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Nikki Carburetor Squeaking / Whistling From Secondary Boosters
Recently rebuilt this Nikki for an 85 12A. Complete overhaul W/ new gaskets. Removed all emission, converted to mech. secondaries, acc pump mod, and extension. Filled, and drilled primary slow air bleeds and drilled to .0456. Carb runs great, and performs well with no dead spots, however cant get it to idle below 1150 RPMs, and I hear an intermittent squeak coming from both left and right secondary boosters. Right at the end of the brass tube. The primary throttle stop is completely turned out, and the primaries are 100% closed. Secondaries are slightly open, and any adjustment closer to the closed position makes the engine hunt, or stumble severely, and die. The exhaust smells extremely rich, and I cant seem to tune it out with the mixture screw. The car has half of an exhaust system with a small muffler after the pre-cat ending under the pass seat. No other mods aside from the intake which was striped of the ACV, and shutter valve. All holes and unnecessary vacuum ports have been pipe plugged, and or welded shut with JB weld quick steel. I have a feeling the engine is idling off of the secondary slow circuit, and maybe thats the cause of the whistle from the boosters, however that would mean the primaries are not doing anything. I have tried to open the primaries with the idle screw, and then close the secondaries and adjust the mixture screw. It will idle down a small amount however begins to run very choppy, and will eventually die. At that point the mixture screw is usually maxed out, and I have to bring it back to where it was to get it to idle again. I did have the secondary throttle shaft stop out, and its possible that it is out of adjustment, but I did mark it before removing it, and it looks close to where it was prior to removal. I also had the booster out to polish the venturies during the overhaul. Float levels look good, and I have verified that there is no external vacuum leaks from the carburetor, or intake manifold. All unnecessary vacuum ports on the spacer plate have been plugged, and the remaining ports (evap, and vac adv) on the carburetor and spacer plate have been capped for diagnostic purposes. The carburetor would not idle prior to the overhaul, and the shutter valve was leaking.
Any help, suggestions, ideas appreciated. Im sick of scratching my head, and losing sleep over this thing.

59bomb

Look down your carb at the secondary butterflies, it sounds like they might be very slightly cracked open from all the symptoms you are describing.

Im not overly familiar with the mechanical secondary mod, but it sounds like whatever is tying your primaries and secondaries together is also holding the secondaries open. Disconnect that link and push the secondary butterflies closed to see if you can get your idle down to a respectable level.

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The secondaries are slightly held open by the secondary throttle shaft stop (the small stud and 7mm nut). When I try to close the secondaries up a bit bt adjusting this stud, and nut it does not take much (thousands of an inch) and the idle quality spirals into the abyss. Just plain wont idle. I usually have to pump the pedal and hold it to restart after it dies. Are the secondary butterflies supposed to be closed 100% at idle, or do they need a slight crack for the slow circuit to function properly on the secondaries. Any specifications or procedures that are known to work to reset that stop. I knew I should have never touched that thing lol. I assumed the stop was just to keep the secondary butterflies from becoming stuck in the barrels, and they should be closed, but I have not found any definitive answers, or good info.

59bomb

The secondary throttle plate stop screw is there to make sure when the secondaries are closed/are closing, the butterflies aren't resting or being stopped by the carburetors bore, but are resting/being stopped by the throttle plate stop screw. This prevents damage to the delicate brass butterflies.

Yes, they should be "completely" closed. Turn your stop screw out until it's not touching, then slowly turn it in until it just contacts the secondary linkage. Then turn it 1/8-1/4 of a turn. This is essentially completely closed but still making sure the secondary linkage is resting on the stop screw and not the carburetor bore. You get the idea.

Reset your idle speed and mixture screws while you're at it. While troubleshooting you may have adjusted them way out of range to where the car won't run even with the proper secondary clearance. Check the carburetor shop manual for that procedure, can be found on foxed.ca

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Thanks again for your help, and suggestions 59bomb! I did close those secondaries up, and that did take care of the whistle I was hearing, and also helped to bring the idle speed down. After all was said and done I have got a fairly steady idle at roughly 900 rpm. Much better than 1150 - 1200, however all of the throttle stop adjustments are still maxed out. The secondary stop is borderline touching the the stop arm, and the primary idle stop is all the way out. The idle will fall to 500 rpm momentarily when coming to an intersection to stop, however will come back up and rest in the 850-925 range. Is it possible that one or both of the throttle shafts is twisted and causing the lack of adjustment in the range I am looking for?

59bomb

Good, I'm glad I could point you in the right direction. Unless you modified/shaved your throttle shafts in the carb(like a Sterling car), there is very little probability that your primary or secondary throttle shafts are bent. At any time, did you remove the brass butterflies from the shafts? They are held on by two screws.

What you are describing now sounds like a small vacuum leak somewhere which can be frustrating to track down. Even a pinhole can bring the idle up. Check to make sure all capped vacuum ports and remaining vacuum lines aren't cracked. Check the tightness of your carb main body to baseplate, and baseplate to intake manifold. I've found that when installing a new gasket/after rebuilding a carb, after a few heat cycles they need to be re-tightened. But don't go nuts and snap bolts off, check for uniform tightness.

Pay special attention to the vacuum lines that run to the brake booster, since you mention the idle drops when you are coming to a stop. When the car is just idling and you press the brake with good force, does the idle drop/change?

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I did not shave the shafts, however I did have all of the butterflies out. I removed the shafts to cut the ends, and remove the excess shaft length left over after removing the throttle position sensor, and OMP linkage parts. I replaced the original screws with shorter length button head stainless screws, and used blue loctite to help secure them. I have all of the remaining vacuum ports capped with new vacuum caps for the sake of diagnostics. The brake booster has been checked to make sure the hose, and pipes are not leaking, and at a stop depressing the brake pedal does not affect idle speed. I have replaced the throttle plate gasket with a home made version (1/32 vs 1/64) that omits all extra vacuum ports coming from the throttle plate. The spacer plate has been filled with quick steel, and only the evap, and vacuum advance ports remain open, however are capped for diagnostics atm. I will check the bolts for uniform tightness again just to be sure the throttle plate gasket is crushed evenly and has no chance to leak. Thanks again for pointing me on the right direction. One other thing that I noticed is that when the snapping the pedal to the floor from low rpm with, and without a load the engine bogs severely. I did drill the ac pump "squirter" to a #55 AWG size. Later reading that this may have been to big for a stock port 12A. I would assume this is the cause of the bog, or hesitation from 0 to go. This isnt the first Nikki rebuild for me, but it is the first modified Nikki for me. As always with anything new there is always that question of "did I do something wrong?" when it does not work right. This is one of those lol.

59bomb

Ok all good information. Sometimes I've seen in the past, when removing the butterflies from the shafts and reinstalling then, they aren't 100% centered. If they aren't 100% centered on the shaft, one side of the brass butterfly could be contacting the carburetor bore, while the other stays open. It's hard to explain but look at your butterflies again and make sure both sides are closed/have equal distance to the throttle bore. If one side is contacting while the other isn't, they might be off centered.

Sounds like you paid your due diligence when sealing everything back up. It might be useful to go around all the nipples and hoses with carb cleaner or a propane torch to check for leaks.

Your bog issue is definitely from how your carburetor is running. Too rich or too lean and it will initially bog when you give it gas.

The hissing tells me there is still unmetered air entering your system from somewhere. Keep at, you will find it after enough time beating your head against it.

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Thanks again for all of your help and suggestions. I did finally get back to this carburetor to repair it, and get it running right. I ended up replacing the throttle plate with an untouched factory adjusted throttle plate, and whalla.. problem solved. I think it was as you mentioned the butterflies not being centered in the bores properly, or a score or some slight imperfection on the butterflies causing a slight VAC leak. The new throttle plate is well within adjustment, and idles perfectly at 750 rpm's. Now I can move on the the secondary stumble and this carb should be SET!! Lesson learned, I cant thank you enough for your help with this!!

59bomb

Excellent news! I'm glad I could help in solving your problem and hopefully this helps anyone in the future with a similar problem.

Enjoy your freshly tuned carb and happy motoring!