Tech Help! Very Rough Idle At Low Rpm...why? - Carb Guys

My first two guesses would be Vacuum leak or flooding (since you get a strong fuel smell). How do your plugs look? Are you getting any other symptoms like backfire or pre-ignition?

If it is a true ‘backfire thru the carb and not just a sputter then your timing is advanced too much, Valves are adjusted improperly, valve sticking (intake), cam one tooth out of time?

Could be a vacuum leak at the base of the carb or a crack in a vacuum hose or vacuum cap/plug. To check for leak at the base you can spray carb cleaner around the base of the carb while running. If there is a leak it will change rpm when you hit the area that’s leaking.

By flooding I mean running way too rich – too much fuel for the amount of air taken in. This can have a couple of causes. Stuck or misadjusted float, Choke set improperly, excessive fuel pressure.

The fact that you smell fuel and are getting black smoke is why I went there and you are probably not burning off that excess fuel so its also carbon fouling your plugs as well. Id pull them and see how they look and they’ll probably be black.

Do you have electronic ignition with a strong coil or stock setup?

I would start by backing your timing off (retard) a few degrees till you get rid of that backfire. If that doesn’t go away then you have other problems.

Then I would turn your mixture screws in all the way and back out 1 full turn and go from there.

Check the choke adjustment and operation.

If you are sure you have no vacuum leaks I would do the following:

Disassemble the carb and make sure it is clean and all passages are clear. Set the transfer slot exposure such that .020"-.040" of slot is showing under the butterfly. The slot will look like a little square when you are done. You also need to make sure that the secondary butterflies are closed at idle. The carb is likely rich because it is idling on the tslot instead of on the idle discharge hole which is also why the idle is so high. Set the idle mixture screws 1-1/2 turns out. Reinstall the carb.

Start the engine and set the initial timing using a timing light. Don't guess based on vacuum readings. For that cam I would set the initial to between 12 and 16 degrees. Once you have the initial timing set you can set the idle speed and dial in the idle mixture setting using a vacuum gauge. I like to adjust the mixture screws to highest vacuum and then turn them out (richer) 1/8-1/4 of a turn. Reset the idle speed if necessary.

Setting the timing with a vacuum gauge can lead to timing that's to far advanced. I'd put a light on it to verify. If it's blowing black smoke as you hit the accelerator I'd think the accelerator pump needs adjusting....

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Is the distributor vacuum advance? Check the diaphragm. Also make sure it is hooked up to manifold vacuum. I prefer mine to be hooked up that way. Much easier for me to get the idle satisfactory for me. You will have to re time it. Also check the foat level.

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If you've backfired through the carb the power valve has probably been damaged. Guessing the carb is older than 1988 you will need to install a check valve to prevent blowing the power valve again if not already installed.