Jeff20B

iTrader: (13)

I need to boost prep a Nikki.

We always refer to bad83 on this forum who succesfully boost prepped a Nikki. He doesn't give a step by step, but seemed to give a brief explanation. If you ask him nicely he'll provide a link to some pictures on a photo bucket site.

I'll give you a brief rundown as far as I understand it (in no particular order).

•Get a carb rebuild kit:
New needle and seats (should be in the kit)
new accel pump diaphram (should be in it too)

•Rebuild the carb as the little site glasses could leak under boost.

•Get some large secondary jets. I'd say a set of 170, 180 and 190 to start. If more fuel is needed, up it to 200 or 205 etc.

bad83 says to leave the primaries alone. He also says the Nikki is too small for a 13B.

•Take out the little fuel strainers above the needle and seats (at the banjo bolts). Don't reinstall new ones = more fuel in. Let the main fuel filter do your filtering.

•mod the accel pump like Sterling (Search)

•remove the bulk from the carb:
remove choke flapper valve and rod - block holes
remove associated choke hardware - I think the fast idle can be retained for easier cold weather starts (maybe?)
remove altitude compensator - block holes
remove vacuum secondary actuator housing and block the hole

•Do the mechanical secondary mod (Search)

•The manifold:
take out the shudder valve
block remaining hole after removal
cut a channel to mimic other side
block the hole on the rear secondary runner
remove ACV, add a block off plate, or better yet add quicksteel to your intermediate plate ACV port and trim down the ACV section of the manifold for more clearance for your turbo - that's what I did.
Add 20mm freeze plugs to the rotor housings while you're at it.
Get a new gasket and yes two new o-rings because your abilities to install perfectly sealed freeze plugs will really be tested here.

Did I make any glaring mistakes? Did I leave anything out? Please feel free to correct me and/or add relevant info to the thread. Thanks.

placd1

iTrader: (3)

Interesting info. Its a good starting point for anyone serious about boost prepping their Nikki. bad83 is indeed correct, the Nikki is too small for a 13b boost application even with the largest primary and secondary jets available.

Jeff20B

iTrader: (13)

Yeah, it's not really about the jets. It's about the size of the venturis. For example I have a race prepped Nikki which has massive venturis that I'm going to use on a 13B boosted. It's almost boost prepped as is. I only need to up the secondary jet sizes from their current 175 to probably 200 or so.

This Nikki is indeed big enough for a 13B. But now I'm discovering I'd like to do another blow through turbo on a another 13B but I only have stock Nikkis to work with. Do I try to enlarge the venturis like the racers did or just use this on my very stock ported 13B and throw the racing Nikki on the 74 ported 13B (74 spec closing is at 50 degrees).

The other issue is the race prepped Nikki has grose jets. These are vibration and shock resistant/can't get stuck and flood etc, which is perfect for the racing streetported 12A application it came from, and will work with the stock ported 13B in the rotary baja. But it also has glitches in its fuel delivery curve due to the huge venturis. It's mostly at low RPM and small throttle openings though. High RPM pulls well enough and smoothly. Not an issue if NA but could be a problem if boosted. Maybe?

I'm still learning. I'd like to pick the best candidate for the application and build from there. It's just the choosing of a carb right now that's proving difficult. It also doesn't help when I try to think about this stuff when I'm tired.

Jeff20B

iTrader: (13)

Read this thread to find some quality rebuild kits with actual aluminum crush washers (kind of required for boost prepping). https://www.rx7club.com/1st-generati...ld-kit-986173/

Oneiros

Hi Jeff, I was looking into this a few years ago and will share a few extra thoughts -

Go with an early model nikki as you have fewer holes to plug up. The main body on those early Rx2/rx3 carbs is sexy smooth, if you can get one.

You may want to premix to avoid potential OMP issues. I believe there is a thread around where bad83 said the stock OMP setup is fine up to 10PSI.

You may need to run a 3/8" return line to accommodate the boost referenced fuel pressure regulator. Also I remember seeing a pic of someone (probably bad83) using one of the later model air horns with external fuel lines, which he replaced with some pipe straight to the FPR for better flow.

Anything you can do to reduce air pressure loss through the carby is going to give you better performance, so I'd be improving the main venturi cuts and thinning the throttle shafts (but not too much).

I'd be tempted to ditch the gasket between the air horn and main body and use some kind of sealant instead. You may get away with it but that gasket seems a bit flimsy in key areas.

Hope that helps.

P.S. You know if anyone is selling modified nikkis in the USA right now? Last I checked CobraStngSVT had taken over from Sterling but he seems inactive now.

Jeff20B

iTrader: (13)

I had a couple of early Nikkis, but not any more. But yeah they are the better choice, probably. However something Sterling once said on his website is the latest carbs, the 84-85 specifically have the best idle circuit or have the best fuel delivery and are therefore the best carbs for boost prepping. It's been years since I read that though so I might have remembered it wrong. But I have set one 84-85 carb aside for boost prepping just to try it out for myself. It's currently in use to get a fresh rebuild up and running but will be coming off soon for boost prepping.

OMP fine up to 10psi? With no other information available, I'll go with that. Actually the carb OMPs are only ideal for fully open nipples in the Nikki. The moment you try them on any EFI engine with oil injctors in the rotor housings, it will blow out the seals inside the OMP (some o-rings I think) and it will leak a lot after that. I've seen it several times. Not only do you have to custom make some oil lines, as none exist for this faulty mod, but the orifice in the rotor housings (little brass jet with a practically microscopic hole) is such a restriction that it blows out the OMP. The oil has to go somewhere, so it leaks down onto your front cover and then onto the ground. So to compare this major flow restriction to the potential limit of 10psi while hooked up correctly to the carb nipples, I'll choose the latter. It's not like you're boosting 100% of the time, and even then, 10psi vs the already known flawed microscopic rotor housing option, I think the OMP will work ok at 10psi or even more. Thoughts?

Sorry if the above is difficult to understand. I don't know if I've ever tried to explain it before. Think of it this way. Keep a carb style OMP with a carb, and if you have an EFI type OMP, they were used on turbo cars, so you're fine, but they don't fit the 12A front covers. You'd need a GSL-SE front cover for that.

Yeah when I first tested a Mallory 4309 fpr, it would only go down to 5psi which is too much for a Nikki. It had a 3/8 send line and a 5/16 return line. D'oh! Exactly opposite of what I should have done. I didn't want to swap them around because it was some work getting the 3/8" line in there and everything was set up perfectly. So then I started to realize the fuel pump I was using was the problem. The walbro 255 is too big for this app. Others found this out the hard way too. I switched to an MSD pump and everything is happy. I can get 2.5 psi and the Nikki runs fine in NA mode. The 5/16" return line seems fine now too.

If I ever need more fuel volume, I'll double up the MSD pumps. But then a 3/8" return line might be required.

That sounds like a lot of work to replace the banjo bolts on top of the carb with a set of fuel lines. But then again, if it worked for him... I figured just remove the fuel strainers and you're good.

Interesting about improving the air flow through the carb. I know the FB Nikkis are terrible. The older ones from SA and earlier have throttle shafts with smaller screws made of brass, not large steel screws. I was planning on grinding the screws down a bit but not thinning the shaft.

I have a race prepped 79 Nikki that has big cut venturis (26mm). It also has grose jets and old school floats. If Grose jets can handle boost, this carb would be amazing. It's already somewhat boost prepped. The low RPM driveability is a bit imperfect, but seems to be ok when you get on it. It might work for boost. I can always try it out and keep an eye on a wideband. The point is it's got very little air restriction, like you were talking about. I even chopped off the booster supports and reshaped, Sterling-style. Oh and his accel pump mod. It needed it.

I wouldn't ditch the gasket. What happens when you need to open it up for a jet change durring tuning? Sealant would suck. I'm thinking a sheet of neoprene. But I'm going to try the paper gasket in a rebuild kit first.

I kinda figured that CobraStngSVT guy would burn out quickly. He was a noob to this forum and didn't even have a rotary related user name. It's too bad.

Nope, I don't know of anyone currently modding Nikkis. I'd like to learn how to cut venturis. That would be interesting.

Yeah, it did help. Thanks. Let's keep this going, ok?

Oneiros

From memory I was going to run a full 2nd gen OMP setup, tapping the injectors into the stock manifold, or just premix. I wouldn't risk much boost on the stock OMP, but I am a cautious person. If you have spare parts lying around you could always try it. I wonder what the effect would be on the stock nozzles under boost conditions... whether positive pressure would force the oil back down the nozzle, or just stop it from falling down the barrels for a few seconds while you boost until you close the throttle and then you get excess oil in the barrels from the pressure equalising...

Grose jets are a good idea, from memory bad83 complained about fuel flow restrictions so larger needle and seats are great. I think that is also the reasoning behind those revamped external fuel lines. Otherwise if you do a full throttle run through 3rd gear I think you might run into starvation issues.

I used to mod the nikkis and sell them in Australia, like Sterling but on a smaller scale. Thinned throttle shafts was I think the best mod for improving flow (especially those secondary throttle shafts), followed by cut venturis. If you want more info on these just ask. Sterling advised me on how to cut the venturis, and I know he did a lot of testing in that area.

I'd be very keen to hear how you go with this project. I ran out of money and motivation doing mine a few years ago but I always loved the idea of a boosted nikki.

Jeff20B

iTrader: (13)

I finally picked a candidate for boost prepping. 83 Nikki that had a clogged accel pump which barely squirted. It would also flood itself so much the engine would blow a ton of smoke, run on what sounded like one rotor and the secondary barrels were filling with fuel. Hmm, what can I do with this boat anchor?

The first thing was to take out the fuel inlet strainers. They had rust and sediment. Yum. Also went to change the needles and seats, but turned out I had a set of new 79 style. Hmm, these are different. Ok, just swap in the new needles themselves only. Leave the seats. They'll burnish in or whatever. lol

It still flooded but not from the needles. Turned out the float bowl vent solenoid was stuck. Freed it up and it worked, but would continue to get stuck so swapped in another solenoid. Probably some gum and varnish in it. Eventually sprayed some WD40 in the vent hole. We'll see.

It passed the run and idle test after this; Ok it doesn't flood itself to death anymore. Good.

Next phase: make this carb flow enough for a 13B, and work with boost.

I took a die grinder to the primary venturis. Is that a big no no? Hogged them out to 24.5mm. Just had to go bigger than Yaw. Actually after the final shaping and smoothing, they ended up closer to 24.9mm. Works for me. Still not as big as the racing Nikki's 26mm though.

I decided AGAINST enlarging the secondaries. Why? One word. Boost. Actually I wanted to maintain the stock air fuel delivery curve so the things bad83 discovered would apply to me. I also recall what Carl said about hand-cut venturis, that no amount of tuning will correct an air fuel curve if the hand cut venturi shape isn't textbook perfect. Plus because it's boosted, I don't really need to make the secondaries any bigger as boost will cover for all that anyway. All I needed was to enlarge the primaries to work with a 13B because the stock 20mm primaries are pathetically weak, even on a 12A, and can't hope to deliver enough air and fuel on a 74 ported 13B, boosted or not. Plus I love the way the racing Nikki drives, except it has a couple glitches that I might be able to fix... More on that later.

Up next was to do the airfoil mod to the venturi boosters. Won't bore you with that.

The accel pump was interesting. Connecting piece: Sterling says to do a set of holes 1 1/4" apart. I found 1 1/8" was better for the 79-80 accel pump diaphram I had on hand. They have slightly more travel. This lets me use a shorter connecting piece which gives a faster stroke = more fuel. It is now matched to the throttle opening. If you have a regular 81+ accel pump diaphram, just use Sterling's 1 1/4". Also the three gaskets on each side, longer screws etc.

Drilled the two banjo bolt holes to 1.18mm because I had that drill size. Used a 79 banjo bolt because it lacks the small jet at the bottom, but you can easily drill it out on the 81+.

The squirt is righteous. It seems to get stronger the further it goes. It squirts tons as the secondaries open and continues until fully floored. This is important for boost.

There is still a slight hesitation as the secondaries open. There has been no accel pump tuning attempted yet. Hopefully it will be ok in boost. Will test with a wideband.

Mechanical secondaries of course. Won't bore you.

Otherwise how does it drive? Getting back to the glitches statement from earlier, this carb doesn't seem to have them after some critical troubleshooting I had to do today. It seems the first setup had some problems with not so obvious solutions at the time. First the primary jets I had on hand were too small. Also the carb spacer on hand is from the racing Nikki, which is an open hole design. Well the FB Nikki like this 83 wants a separate runner spacer on a channeled manifold. So grabbed a proper spacer.

I drew up a jetting graph to show sizes ranging from stock 20mm venturis with 92 jets, on up to the racing Nikki 26mm and 155. Turns out 140 is pretty good for me. Anywhere from 135 to 145 could work, but I had 140s on hand in a carb so I grabbed 'em. I also had to swap out the primary slow air bleeds from 180 (Stock) to 170. Still way too big (idle too lean). Swapped in a set of 150 which I had on hand. Again still too big. The idle was too lean for smooth running and a glitch free rev from a low RPM. So I did something Sterling-esque but improved for this day and age or whatever. Anyway I took the tiny 1.18mm drill bit and drilled a set of nickel plated 60 secondary slow air bleeds, then scratched off the "60", tossed them in and viola! Finally the carb is calibrated well enough that it runs glitch free, is fast reving (saw 8k on the tach for the first time since getting this car) and it returns to a perfect idle without needing any safety revs or dash pots. Incredible.

If you're kinda gutsy, have a spare Nikki around, and want to fill the gap in your rotary life after Sterling and Yaw left the "Scene" (hate that word), and want to save some money, go for it. It took me three days in between rain storms.

Jeff20B

iTrader: (13)

Here is the Nikki primary jetting graph I drew up in MS paint. It shows the jet sizes to use according to how big your new bored out venturis are. Below it has some added commentary about Sterling's prefered primary jet sizes in his "Sterling" carbs ie the ones with his bored out venturis at either 22mm or 24mm (not sure what his final size ended being). I just converted from his annoying SAE sizing scheme over to a correct metric way of doing things in a metric carb. lol


Legend:
vertical - venturi size in mm
horizontal - jet size in metric (where 140 = 1.40mm)
steep lines = leaner
less steep = richer
purple - 26mm @ 160 possible rich tune in the racing Nikki
green - 26mm @ 155 current tune in the racing Nikki
light blue - around 24.9mm @140 where my hogged out boost prepped Nikki venturis ended up when finished.
grey - an in-between line to get a feel for it
black - slightly steeper angled line to work with the ones below
orange - another line at an even steeper angle to get even closer in angle to the steep ones below
red and blue lines - these jets are what the stock 22mm hitachis used
light grey - original FB Nikki 20mm venturi and 92 jets with the steepest angle, or similar to the stock blue line above

Notice a theme here? It seems the larger the venturis, the less angled the line. This is what I noticed while making the graph. I also now see that my 140 primary jets, which I had on hand, and work very well, may be a little on the lean side, if Sterling's use of 135 and 140 at either 22mm or 24mm venturi size he uses are any indication. Makes me think if I had a set of 145 jets on hand, they'd definitely be worth swapping in and observing the results. Wouldn't you think? I'll probably get a set, or a multi pack with various sizes for tuning. Mikuni, though. Not gonna bother with Holley air bleeds and have to convert the numbers all the time. That'd get annoying quick!

I do have a set of 150 jets, but that sounds a little on the rich side. At least for NA use. Would probably be ok for 25mm+ venturis... I'll see what the wideband says once the turbo and the new 3" exhaust go in. There is no bung in the RB long primary exhaust and it would only measure one primary anyway, unless I added it to the collector which is above the rearend, then I'd have to extend the wires and all that. Nah, I'll just use spark plug and exhaust tip color for now. Oh and how rich it smells too. Can't forget the rotary fragrance. That's always important.

Jeff20B

iTrader: (13)

See this thread for Mikuni jet info. Can't believe I went this long without knowing about these things. They look absolutely perfect for us Nikkiphiles. Or are we Nikkrophiliacs? https://www.rx7club.com/1st-generati...nikki-1057894/

Cookboy

iTrader: (10)

Thanks! Valuable info indeed!

Jeff20B

iTrader: (13)

You're welcome. I had never seen a graph layed out like that before. It would have helped me a lot in the early days when I first started experimenting with jet changes. I had to learn the hard way that jet size corresponds with venturi size in a logical progressive, basically linear way. But I needed a visual aid, the graph, to see how the larger the venturis get, the ever so slightly richer the jets need to be. It makes sense considering the bigger they are, the less vacuum signal is available (if you read any of Sterling's writings, which is why he didn't go bigger than 24mm in an attempt to not damage low RPM vacuum signal too much) and thus there needs to be more fuel available to make up for the shortcomings. Also a good reason to throw in smaller primary slow air bleeds, as Sterling was known to do on some stubbornly bad idling carbs he had completed. He said it richened up the idle a lot using nickel plated 60s.

The other side of the coin is the original stock 20mm Nikki was trying to be a fuel saver because rotaries are known to be thirsty, which let Mazda go pretty big on the secondaires. If you needed power, it'll be there for you, just accessing it would sometimes be less than a rewarding experience ie vacuum secondaries sometimes not opening if conditions aren't right ~ around 10% of the time. But then you're left with a slow dog at low to mid RPM the other 90% of the time and thus rotaries also became known as lacking low end torque. Plus Mazda's insistance on using the 12A as long as they did, even up into the 1st gens, didn't help change the minds of anyone - it only served to reinforce it.

Of course with the GSL-SE, torque really improved in more ways than one. Obviously the 13B is a bigger engine and will always have more torque than a 12A at the same RPM. It also had 6 ports so the smaller ports can supposedly help with low end torque at low RPM, and all that (which I'm not so sure about anymore, these days). However the other reason to have small primaries was to keep velocity high for the injectors to be more efficient at a low RPM (idle) where incoming air speed is low and fuel atomization/mixing with air might not be the best. But enough about that.

Any plans to do a hogged out Nikki any time soon? I'd like to do another one while it's still fresh in my mind. I might just pop out a set of venturis from a questionable carb and do the deed. Then worry about Nikkromancing it back to life later on. Or swap them into a known working carb.

Jeff20B

iTrader: (13)

I have some new information to add. I tested this boost prepped Nikki on a 12A yesterday. Before I pass judgement I'll say the over all impression is this carb is too big for that particular application. It consisted of two very good things and some not so good.

First the good: a 74 ported Y 12A with 83-85 rotating assembly. Aluminum flywheel. It could see 7k easily and even took it up to 8k I think.

Now the not so good: Stock FB intake manifold. Stock fuel pump (we know these are too small for a 100% stock setup, and ran out of gas at 6500 or near the top of 2nd gear). Stock fuel filter.

The in-between: I tested this setup with an RB spun aluminum air filter with an ok for testing K&N filter. The exhaust consists of a short collected RB 12A header, chopped and redone into a 2.5" collector, then into a 2.5" Magnapack, then contracts down to 2" all they way to the restrictive aftermarket replacement muffler meant for stock applications. In other words, restrictive. But the gasses cool somewhat back there so not too terrible over all. Just not really ideal for the higher flow potential of the modded Nikkis. Plus it's not a long primary which as we know you kinda need for the wide rev range of these things.

Over all impression of this hogged out Nikki on a 12A: good but not great. For the amount of CFM the exhaust can flow, as well as the crappy FB intake manifold, this carb is honestly too big for this application. I could tell when I swapped the stock FB carb back on. Stock one being too small for the app while the hogged was too big for the app. A nice medium size carb of 22mm to 23mm would have probably been perfect. But of course it honestly does need a better intake manifold and exhaust, fuel system etc to know for sure. The 12A is getting swapped out soon and a 6 port 13B is going in, so no way to test whether better parts would allow the modded Nikkis to perform better. Plus I need my boost prepped carb back for, well you know, boost.

I can now see why Sterling limited his venturi size to just 24mm or possibly 22mm (I've seen him mention both sizes). Yaw only did 24mm as well, and insisted on a decent manifold such as a 79 at least, along with quality fuel system components. As did Sterling, but not as sticklery. But they are a safer bet than anything larger because they are more compatible with more porting choices and exhaust system types.

Jeff20B

iTrader: (13)

You've seen me mention possibly wanting to try an even bigger carb (the racing Nikki) on my rotary baja if I'm keeping it NA. Well the problem here is the exhaust has like zero room for all the length of pipe and components necessary for a proper long primary. The only NA solution left to me is to do a small tubular manifold based on a stock cast iron from an FC (they had around a 2.5" outlet and could make up to 146HP from a stock S4 NA EFI setup, but the outlet angle is all wrong and I doubt I could weld cast iron). Fellow forum user rotorbugg is dealing with this issue right now. He figured it out, but it is not ideal. https://www.rx7club.com/old-school-o.../#post11667285

The other problem is how badly the hogged Nikki performed on the less than ideal 12A's setup, which being technically smaller/lesser flowing than the racing Nikki, doesn't give me much hope for the baja's NA setup. If I had ported the baja's engine out to 74 spec, at least that could have put me more in the range of the carb, but then the later you close the intake ports, the higher in the RPM range the torque peak shifts. Not a good idea for this application where low end torque is required. And unfortunately without a long primary, I just don't see that happening if staying NA. But man that carb certainly performed with the RB long primary on the 74 ported 13B in the GSL-SE! Both carbs did, actually.

So the only solution I have left is to go turbo with the smallest packaging/fastest spool I can do. This means an S5 turbo because it hugs the engine, spools fast on a rotary and only needs to be spaced out 7/16" to clear the NO manifold (after proper aluminum clearancing of course). This leaves enough room for the huge 5x11 Magnaflow mentioned in my rotary baja thread. https://www.rx7club.com/old-school-o.../#post11608883

So here is what I see happening. Throw the NO manifold with S5 turbo and 7/16" spacer in the baja. Boost prep another carb or just use this one if it performs well in the GSL-SE. I gotta boost prep another Nikki anyway so I'll test them both back to back. I need the practice. Then the big muffler can go in which simplifies the exhaust dramatically. I'm all for that. A turbo is a muffler that adds power. Also gotta lock or semi-lock a distributor. I need the practice here too. I also need the practice installing an MSD pump and a Mallory FPR and running a 5/16" send line & a 3/8" return.

Hmm, one question I have is about the size of the primary venturis on the next carb. I think if I keep them at 24mm, it will work ok with the stock 12A size ports in the baja's 13B. Thoughts?

Shredduuhh

iTrader: (11)

Jeff, let me add some info..

jb weld up the float vent tube, the holes in the carb body where the vac secondary actuator went, and the holes in the carb top where the choke rod used to go through.. there are a few vent holes around the base of the carb, jb weld those shut as well.. make the primary jets five thou bigger as well, this will lessen the hiccup when the secondary's open
most importantly, eat cheese while turbo prepping carb

Jeff20B

iTrader: (13)

I took it for a quick test drive today. Yes it has a hiccup when the secondaries open. About 5 thou bigger primaries you say? I had a set of 140 jets which were already pretty big. They were showing 10.0 on the widband for normal around town style driving. Is that normal? Looks really rich to me.

I swapped in some 130s because I didn't have any 135s so we'll see if the hiccup gets worse.

I also swapped in some drilled secondaries at 208. bad83 said to use 200s, but I had a drill bit at this size so why not. It's close enough for testing.

I see what you were saying about the float vent needing to be blocked off. I had a vinyl cap on it. I had.

Oh and yes, this.

82transam

iTrader: (18)

Lots of great info in this thread Jeff. I have nothing to add, but I'm subscribing

I'm also moving this to the tech section, since it is full of, ya know... Technical stuff

Jeff20B

iTrader: (13)

My posts usually are techy.

So I tested the 208 jets. Didn't work. It fell flat on its face. I also tested the 130 primary jets. The idle sucked and the higher RPM on primaries was really bad. The wideband still said 10.0 and sometimes 10.1. Hmm, so I went back to the 140s and kept the 208s. Solid 10.0 and still having primary and secondary problems. So Then I figured, not more fuel, less fuel. Why not try smaller jets all around. Maybe the incoming boost automatically richens up the carb for you without needing to add bigger jets? I admit I'm new to turbos so I'm willing to try it because going richer wasn't working. So then I swapped the 118s back into the primaries; the ones that caused the really bad idle before, and swapped in some 150s for the secondaries. Would I blow it up? Might as well take it for a drive. It drove well on primaries. What? The idle was vitually non exsistant though, as expected. The O2 said 13 to 14. Ok, now we're getting somewhere. But what about secondaries? Layed into it and broke them loose in gear with that same hiccup as with the 160s. Ok, the engine didn't blow up and still behaving as before. Hmm, if only I could fatten up the idle a little, this thing might actually work. So I swapped the 118 drilled nickle plated 60s from back during NA testing that made the idle so rich it burned your eyes. Woah, suddenly the idle is perfect! And it doesn't burn your eyes anymore! It starts right up and almost wants to idle without a choke! Much smoother. Drives so much better. Doesn't stall coming to a stop. Primaries were excellent, still in the 13 to 14 range. And as for secondaries, again the same hiccup, then hold on!

I had to back the mixture screw almost all the way out as the tiny 118 slow air bleeds are basically at the end of the calibration range of the screw. I think 120s to possibly 125s would be best, but I don't know anyone with drill bits in those sizes, and there aren't any other air bleeds in those sizes either. I'm fine with the screw out this far. I'll just stretch the spring a bit to keep the screw from backing out on its own.

cliff notes:
140 jets: were ok at idle but too rich otherwise
130 jets: about the same but idle wasn't as good
118 jets: too lean at idle but cruise was perfect (13:1 to 14:1)
Decided to go with 118 primary jets and 118 slow air bleeds

Jeff20B

iTrader: (13)

Today I drilled another set of nickle plated 60s to 118 and grabbed a 79 accel pump banjo bolt and drilled out the two small holes. The nice part about 79 style, is there isn't a small jet at the bottom to drill out. Then I tapped in the hogged out venturis and the trimmed boosters. OMP nipples came next.

I dissagree with those who think the OMP nipples need to be tapped in only just enough to let oil in instead of tapping all the way back in to the stock position right next to the boosters. Reason why is I've tried it with them barely in there and it makes a really big oily mess all over and oil seeps out between the base gasket. Think about it this way. When you hog out the venturis, there is a ton more room for air to find its way past any restrictions such as the booster support arms and the accel pump nozzles. How could the small round OMP nipples offer much restriction? So now I tap them all the way back to the stock position so oil doesn't make a mess and gets to where it was supposed to go.

I have a set of 118 drilled 92 fuel jets that will go in the primary holes. The secondaries will just be stock 160s. I'm not sure swapping them to 150s in the other carb did anything. I'm sure I'll swap those ones back to 160s as well. Then do secondary tuning at some point.

Jeff20B

iTrader: (13)

Got an update. I ordered the Hygrade kit from rockauto. It's your typical black paper gasket material where the holes a slightly off but ok/workable. At least it has metal crush washers and the long travel 79 accel pump diaphragm!

Today I pulled the fuel inlet banjo bolts to clean & inspect. Only a tiny bit of rust and very little sediment. Nice. Tossed out the strainers and threw in some new aluminum crush washers.

Then made a set of block off plates for the altitude compensator and float bowl vent solenoid. Then pulled the choke rod and filled the holes with quicksteel. A little went into the float bowl vent tube but I may still cap it off just to be safe.

Jeff20B

iTrader: (13)

I dug into the 6 port Nikki today.

This one is weird.

It has 18mm primary venturis and 79 primary fuel jets. I've never seen these this small!

The secondary venturis appear to be normal size but the air bleeds are 100! And the fuel jets are 120!

Hold on, 120? That's the size I'd like to try in the primaries of my other carb.

It also has a set of small holes, one in each float bowl, that communicates with the bottom of the secondary slow air bleeds for some reason. I think it has to do with the aux ports not being open all the time. I hope it won't cause a problem with boost. Only one way to find out.

Some other oddities are the boosters. The primaries are small to accommodate the small primary venturis. The secondaries appear normal size but they have a brass tube going all the way across that has several holes in it. Not sure if that is supposed to work better at low throttle openings than a typical short brass tube in a common FB Nikki, which don't start to flow until higher RPM. Remember the 6 port has aux ports. Still wrapping my head around it.

It's my understanding the 6 port 12As only had low end torque. I recall two conversations (one on the forum and one in real life) where someone who had driven one said they only have low end torque but nothing else. I was going to change all that with mine, but I don't really want to stay with a Nikki because I think if I did, I'd need to keep this one similar to how it is now. But 18mm venturis? 79 primary fuel jets? 120 secondary fuel jets? C'mon people! I can do better than that!

Fortunately I have a set of 24.5mm venturis to swap in and some already trimmed common FB Nikki boosters I set aside for another carb. Nothing says I can't use those ones here. But then will this carb still be ok for the 6 port 12A? Not sure yet. I'm not even sure how well this carb would work with the weird holes in the float bowls under boost. It might push a bunch of fuel up and out of the slow air bleeds... or let a bunch of air in. Maybe this is why I think Mazda did this: it was supposed to help with a smooth transition when the aux ports opened. But the question remains: what will these holes do under boost? And will it be an issue?

Jeff20B

iTrader: (13)

I'm experiencing a new problem that only shows up during boost. The base gasket leaks. It seems the thin paper is just too thin. We doubled up the gaskets which solved the problem. So now I need to cut my own base gaskets out of thicker paper. Fun!

Has anyone else experienced this?

Jeff20B

iTrader: (13)

I guess not.

I made some gaskets and they solved the problem. No more boost leaks.

I'm posting this as an update because I'm getting PMs about modding carbs.

I'm done modding carbs right now!

It takes a week or more of work. I've modded six carbs total (two are NA only and none are for sale so don't ask). That's a solid 6 weeks of work that I wasn't able to spend on anything else. I was beginning to go crazy, so I'm done. I may pick it up again in the future but I'm done for now. Please stop asking.

Just so I don't leave you hanging, here are the final mods with explanations.

Get a hygrade kit from rockauto or a local autoparts store. You need this kit because it contains a 79 style accel pump diaphram with the long travel needed for the Sterling style accel pump mod.

Drill the steel strip for 1 1/8" hole spacing and file one of the holes to an oval for the accel pump rod. Bolt to the accel pump lever with a nylock nut or at least a lock washer and a 8-32 screw or whatever. Bend the steel as Sterling shows.

Drill accel pump banjo bolt to what Sterling recommends of .050". I have a .046" drill bit (117 - 118 in metric) which works for me. Also drill the AP nozzle squirter to Sterling's .050" (I drilled mine to .046" and the stream is huge now).

I averaged hogging my venturis to 24.7mm which works perfectly with the 118 drilled jets and a set of 90 air bleeds on my 74 ported R5 13B. My tools stop wanting to cut after about 24.5ish but I keep at it and smooth the inside until it's at 24.7mm. Your requirements will vary due to your engine (12A wants smaller venturis probably, your porting, etc).

I leave my secondaries alone. This keeps the AF mixture calibrated for boost. Of course you can experiment.

I also leave the throttle shafts alone. I feel no need to mod them and I get plenty of air flow up high and low end torque. But don't let my experience change your mind. Mod your carb the way you want to.

My handy 118 drill bit was the perfect size to drill out my primary fuel jets to keep part throttle cruise at like 13 to 14 with my enlarged venturis but was not compatible with the stock primary slow air bleeds for the idle circuit requiring the mixture screw to be cranked way out of its range. Then I drilled a set of spare nickle plated 60s from the secondaries of a parts carb, to 118 and put them in the primary holes. It brought the idle back into calibration. Not sure if Sterling ever tried this as I know some of his carbs had unsolvable idle issues. None of mine did after this mod.

Anything else? Just standard boost prep mods such as removal of the choke and shaft, fill holes with quicksteel. Remove the secondary vacuum box, fill hole with quicksteel. Weld the secondary linkage or tie it together with a steel wire or do Sterling's flip-o-matic, whatever that is.

One last thing. I tried the accel pump mod on a couple of 79 carbs. They don't allow for a full throttle opening before they run out of travel. Not sure what that's about. So my advice is to stick with 81-85 Nikkis for this kind of work. I know this goes against what others like bad83 say. Whatever. This is what I did and it worked for me.

That is all. Thanks for reading!

Oh and I'm not taking on any Nikki work so please stop asking! I need to regain a little sanity here. lol

sbeers

Thxxxx