Topic: Back fire
in Forum: C3 Engines

Moderator

Several possibilities come to mind......
1) EGR valve malfunction, or vac. to the egr
2) Exhaust leak at or around the manifold/header pipe
3) Carb not functioning properly(internal leaks/vac. leak)
4) Carb. throttle blades closing too far/idle speed low
5) Ign. timming off
An exhaust leak will cause this, because sometimes, the leak will go away once the part that is leaking gets hot, and expands. (usually a manifold crack, or leak) If the exhaust system is getting cooler air into it from a leak, then, when you let off the gas to shift, there is a "vaccuum" in the system, and it will draw the colder air in, and that's what makes a backfire noise.(Kinda like what happens when it thunders in a storm)
I would start by checking for an exhaust leak when it is cold, and then go on to the other possibilities.


Joel Adams
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I always thought that backfiring (out the tailpipe) when shifting gears was caused by the sudden letting off the gas, sending little droplets of unburned fuel rushing through the exhaust and detonating when contacting the hot pipes.....like I always say.....what I don't know is alot.
JR
Let me ask, is this when the engine is cold? I'm guessing so. Next, is it backfiring in the exhaust or the intake?
If it's the intake when cold, the problem is a choke adjustment. Running too lean cold.
Another good piece of info need from you. Does it do this when you let off the gas to shift, or when you apply gas during the shift?
Backfire in the exhaust on decell is usually due to a rich fuel condition. This would be the case during cold operation with the choke on, but you may be able to lean out the choke a bit and cure the problem. Be sure to check the choke vacuum pulloff. If the pulloff has failed this could be the problem.
Once the car warms up, the choke opens, and the problem goes away.