at it slowly, the banjo is only 50 - 60 to start with. I'd start with 118's or 124's for the primary fuel jets. Fine tuning is nearly guaranteed however, at least if you want the most from your carb. The nickle plated 60's do often get swapped to the primary no. 2 position, however they need to be drilled to ~120 as a starting place. For a final carb I wouldn't use solder to make jet sizes. As Jeff pointed out to me a couple days ago, tin whiskers can form in the solder jets which will be fun to diagnose... I'd suggest trying a 50 for the secondary no. 2 which can be found a few places online (I'll get a pn for you later). With the jet changes you're making to the secondary air bleeds while keeping the venturis stock I think even a 140 might be too big for the fuel. Not sure where to point you there but I'm going to try a 136 to start with 30mm secondaries and the same jet changes sometime soon. I'll explain the long slow solder filling when I get on the computer.

cpt:
I noticed last night that my primary circuit doesn't seem to come online until about 22-2300 rpm on my carb, although it can stay running even if I drop it down to 1900 after exceeding 2300. I'm assuming this just has to do with the fuel not having enough vac on it to get moving at first but the inertia keeping it moving afterwards. Last night while testing I found that as I approach 22-2300 the afr continues to go leaner (about 16 when cold) until the main primaries finally kick in. It is sort of interesting, isn't it. I only have problems slowly increasing the rpm to there when cold, but otherwise it runs pretty well there, albeit a bit lean. That is something I'm still working on tuning.

Thanks, Benjamin4456. I'll definitely do the manual secondaries. I've already done that on my 84 Nikki that is stock jets with the Sterling AP mod and stripped.

No problem, glad to help. Now that I'm on my computer let's see if I can make this a little more organized.


Here's what I'd suggest to start with for jetting, eric. This assumes 24mm primaries with stock sized secondaries.
Primary:
- main air bleed: 70 (stock)
- long slow: 46/70? (stock)
- no. 2 air bleed: swap the nickle plated 60 over and drill to 120
- main fuel: 120

Secondary:
- main air bleed: 80 (modify stock jet to accept 'tuning jets' - I'll explain this shortly)
- long slow: solder fill the upper two holes, this is most easily done from the top - once again, I'll explain
- no. 2 air bleed: smaller is better, to a point. The Mikuni B42/55 jet will fit here. Find the 50 (B42/55/50) and install it. Should still give an improvement while not dealing with solder.
- main fuel: start with ~128? Not really sure. You can always drill bigger. The stock secondaries are jetted a little rich anyway, so I'd just say start small and work your way up.

AP nozzle and banjo: leave stock for now. You might not need to mess with them, who knows. After you get the other mods done and the carb running you'll be able to test this.

Idle air bleed/jet: not really sure what you mean here. Jeff, and perhaps others, sometimes refer to the primary no. 2 air bleed as the idle air bleed - it should be 120 to start. The only 40 jet I can think of is in the underside of the air horn, above where the richer jet and plug are. This jet is also sometimes a 30. 30/40 doesn't matter, it should be filled from the back side with solder to seal it off. This assumes you're blocking off the altitude compensator - if you aren't, well, that doesn't seem like the greatest idea. This and a few other changes/mods have been well documented across the forum and for now it'd probably be better to reference those than me retype a bunch. I do plan on making an abridged version of this thread once I get my carb working correctly, but there are already plenty out there to read through and digest (which it sounds like you have already done). This thread is more of a progress thread, if anything, for me. I must admit that it's not exactly the easiest thing to look through.

Oh, I haven't explained the long slow and secondary main air bleed. Ok then, for the long slow you want to fill the upper 'circuitry' of the jet. In that jet there are two 'circuits' if you will, one that connects the bottom hole to the lower most sideways hole, and another that connects the upper two sideways holes to the top of the jet. You want to fill the top 'circuit'. I did this by clamping the jet and heating it with a butane torch. I then applied solder in the top of the jet until it I could see it coming through the upper two sideways holes. All you are trying to do is seal that upper circuit. Solder doesn't need to come out those two holes so long as the actual path between them and the top of the jet is obstructed, however seeing the solder at those holes is a good clue that the jet has been properly sealed.

For the secondary main air bleed, what seems to be the popular method is to shave/cut/grind down the top of the stock air bleed until there is no more screwdriver slot. At this point make a new screwdriver slot and drill the top of the air bleed with a 4.20mm drill bit. You do not want to drill so far as to detach the emulsion tube from the air bleed, that would be bad. You'll then need an M5x0.8 tap for the new threads you'll make in the top of the air bleed. After doing that the air bleed should now accept Hitachi pilot jets (the same type that is used for the Nikki's fuel jets). The largest size you can get is a 43 so you'll need to drill that out to 80. Now install those jets in the top of the air bleeds and there you go, tuneable (and currently jetted at 80) secondary main air bleeds. Now heed this, I have not done the tapped secondary air bleed mod yet on my own carb(s) so take the prior with a grain of salt. What I have outlined is the process that should achieve the desired result, however I can not say whether there are small improvements to tricks to be made along the way.

Well that should wrap that up for now. Hopefully it wasn't too convoluted.

Convoluted? Hell, that was great! Couple of things to comment. I will plug the air bleed in the air horn (I did that on my 84 Nikki and it solved a major problem). The altitude compensator will be gone, as will all the other items that go into stripping a Nikki. Did that on my 84 and it was fine. Now as to the idle richer air bleed & jet, that's the jet that is in the triangular area above the secondary venturies in the main body. It's the long one on the right of the center hole that receives a plug. Stock is the same as the primary long slow air bleed & jet #1, i.e. 40 jet/70 bleed.
I assume I can contact JetsRUs for the Mikuni jets you recommend. BTW, I patiently filled the upper 4 holes (2 on each side) in the secondary long slow bleeds themselves. It was tedious and I had to file off the excess. I'll go back and fill the top to create the seal.
I've read Jeff's description of the work to be done on the secondary main air bleed...have to say it scares me a little. I have the tools to do it, but...ummm....errrr. I want to take another look at the primary main air bleed. I seem to recall the threads and body being smaller, but it is a 60 or 70 and could be drilled. However, if it won't fit, that would not work. When you said the Hitachi pilot jets being similar to the Nikki fuel jets, I don't understand. The secondary main is a long jet. The fuel jets are very short.

Get Mikuni air bleeds from eBay. Get the Hitachi pilot jets from jetsRus.

As a timely situation, I'm about to install some carefully drilled out hitachi pilot jets in a set of modded secondary boosters. The size I went for was .79 to .80 to correspond with my findings on a stamped jet and/or air bleed is usually about 2 numbers off from actual metric. In this case, if there was such a thing as an actual factory secondary air bleed at with a nice stamped 80 on it, its actual metric size would be .78 to .79ish mm if measured with a calibrated tool that slowly increases in size along its length, say 1mm per 1cm of length. Those would be cool to have a set of. I bet the Nikki plant had fun tools like that. Oh to be a fly on the wall back in the days of factory Nikki development and testing...

As for no 1 slow primary air bleed, I don't understand this 40 70-whatever stuff you guys are talking about. The factory number that is stamped on these is 46 across all 81-85 carbs that are US-spec. The upper hole is not measured as it serves two purposes (it communicates with no 2 slow [idle] and main primary air bleed through different means). The hole at the very bottom is about .46mm and has more wiggle room on the .45mm micro drill bit I used to double check several 46 bleeds compared to a questionable 48 I came across in a modded carb a week ago. Might I say a very poorly modded carb at that. The 48 bleed had a lot more slop on the drill bit when inserted into the bottom hole. Ok, I know how that sounds... We all want a naked Nikki that has bottom holes you can insert things into...

Thanks for that info on purchasing the jets, Jeff. As to the #1 primary slow air bleed & jet, my Mazda factory 79-85 Carburetor manual says that the jet at bottom is 46 mm and the air bleed at the top is 70 mm. Same is true for the idle richer air bleed & jet on the main body above the secondaries. And was that an NSFW comment at the end ? :-)

We have secondaries! Well, I mean I've always had secondaries, but now they're 30.0mm. I've got some tuning questions though, if some folks wouldn't mind taking a look. But first here's the present setup.

Secondary Side:
- 30.0mm venturis
- 80 main air bleeds
- 160 main fuel jets
- solder filled long slows
- 50 no.2 air bleeds

First off, they work, and definitely are making more power than before (able to slip the clutch on occasion again, great... Probably got a little damaged from my thrust bearing endeavour). The secondary transition glitch is certainly still present, although I wouldn't say it's any worse than it was with stock main air bleeds and venturis. That is one thing I'd like to reduce though, if possible. I've have however noticed that if you floor it with a little less haste the glitch is far less prevalent.

Secondly, they are a bit rich (which using the stock 160's was to be expected as it was for a baseline), however that's not the main concern. Rather, they get richer as rpm increase. I had a friend watch the wideband and going from 3k to ~6.5k in third saw starting at ~12.5 and ending in the 10's. How is this sort of behaviour tuned? I don't have that much experience with tuning main air bleeds, although that sort of sounds like something they'd be related to...? Curious if some folks could chime in on that. In the meantime I'll do some more research as well, but if anyone has some pointers off the top of their head, fire away.

For now that's about it. New midpipe and muffler should be going on tomorrow so that may affect things slightly, we'll see. Oh, and I'll add some photos of the ordeal tomorrow for "documentation".

Interesting findings, and I thank you for them.

For air bleeds, try 92 stock fuel jets swapped into your sec air bleeds if you have any spares. These will actually be about .90mm and can be a good step up in size of 10 numbers. The factory did steps of up to 20 numbers on the secondaries and 10 numbers per step on the primaries (and is why I like 10 numbers on secondaries and 5 numbers on primaries when I tune air bleeds).

As for fuel jets, as you mentioned, the stock 160s were just for baseline testing, and could be why the AFR goes kinda rich up top. Remember I've done almost all of my testing with boost so I've had to keep things oj the rich side just out of safety for my engine. And I recall I tried 145 back when I was still running stock 28mm venturis and experimenting with air bleeds. Thus I'd say do not go any smaller than 145, but focus between 145 and 160 here, in steps of 5 numbers. There is no need for tighter resolution than 5 numbers per step on the secondaries.

Quick question: did you remove or modify the shutter valve? Just curious if removing the shaft or plate might allow more airflow to the rear rotor since the throttle plate is larger to accommodate for the restricted airflow.

Glad you found my findings interesting, Jeff. Happy to provide further Nikki modding data, although I certainly wouldn't be this far without all the work you've done.

Now as for the changes. First I swapped in the 92's at your suggestion and they made a slight improvement - keeping me out of the tens was most notable, however it did still go rich in the top end seeing the mid to low 11's. The secondary transition glitch was also not affected much by this change.

Tried some 145's and they were too lean, seeing low 14's around 4k once the secondaries were open. Doing this also kept the carb in the mid 12's at redline, although it of course still got richer as rpm increases. The glitch was worse, but not too bad. Next I tried some 150's and they had 4k with secondaries open sitting at about 13.5 afr. This then drops with higher rpm's to approximately 12.2; still richer on top than I'd like and leaner in the mid range than I'd prefer. Since it's summer (and this is a year round car) I'm aiming for wot to be around 12.8 or so. If I should perhaps adjust that target I am open to suggestions; my idea behind 12.8 is that I don't want to see leaner than 13.5 in the cold of winter, but that's just a ballpark based off my limited experience.

For now I'm going to hold off on further secondary tuning as I have a feeling part of the problem lies with my air filter - let's just say it's uh... more than a few years old..... Pretty soon I'll install my K&N E-2700 and RB airbox, hopefully that'll let the top end breath a bit more.

I'm starting to wonder about AP mods. Other than the arm extension and extra gaskets my AP system is stock (nozzles, banjo, etc). I'm curious if I should consider modifying that, and if so what I should try first. I have a fair number of spares, and if I could reduce that secondary glitch I'd be game. To me it seems like it can't get the volume out fast enough, because if I floor it more slowly the glitch is sometimes completely undetectable, but if I slam down the pedal you almost always feel some degree of a hitch. My secondaries also open really fast and really late, so that's probably part of it.

I've also been contemplating trying some 60's or 65's in the primary main air bleed position. On my carb the main circuit doesn't come online until about 2200rpm, and that just seems a little later than optimal. I suppose I could drill and tap a set, although I'd need to pick up some more jets and drill bits. It drives fine around town, although when slowly increasing rpm it does go quite lean just before the mains come online. I am also still tuning the transition/idle circuit, although I haven't had much time to do so recently.



907RotaryFB, I did remove the shutter valve. I've been seeing more opinions lately that removing it is not necessary, although I don't think it does any harm. The primary throttle plates are the same size, although the rear runner is slightly larger (in only that area) to accommodate the shutter valve. Since both primary runners choke down to about the same size I doubt it makes much of a difference, for better or worse. As I cut the channel on the rear runners (like would be found on earlier manifolds) I would consider removing the shutter valve necessary, although some folks even suggest the channel may not be worth it - I personally think it is. With my intake mods the car sounded slightly different and power delivery definitely felt smoother, but that's just my opinion. If in fact the shutter valve does not make a difference for better or worse, I would remove it anyway to avoid the possibility of it failing (assuming you are no longer running smog equipment).

In response to that quote you included, the rear runner now only runs about 40F hotter than the front which I would consider satisfactory. I believe the majority of my issue was air in the cooling system. Also my t-stat was a little sticky, so that undoubtedly contributed as well.



Oh, and I almost forgot photos. Here's some of the new secondaries and other carb things.

https://cimg0.ibsrv.net/gimg/www.rx7...bcc8927a01.jpg

https://cimg8.ibsrv.net/gimg/www.rx7...1398cd4983.jpg

https://cimg1.ibsrv.net/gimg/www.rx7...abd9b65194.jpg

https://cimg9.ibsrv.net/gimg/www.rx7...241ab57fe6.jpg

https://cimg7.ibsrv.net/gimg/www.rx7...3f38904dc2.jpg

https://cimg6.ibsrv.net/gimg/www.rx7...14e3d2797e.jpg

It's been a few months and I figured I'd get an update in here so, here we go.

First off, I did end up installing the new air filter pretty soon after that last post of mine and it certainly made a difference. Aside from the wonderful sounds that now emanate from under my hood, it's also keeping the top end from going quite as rich as before. I don't have an actual number (I used to but it's been a while) but I do recall it making a notable improvement.

The secondaries are still a little glitchy when they engage, particularly at low rpm. Anything above 4k is seamless with it getting progressively worse as the rpm drops - I haven't done any changes with the secondaries since last the last update. Still hoping to mess with the AP a little, although I haven't had the time yet to do so. Regardless, the glitch is completely manageable and is actually kinda fun once you're right on the edge of the "glitch zone" - you get that sort of nitrous shift pop and then it takes off without much any bucking. Probably not the best thing and I'll certainly deal with it before boost gets anywhere near this thing, but for now it's not my main concern.

Now then, the idle/transition circuit... So I've been doing some fiddling and I've discovered that 120's are too rich (for those just tuning in, we're talking no. 2 air bleeds so smaller is richer) for a stable idle and 135's are too lean for a competent transition circuit. I had been driving with 130's for a while and they did pretty good, although I often needed to blip the gas to prevent it from dying when returning to idle. Today I swapped out the 135's which I put in a few days ago for some 125's, and while the idle is harder to get right the transition circuit is a bit happier, although it also has the mains kicking in a skosh later which seems backwards, but oh well. I should note that the 135's drove fine with the exception that low rpm cruising was a little twitchy, although it was the cold starts that made me want them gone. So far it seems like the smaller the no. 2 air bleed the easier it starts although the more the idle quality/tunability suffers. 135's had the car dying multiple times on a cold start if I got anywhere near 1.5k rpm. With the 130's I could pretty comfortably sit at 1.5k while warming up with some occasional lean misfires. Tomorrow I'll find out how the 125's fair in this regard. If the car wasn't a daily I really wouldn't care so much about this, as the car drive quite well. It's just those pesky cold starts that have me fiddling with these darn idle air bleeds.

I really wish the Nikki had a way to adjust the idle mixture (not the mixture screw, that's different, and the same, hold on I'll explain - no rhyme intended) and the transition circuit separately. It even has me wondering about the SA throttle body as it has a seperate air screw, but I don't know if that affects only the idle mixture or also the transition circuit. For those who don't know, the idle mixture is dependent on a few things, the top two being the mixture adjustment screw (MAS) and the no. 2 air bleed (sometimes referred to as the idle air bleed). My problem is that the no. 2 air bleed needs to be larger for a stable, less temperature dependent idle, but smaller for a properly functioning transition circuit. Now don't get me wrong, the carb works quite well, I'm just being nitpicky about low rpm operation since this is for a daily driver. This carb runs wonderful above 2.5k and screams to redline. With the 135's I had a pretty darn stable idle, and the 130's were quite similar. Unfortunately the 125's are getting to that point where the mixture adjustment screw becomes a little too sensitive and so the difference between rich misfiring and lean misfiring is practically a quarter turn. That said, my engine is also quite picky about the idle mixture - only likes between about 12.8 and 13.0 for a smooth consistent idle - and that may point to something like my ignition system being a little under the weather, but that still doesn't address the fact that the MAS gets too touchy with the smaller no. 2 air bleeds. Currently I have the MAS sitting at right about 2 full turns out from seated, which in my mind is too little for the Nikki. I was aiming to get more towards the factory 3.5 or so, and approaching that does make the idle quality improve (achieving this by upping the size of the no. 2 air bleeds; idle afr remains the same) although the transition circuit takes too much of a hit to justify it. If anyone has ever run into this issue I'd appreciate some input. So far it seems like I may be one of the only people to encounter this, so perhaps there is something else going on or I just have a special Nikki, who knows...

So anyway, this is getting a bit long winded and I applaud those of you who are still reading this. For the time being all I really have left to mess with is perhaps putting some 130's back in, and then some AP fiddling for the secondary glitch. I might note that on my road trip earlier this summer I managed to hit nearly 25 mpg on the highway going through the coastal range. Definitely wasn't going that easy on the car, although I wasn't going ham either. Typical cruising afr on the trip was 13.5 or so - who said rotary's can't have somewhat respectable gas mileage ;). Anywho, I know I've talked about making a condensed version of this thread to cover the meat a bit more efficiently and I still hope to do that at some point. Of course we've already got a number of resources out there thanks to Jeff and all the others. Perhaps I'll get it written up once I toy with the AP some. For the time being though, happy Nikki modding!