Awesome info, thank you. I had been taking a break from carb work to try and fix my ever lasting misfire issue, but it turns out now that I'm going to have lathe access this coming Saturday. That said I'm going to try and get my modded Nikki fixed up and back on the car in just under two weeks. If the lathe works well, great, if not I'll probably keep doing them by hand.

With that I have a couple questions. If going with 24mm primaries, what would be a good starting place fuel jetting wise. I believe I saw 120's somewhere a while back? Same question regarding 29mm secondaries - earlier in this thread Jeff, you suggested 145's with the 80 main air bleeds on a stock 28mm secondary. So perhaps somewhere around a 150 as a starting point?

I do have the 120 drilled nickel plated 60's on the primary side for the secondary air bleeds, so that's something to keep in mind.

I unfortunately still do not have a wideband and so I don't expect to get everything tuned in perfectly until I do. Hopefully I'll pick one up around or before the holidays, though it seems like I've been saying I'll get one soon for a while now...

About how many sets of jets would make sense for test and tuning (# of primary vs secondary as well?) - particularly considering I don't have a wideband?

So anyway, those are my questions for now. I'm looking to order the jets today or tomorrow so they will be here in time. I do have .5mm incriminate bits if they are needed.

24mm might do well with primary 118 to 120 jets. 28mm with smaller air bleeds like you mentioned could get away with 150 jets to start.

WIDEBAND SOON!

Great, thank you.

Yes, I know. I'll get one eventually here, hopefully sooner than later.

Well wouldn't you know it, my lathe access fell through once again... Seems I'm destined to not use one for this project. Regardless, I've started to do some more work by hand (I have now ditched the rotating holder idea for the most part) and so some progress has been made on that one primary venturi I started a little while back. With the burrs I have the going is slow, but I think it'll work out. Here's a photo of about how good I can get the surface (note that I only smoothed that one small vertical area directly opposing the camera - as you can see the rest is a bit rough).

https://cimg4.ibsrv.net/gimg/www.rx7...270f571ca1.jpg

I might be able to get it a little better, but I'm not entirely sure what I'm aiming for. It feels quite smooth so... thoughts? I'm looking to get the carb all done with the new venturi's by mid next week. Not sure how realistic that is, but we'll see.

Go at it slow and be open to changing your approach. It is one of the hardest car-related things I ever learned how to do in life. Use any and all tools you have to check the shape and ID at all levels. I like to use washers of various ODs as depth and roundness guides. Stick them on a stud with a set of nuts to hold them, then use the edge of the washer to rub or scratch the ID to show where the high spots are, so you can attack them with the burr. Then smooth and check again. Repeating as many times as it takes.

And it takes a lot. There are no time-reducing shortcuts for a noob and very few for a seasoned veteran.

Ok, makes sense. I'll look to see what I have that can be used to check consistency. That aside, how does that one area look in terms of finish - and just finish, it's not anywhere near done yet? I know there are some visible divots that I can work out, but otherwise? I could get another photo if that'd be of help. Right now I'm using two burrs, both cylindrical, and that's working okay for now. I have to work bottom up though because the back edge of the burrs leave small marks, so not the greatest solution, although budget-wise different style burrs aren't really an option.

Speaking of making guides, I've already made one that's simply a nail in a board so I can visually see how even the height of the choke point is across the venturi. I'll figure out something to check roundness as well - might have to do a parts run for some washers.

Splurge on the burr I mentioned before. The SF-14 single cut, which is nearly the perfect shape for venturi cutting by hand.

Regarding what you have cut so far, I don't know how well a cylindrical shape will work. My guess is not very well. You can make life easier by using a tree shape with starts you at abetter angle to more closely matches the tape and shape of the venturi. The SF-14 is also pretty big, which helps keep divots from occurring to a minimum.

I would, except I really can't shell out over 40 bucks for a burr that will rarely see use, at least I can't in this moment. I looked at getting a small (read cheap) tree burr as I thought the same as you concerning the divots and working angle, but again I couldn't justify it - unfortunately I don't have the means to splurge on much right now. And yeah, I checked into the SF-14 back when you first mentioned it and it is indeed large. I'm surprised you can see much of what you're doing, at least on the primaries.

The cylindrical shape really isn't the right one for this job, but I think I can make it work. The finish probably won't be quite as nice compared to if I had a larger, conical burr, but with some finesse I should be able to keep the divots to a minimum. I still have well over a mm to go so I have a little time to get my system down before I've got to be on point. I should also note that all my burrs are single cut.

If you were to choose a 'venturi burr' over again, would you still go for the SF style, or maybe an SL instead? It seems like the flat side of the SL burrs would make it easier to keep everything straight, but then again maybe that would also make things more tedious since you couldn't focus as easily on a small section. Just thinking out loud for future purchases.

It is hard to cut the primaries with that bit but it's all about rough cutting out to 24mm at least. This opens up enough room to begin shaping and correcting all the ugliness from the rough cutting stage.

I'd have to look up the SL style. Having looked, that is a good question. And I believe your conclusions are correct. Now if you had access to a lathe, it is possible that the SL series would be best. But doing them by hand, SF is probably a lot better because you can zero in on problem areas like divots, which will occur when doing this process by hand.

Just remembered I measured my primaries from when I did the hogging out process using a hand held dremel and valve grinding compund and its at about 22 mm, so not 24. It does still retain most of the stock curve/angles. I did not remove the venturis from the body either. I know, I cheated. I run 105 jets with none of the other jetting changes that would be required for higher jet sizes. I think I have 120s in the secondaries. I too need to get a wideband but my plug colors and fuel usage are about right, 17 city 24 hwy and it's really fun to drive.

Good to see you making progress on this.

Ah, thanks TG. I've always wondered what size your venturis were. If you ever feel gutsy, you might want to give 24mm a try. It really wakes things up. :) Do your secondaries too and borrow a wideband to get 'er tuned up to find out whether 120 jets are too small (I suspect they are).

Inlet angles are important. I doubt you can get a very good angle if you leave the venturis in the main body. It's worth a revisit at some point.

Good greif auto-save. Sometimes typing forum messages on mobile can be quite the task...


Anyway, I was saying that I've been using sockets as roundness gauges and that I found one that is exactly 24mm in diameter which I'm using to check the shape as I get close to finishing.

That said I've started to lighten up on the pressure to prevent any more large divots from developing. Unfortunately I'll probably end up with a few small ones just due to the nature of the burrs I'm using, but so far you cannot feel them by hand - and I still have a good bit more finishing to do. Here's a couple shots of how they stand at this point - I haven't cleaned up everything yet, and you'll notice the choke point is still a little wavy. I'm going to essentially finish the bottom side before I worry too much about the last 0.5mm of wave I've got at the moment.

https://cimg0.ibsrv.net/gimg/www.rx7...5feaba43d2.jpg

https://cimg1.ibsrv.net/gimg/www.rx7...9e699a7fbd.jpg

https://cimg5.ibsrv.net/gimg/www.rx7...e58bafeb5e.jpg

Unfortunately I probably won't be able to finish it today, so that will mean I can get back at it on Saturday. I hope to have both done by Sunday so I can get them installed and tuned in Monday and Tuesday. I figure the second one will go a lot faster in the rough cut stage now that I know where I'm ending up. The secondaries probably won't happen just yet, but I'll work on them with free time over the next while, right now I just want to get the primaries in while I have some down time in case things don't go as planned.

And neat info both of you. I'll be sure to pick up an SF style burr if/when I do more venturi's by hand farther off the future.

^Not bad.

My next one will be more extreme but I won't do it again until I have a wideband setup to use. It seems pretty woke now. Dipping into the secondaries feels like a boosted boinger.

What you say about the secondaries tells me that your primaries are too small. ;)

In response to no previous post... wow. I'm impressed. I honestly didn't think they'd work, no clue why, just some weird gut feeling. Not only do these venturi's work, they work really well. I'm spinning the clutch in gears 1-3, but good grief, when it hooks up, haha, wow... Pardon the excitement, I'm just so happy to be rid of that boring stock carb and back to my modded wonder. More power sure is addictive isn't it.

Ok, in all seriousness I'm extremely happy with the result (and I still have yet to do the secondaries :)). The primaries ended up at 24mm with a deviation in roundness of only 0.08mm in a few places - that was a painstaking achievement. After finishing them up I threw them in freezer and after an hour or so I pushed them in place by hand. This carb is just about done except for the secondaries, which now feel relatively weak in comparison. Oh, and I still have to drill and tap the secondary main air bleeds to accept 80's. Unfortunately both of those probably won't happen for a while (maybe around the holidays? - perhaps a wideband around then too...) which is a little saddening, but I think my clutch will thank me for the time being. It's too bad that my brand new clutch (flywheel resurfaced, new pressure plate and all) can't hold the power. I guess it was an OE replacement, but it seems like it would have held a little more. A stiffer pressure plate may be in my future - probably a ways down the road though...

Getting back to things, there is a noticeable gain in low-end torque, and power is improved across the board. It really starts pulling at around 4k (if the clutch decides to stick that is) and then screams to redline, or past if I'm not careful. The 120's are definitely a good starting place, but as soon as I get a wideband I'll dial it in a little more, though it does seem really close as it stands now.

Here's some photos of the venturi's. If you see spots or shiny patches it's probably a last minute touch up.

https://cimg9.ibsrv.net/gimg/www.rx7...6cce39bc15.jpg

https://cimg0.ibsrv.net/gimg/www.rx7...f0b49fbe45.jpg

https://cimg2.ibsrv.net/gimg/www.rx7...2043b61b4e.jpg

https://cimg3.ibsrv.net/gimg/www.rx7...b2f76c42d8.jpg

https://cimg4.ibsrv.net/gimg/www.rx7...670fd802ca.jpg

So there ya have it. This thread won't die yet - I still have the secondaries and air bleeds to do after all - but it might go dormant for a while.

All questions, concerns, and the like are all welcome as always. Thanks for all the help getting here.

.08mm is definitely an achievement! Congrats. They even look good.

I'm glad you get to see what I was talking about regarding how 24mm gives low end torque and makes the stock secondaries feel less transcendent than they originally do. Hint hint at TG...

Your boosters look beautiful. I bet they took a long time to shape and polish like that.

The gasket you punched has a lot of unnecessary holes. I only punch the minimum required for a stripped and boost prepped carb. Yours will still work but it saved time and effort to only punch the needed holes. Maybe next time.

If you still have stock air bleeds in your secondary side, you should keep the stock 160 jets as well. You are getting a ton of air through them so you need stock jets in order to not go lean. And I bet you are getting a little bit of a bog when you open them at perhaps anything lower than 4000. Right? With corrected air bleeds, I can open mine at 2000 and never get a bog. There is no power down that low, but it doesn't bog ether.

I think your flywheel step height may have been resurfaced incorrectly. A stock duty low mile disc and pressure plate won't slip at your power levels. Makes me think something is wrong.

Yeah the metal will dull over time. Freshly cut venturis are so pretty with all those micro facets that reflect light when using a tree shaped burr. Yours probably looked good with your cylindrical shaped cutter when they were fresh too.

Good job!!! Oh and by the way, those cheap aftermarket parts store rubber caps will crack in a very short time. Unless you used OEM ones. I like to take some of the old OEM tubing and push a short piece of 3/16" aluminum rod in to block it. Then shove them onto the nipples. They work with boost and last basically forever if they were in good shape to begin with.

Enjoy your carb and keep an eye on those caps. Also see if you can tell me how it feels when you open the secondaries at various RPMs and loads. Thanks.

Oh, by the way, I noticed a little stamped "6" on your accel pump banjo bolt. I have a carb with one of those too. It was was a factory .60mm set of holes compared to the more common smaller size of .5 or .55 or so (I'd have to get out and check what it actually is but suffice it to say, "6" is the bigger one). My hogged out carbs with the accel pump mod need a bigger set of holes drilled. I range from .95mm up to 1.18mm depending on venturi size and what the engine wants from the carb I'm testing. I never go any larger than 1.18mm. I hope this helps.

Wish we were closer so we could all swap cars and get a taste of various levels of mods to the nikki. I'm fine with what I have, really. It works great, no bogs anywhere and it screams when I tip into it. I am getting some leakage around the primary areas on both sides and I think maybe its leaking at the primary throttle rods but can't tell for sure. I may try cutting a better, thicker gasket for the carb body to throttle body to see if that addresses it.

/me whispers 24mm TG, 24mm...

Any benefits to milling primaries and secondary's down without modifying the rest of the carb and losing the rats nest? I need to keep the rats nest for AC and emissions. I also have cats and mostly stock exhaust which I need to keep as well.
Thanks

Yes there are benefits such as increased low end torque but a) you have to do the mods well and b) the exhaust needs to flow well. I don't know how well cats flow in a stock exhaust but it might be the main limiting factor. Upgraded ignition to direct fire is a good idea too. The rats nest is fine to keep.

The cats are hollow after all these years, but not sure how much better they flow. The muffler is newish and has good flow it seems.

Maybe a project to tackle this winter when I won't be driving it. I've got a good metal lathe and my new neighbor is a machinist so we can figure it out.

Maybe I'll buy another carb to have in case it doesn't work.

Thanks. They sure did take some patience to get there (and I wasn't even using the right tool at the time). I trimmed them down with the Dremel like most probably do, and then wet sanded them by hand in three stages working up to 2000 grit paper. Finally I finished them off with some polishing compound and a mini wheel on the Dremel. I was set on making them look as though there never was an arm on the other side of the booster, so that took a while as well. I also polished the outer face of the boosters and the side of the air bleed 'block' that faces inward. I think there's some photos a ways further up in this thread, but it was a while ago.


Yep, it sure does. I punched the holes I thought I needed - from memory - and then when it came time to install it I realised I punched many holes I didn't need, and forgot to punch a couple I did. Oh well, I'll check before I do it again next time.


Secondaries still have their stock 160's in, so yes, I have kept them in place for now since I haven't had time to do the air bleeds. In terms of a bog, yeah, anything below 3k it's noticeable, but there and above I can actually nail it and nothing major, maybe a slight hitch every now and then. 4k and above is seamless. I do have the solder filled long slows in so they're probably helping, but that's it for jetting mods on the secondary side. Looking forward to a boggless experience as soon as I can get those bleeds done. Strange too that I ordered the drill bit and taps over a week ago and they still aren't here; I need see what's up with that. For reference, the proper thread size of the Nikki/Hitachi fuel jets is M5x0.8 which is something that I wished I knew a long time ago, though until recently I never thought to check conversion charts... Also, for anyone who is tapping that size, use a 4.20mm drill bit.


Yeah, that thought crossed my mind too. With the brand new clutch it engaged pretty quick but now that it's been a couple months it's engaging right near the top again and then pedal feel is sort of bizarre. Sometimes it feels like things aren't centered (yes the pilot bearing is in there, I replaced it recently) when they mate because I get some random vibrations that usually coincide with shifting, with it then disappearing on the next shift - not related to any specific gear. My transmission is toast which in no doubt has some to do with the noise and all, but something with the clutch just feels off. Something in the assembly is squeaking too, it sounds like the clutch fork (it's audibly not the master or slave, both of which are new) or something else inside the bellhousing. Whatever is going on is weird, but it works well enough for now. I just try not to romp on it if things are feeling amiss at the moment. I had the flywheel resurfaced at NAPA and they said they took off the same amount from both surfaces. Unfortunately I had never driven with this flywheel before I don't know if it was an already existing issue or if it's due to something NAPA may have done. Surely there's a spec for the offset, probably somewhere in the fsm. I really don't want to drop the trans just to check that, but heck, something's got to happen with it.


Thanks again! And those caps are actually OEM ones I've stolen from my assortment of vacuum racks - it seems like Mazda used a few different ones across different years or sections of the vacuum system as they are not all identical. Or perhaps some were replaced with decent quality ones in the past, but regardless they're holding up fine. I have used the cheapies you're talking about in the past and yeah, they sure don't last at all. If I ever need more I'll do that aluminum rod trick of yours - I haven't thrown out any original vacuum lines so I have plenty of candidates.


Ah, even though I had read about them I never actually noticed I had one, interesting. So far my carb seems happy with the stock banjo bolt and nozzle sizes, but maybe later down the road it'll need some changes.




That would be neat, too bad we're quite a ways apart. I'd be interested even to just see a smoothly running/idling first gen, if it has a modded Nikki even better. That said I'm still fighting my intermittent misfire, backfires, and surging on decel under 3k. Though I have to admit, with this modded carb the symptoms are no worse, perhaps even better, than my stock carb once again leaving me stumped. Cold starts are still a little difficult too, but I'll save the details for my other thread on the subject.

My suggestion would be to mod the venturi's from an extra carb (if that means picking one up before starting, so be it) and then swap them over when you're satisfied with them. That way you save time and hassle if something doesn't go to plan, and it also reduces the chance you'll ruin your good, working carb.

Well anyway, that's my opinion, and is also what I did.


Edit: Or... you could pick up an extra carb and go all out on mods which is actually more close to what I did. The whole time I was working on my 'performance Nikki' (which at this point has been the better part of a year) I was still driving around my car with another stock carburetor I had rebuilt from my parts car. That's allowed me to still drive my car even if I don't trust or have time to dial in my modded carb after a new change - keep in mind my 7 is my daily so this system is important for me.

Regardless, hope it works out for you if you decide to go down this path. :)