Rookie Question

Will someone please explain what actually happens when the engine "backfires through the carbs"? What causes it? My car runs beautifully, but on a rare occasion (usually after a downshift from 4th to 3rd on cold days) I hear a little pop come from the engine compartment. Is it possible that's what's happening?.

Thanks.

It could be normal. In my case, I hear the backfire sound on a regular basis on downshift -- but I know my setup is fine. I have an extremely short header system and a tiny muffler.
Might check your idle jets for obstructions, though.

Brian, Backfiring thru the carb means you are momentarily lean.
That's why it happens when cold and your choke is not fully compensating for the rich mixture that is req'd. Once warm your issue
goes away=Normal. If it continues to happen with a warm motor, choke off=you're lean. If the "backfiring" is out the exhaust its caused by a mix. that's too rich and these are the unburned gases "exploding in the exhaust. If your exhaust is too short or there is not enough
back press you will also get "popping" during down shifts.
Ever hear a straight piped harley down shifting to a stop sign=
no tenough back pressure.

Also could be a sign that there is a leak on the exhaust manifold. So could be many things. Normally there should not be any fackfires.

Bian,

Back Firing on Down Shifts usually are linked to a leaking exhaust flange connection. There will also be a trace of Soot/Exhaust at the location of the exhaust leak.

Backing through the Carburator(s) can surface during cold start as well or if the ignition timing is very "Off."

I'll focus on the exhaust if it happens on down shifts.

Jack

Backfiring can be caused by mixture screws not properly adjusted, or leaks in the intake manifolds to head gaskets or carb to intake gaskets.

Like I used to ask with my kids "what did you mess with last"? If you have done anything, there is a good place to start. If it worked ok in the past, no backfires, and it just started doing it on its own, I would suspect either dirt in the mixture screws on the carbs, or a leak in the intake system. While exhaust leaks at the gaskets on the heads can cause backfiring, that is less likely than the other 2 in my experience(s)

Just about everytime I pull my mixture screws I find "stuff" that does not belong there, and I wonder how it got there as I have a huge and very fine filter on my gas lines.....

I had a similar problem w/ backfiring, I finally fixed the problem - dirt/junk in mixture screw (had to take all side shroud out to get to the screw pain in the !!#**!) regained mid to top end power no more backfire. Cabanaboy fiiinnnaalllyyy!!!!