blwfly

iTrader: (4)

so how much mpg did you get glazedham with your carb like this?

in the past i got 20mpg without accel mod and drilling but the rest of the mods above

in my current 83 i saw 23-26 on a stock setup with rats nest

just curious
thanks kiwi.

Oneiros

Interesting mods. I liked those booster venturis. However I don't think you will see huge gains. Thinning the throttle shafts and cutting the main venturis were the biggest improvements for me. I see that you have done something to the main venturis. How did you cut them? Be careful because you may make things worse instead of better. If you are interested I can give you some specifications of cuts that Sterling advised me on, as I used to modify and sell nikki's in Australia. I was using a lathe to secure the venturi, but a dremel to make the cuts because I had no training with a metal lathe. This method was fast and effective. However the mod I would most recommend is thinning the throttle shafts. I can give you more info on this too if you want.

MosesX605

I'd love to see some detailed discussions of the common mods to the Nikki. If I can get ahold of a spare carb this winter, I'll try some of them out and see where I get.

Thanks!

Cookboy

iTrader: (10)

There are dozens of threads with hundreds of posts by knowledgeable people regarding Nikki mods and tuning.

MosesX605

You're right, of course. I found a bunch after I posted.

fbse7en

Yes please! I've since thinned the trailing edges of all the boosters even further (nearly knife edge). The primary & secondary venturi have now been re-profiled. I've done it all by hand, just a dremel w/flex shaft, carbide rotary cutter/files, hand files and sandpaper. I do plan to thin/re-profile the throttle shafts, just not as aggressively as some have. The primary venturi are currently just over 21mm internal bore/dia. I post up another vid after I've done some more sanding & polishing.

Oneiros

Here are some pics of my throttle shafts. I went to 2mm on the primaries and 1.5mm on the secondaries, also completely removing the half of the secondary shaft that sits between the butterflies and the screw heads. Usually I cut most of the material with the bench grinder then started filing from rough to fine then used the wet/dry from rough to fine grit. I also found some button head screws on ebay which look much nicer than the standard ones.

When reseating I'd recommend using some threadlocker (but not the full permanent) for extra safety. And I liked to keep each butterfly in the same barrel to avoid slight mismatches when reseating. To reseat I usually flick the throttle shaft open and closed many times, putting pressure on them when closing. If you can use spring pressure (easier on the secondaries) to close them while seating, that is even better. The butterflies can rotate while reseating so it can take a little fiddling to get them as flush as possible.



For venturis I wouldn't recommend boring them any. I see that sterling's site is down, however he did a lot of R&D on venturi cuts and found that working on the inlet angles was enough to improve flow AND speed, which gives you better power down low compared to boring. I don't know what measurements he used however he did give me some guidelines on my cuts which I can pass on. Basically you are aiming to remove the red material below. A good point of reference is the vacuum passage holes on the inside of the venturi - cut down as far as these on the interior and aim for a straight angle, not a curved mess like the standard cast. The idea is to sharpen the inlet angle (remove more material from the bottom edge, and none from the top edge) - hope that is clear enough from the pics.



Both mods are not for the faint hearted but they are IMO the most effective mods for improving carb performance.

Oneiros

There is one other thing I forgot to mention, which can only be achieved with mechanical secondaries. In order to avoid twisting across the primary throttle shaft at WOT (being thinner and weaker as it is), I prefer to use the stopper on the secondary mechanism to adjust the WOT position. There is another stopper on the primary shaft but on the opposite side of the carby to the throttle cable (part of the OMP mechanism). If you press down too hard on the pedal, you are applying pressure on one side of the shaft while the stopper on the other side of the shaft is applying pressure in the opposite direction which may lead to twisting and weakening of the shaft. By using the stopper on the secondary shaft you keep it all on the same side. So for that reason I would set the stopper on the OMP side of the shaft slightly behind the WOT position so it is only a backup and doesn't receive most of the tension at WOT. Hope that makes sense.

MosesX605

I am seriously impressed by the knowledge that is being shared here. What a great community.

Now all I have to do is find some spare carbs that I can play with.

Cookboy

iTrader: (10)

It is that. My stripped and chokeless Nikki fired right up this 26 degree morning, took less than 2 minutes to idle. Reading posts is all it takes.

fbse7en

Thanks for posting again! It is nerve racking doing most of these mods! But once you commit, it feels right!

^Glad to hear you aren't having problems w/cold start, it's been my biggest fear w/this project.

Oneiros

Sterling removed the choke setup from all his carbs and never had any problems IIRC. He just retained the fast idle mechanism, which simply holds the throttle open when you pull on the choke lever. I did the same however Australian weather is much warmer so it's less of a risk.

fbse7en

This might seem to be a silly question but has anyone ever flow tested a Nikki w/o the primary & secondary venturis installed? W/all the boosters in place like they should be, just w/o the primaries & secondaries. Wouldn't have to deal w/re-shaping them, just smooth-out/blend the lip in the bores...which if done carefully would give the bores a slight venturi-ish effect (barely..maybe)

-Just an idea that came to me while checking booster position...and after trimming approx 3mm off the OMP tubes sticking thru the carb body..

Oneiros

Not sure what the purpose of this would be... If you remove the main venturis, the car will most likely die as soon as you open the throttle. And if it runs it will be way down on power. Venturi design is fairly precise and if you start messing with them without knowing some of the theory behind it, things will go bad fast.

The stock venturis have more than enough flow for a stock port 12A anyway. The mods above are about improving efficiency as much as peak flow. Only larger ported engines will take advantage of the flow benefits, while all setups will benefit from the increase in volumetric efficiency.

Since I managed to find a historical version of Sterling's site, I would suggest you have a read through it. It's a great resource if you are interested in how carburetors work. http://web.archive.org/web/201103170...etalworks.com/
Also check out the carburetor manuals here for more details Foxed.ca - Mazda RX-7 Manuals

Sorry for taking this thread way off topic, but hopefully the information provided will be useful.

fbse7en

Oneiros, I agree w/you about venturi design being fairly precise. Especially for optimum performance. But one close look at ANY of the venturis (including all the boosters) in the Nikki carb shows that they're far from precise. I think it'd be very possible for it to work nicely w/o the primary and secondary venturis (on BP-PP). I'm thinking of the whole carb as a venturi from the mouth; where the air filter box attaches to it, to the base; where the 4 barrels exit...

Thanks for those links, I just wish the pics on the Sterling site still worked.

The following links are just to show how simple carbs can be.

Dave.Martin2008

So I have stripped down and set up my carb, drilled out the jets and should be ready to test on the car. Switching it out today and I will get some video uploaded tonight for showing what I have done.

Dave.Martin2008

Well the carb I stripped has an issue. #2 rotor primary and secondary drip gas into the venture all the time when the fuel pump is on so I will have to get back to it tomorrow and work some more. Here is I have done so far...

Oneiros

The needle on that side of the carby is sticking which causes flooding, a common issue on fresh rebuilds. Best strategy I found was lightly sanding the corners of the needles (just the parts that contact the seats) with some very fine grit wet/dry sandpaper. Then you may have to whack the carby a few times to dislodge the needle if it gets stuck again.

If problems continue, you can also remove the small clip that attaches the needle to the seat, as this can sometimes cause binding if slightly out of place. Another thing to try is adjusting the float drop (not float level) so that the floats don't fall so far and create a sharp angle for the needle against the seat.

This is the most frustrating and fiddly problem with these carbies.

blwfly

iTrader: (4)

i dip the needle ends in some motor oil and it usually works for the sticking problem. hitting the carb with a rubber mallet has worked as well but only recommended if your mad.

Jeff20B

iTrader: (13)

Dave.Martin2008, why did you chop up your ACV? Lots of work and it still might leak. Better to use a flat plate of aluminum or steel. Cut one out using a gasket as a template. Similar amount of work but looks better and works better.

Better yet, since you're blocking it off anyway, fill the ACV port in the engine with quicksteel and pop a freeze plug into each coolant port if you're taking the manifold off. Otherwise don't bother. Of course you have an anoying shudder valve in the way... Better to get rid of it since you're doing other mods anyway.

Dave.Martin2008

Good to hear from you Jeff the ACV got chopped because it was a spair engine I was modding and there is a thin tin plate in there also along with a make a gasket sheet so its completly sealed. As for my mods I got all winter long to tinker and make this better. I refuse to put my RX7 out on the crap they sray the roads with here, I know for a fact it rots cars away... I also did a sterling carb stripping via the papers shown on this thread. Still need to fix the drip. But this is all an extra setup so not too big of a deal if it fails. I still have to pull the AC system out also and finish my direct firing ignition install. Time to make the engine compartment look clean.

Dave.Martin2008

So dripping gas has stopped... I can start the car on choke almost full on but engine races when choke is pushed off. Rich mixture I would guess. So would the holes that got drilled out on the aux jets cause this, because I have 3 sets and only drilled one as per the papers said at the beginning of this thread. also I did not grind out the venture tubes or file the throttle plate rods. I did make the secondary mechanical via wire tie. But this thing is just running supper rich ATM. so I pulled the carb off and I will get pictures up later tonight of the work so if I did miss something I hope someone can spot it and let me know. I did try and adjust the screw on the side of the carb for fuel mixture and it did nothing to adjust the mixture that I could tell with the way it runs right now. I don't think I have a vacuum leak at this time since I tried the carb spray test and did not rev the engine when sprayed around the outside of the carb. other that the air filter being off when trying to run it but that never had this type of effect before with the rats nest installed.

Dave.Martin2008

I have the direct fire going again and also pulled the rats nest. I will give the stripped carb a test run after xmas and see what I got now that I have a good running base. I was worried that I did the rats nest removal wrong but no it works. So now back to the stripped carb. I will keep you all updated on the progress

fbse7en

My progress so far..

nrx

a simple question ... and what modification to the Nikki will gain most power to a stock port 12a,
venturis, butterfly shafts, mainfold cleaning? or...

Have header and 2,5" "free" exhaust, stock ignition