Backfire
 

I am running two Stromberg 97 carbs on my '39 coupe. Runs great, but backfires, thru exhaust, while idling. This usually, I am told, means a lean mixture. However, the backfire seems to go away when I use the choke (don't know whether it's the higher rpm or the chocking that does it). Recommendations?

Re: Backfire
 

First thing, check for vacuum leaks. I use a spray can of starting fluid (ether) around possible sources of vacuum leaks: ends of throttle shafts on both sides; around carb base gaskets; wiper and distributor vacuum connections, manifold gasket edges. If it sucks up some ether anyplace, the rpm will jump up.

Second thing, back completely off both throttle idle speed adjust screws and check down all 4 barrels that the butterfly valves are exactly closed. If not adjust the linkage between the carbs until you have that right. Then start it up and adjust both of the idle speed screws the same amount a little at a time, until you have a slow but stable idle.

If that did not solve the popping at idle, try adjusting the idle jets. Close them all lightly, and then open each 1.5 turns. Then try each one to see if closing it a bit slows the idle. That is to be sure all idle circuits are clear and working. If you find one that does not affect idle, that carb will need service to get the idle circuit clear.

If all the idle circuits are affecting RPM, then slowly close each one until it slow the idle, then open it back up half a turn. Do that on all four idle adjust screws. If still popping, open them all another quarter turn. If all of that does not fix it, come back here for better ideas.

Re: Backfire
 

I just wanted to give an update: I finally had a chance to do the steps that drolston provided above. I completed step one and was about to proceed to step 2 when I noticed that my two air filters were dirty. I replaced those with a couple of new ones that I had in the cabinet and lo-and-behold, my backfiring thru the exhaust (actually called afterfire??) stopped!!! Haven't heard a peep since.

Thanks to drolnston for his recommendations...I will be sure to retain them should they be needed in the future.