1967 chevelle backfires

I have a 67 chevelle and it runs great but after driving it a while it starts backfiring. I think it is when it gets hot, is this possible and why is it running hot. It is a straight 6. 3 on a tree. Smaller engine like a 250 or something. Original engine and everything. I have had the car since I was 16, now I am 40. Somehow I still don’t know how to fix it.

If I remember correctly, this year has a continuous AIR pump injecting air into the exhaust manifold. You also should have a gulp valve that will inject air into the intake manifold and dumps the exhaust manifold air into the air cleaner when the throttle is lifted. You might try disconnecting the AIR pump belt and/or the gulp valve small vacuum hose and see if the backfire abates.

You might be running a little rich when the engine is warm due to the choke hanging closed when the engine is warm or the carburator running rich. You might need the carburator rebuilt.

You don’t mention the mileage on the engine and the general state of tune of this car. The engine might be ready for rebuild or just a major tuneup.

Hope this helps.

I would look at sticking centrifugal advance for the points, Located in the distributor they should be able to move freely as rpm advances. They are weights more or less that swing out to advance the spark as rpm increases.

Yup, I would take a close look at the points and condenser…Does the distributor cap have a little metal window you can open up in order to set the points with a dwell-meter? a 6 cylinder 3 speed Chevelle …Now that’s a pretty rare car…California models MIGHT have had air pumps and EGR valves, but outside of that state, 1967 models had little if any emissions controls…

So many things, but usually if a mechanic of the day could hear the backfire and the conditions that it occurred, they would know exactly where to look. Under the conditions you describe and absent any other issues, I would look for a leaking needle and seat or saturated float (carburetor).

The mechanical advance weights are another good suggestion but on this car the weights are just below the rotor so they are very easy to check and lubricate.

Other possibilities

sparkplug wires
defective choke pull off
fuel pump leaking into the engine.

If the previous stuff doesn’t fix it, how about a worn/skipped timing chain (does the 250 have one?)

Coil breaking down when hot? Quite common back in the day. Or bad fuel filter, fuel pump or ruptured accelerator pump in carb.

"and why is it running hot"

Define “hot”. Do you have a red light on the dash that is coming on or a gauge that is going into the red? Or is hot the normal running temperature of the engine?

Is the backfire coming out the exhaust or up through the carburetor or can you tell?

When and how does the engine backfire? At steady cruising speed? When accelerating at speed? when pulling away from a stop? When coasting down from cruising speed? And does it backfire out the exhaust out the carburetor?

I have a 03 impala which i have put a top intake and took out the water and won’t start it an 3800 motor new plugs crank sensor cam positioning sensor and still wont start please help mr

Where was the water and how did you take it out…

It would help if you started a new thread though. At the top right you’ll find ASK A QUESTION.

Please don’t encourage responses to trolls

Cool. An all original straight six three-on-the-tree. Haven’t seen one of these in…years!!!

I think it’d be worth it to bring this to someone who knows carburators. It may be time for a rebuild. Remember that as the engine heats up, so does the carburator and the circuits underneath it (if there are any…doubtful on that engine). Wear in the parts, including the needle valve in the float bowl and the float components themselves, could easily allow excess fuel to be pushed through the venturi that could cause, among other problems, backfiring.

Valves hanging up (sticking) are another high possibility…I bet the valve springs have, like myself, lost much of their spring. And I’ll bet there’s a touch of carbon in them there valves.

The good news is that a knowledgable shop should be able to get this fixed up for you without breaking the bank.