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Topic: Back fire

in Forum: C3 Engines


Back fire

Posted: 10/16/05 8:27am Message 1 of 8
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Rocklin, CA - USA
Joined: 4/2/2005
Posts: 484
Vette(s): 1970 T-Top350/350hp numbers matching 4 speed. It's a mess and 1970 Convertible 4 speed.
Hey, When I first get in my car and drive down the road my engine backfires when I change gears.  It only does this the first 5 minutes of driving. Is this normal?


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Back fire

Posted: 10/16/05 4:14pm Message 2 of 8
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Duncanville, TX - USA
Joined: 11/8/2003
Posts: 20214
Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
Normal?! No.
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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Back fire

Posted: 10/16/05 7:56pm Message 3 of 8
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Rocklin, CA - USA
Joined: 4/2/2005
Posts: 484
Vette(s): 1970 T-Top350/350hp numbers matching 4 speed. It's a mess and 1970 Convertible 4 speed.
I guess the first thing would be change the EGR valve. That is not hard, then I would bet it would be exhaust leak because I did have a leak around the the manifold and that was a different sound. I will keep you posted. Thanks for your time guys are the best.


Back fire

Posted: 10/17/05 4:59am Message 4 of 8
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BINGHAMTON, NY - USA
Joined: 7/19/2003
Posts: 3808
Vette(s): ......
             soak the HECK out of those EGR bolts b4 you try to break them loose


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Back fire

Posted: 10/17/05 8:58am Message 5 of 8
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Miramar, FL - USA
Joined: 3/16/2004
Posts: 672
Vette(s): 1972 coupe/350 cid/mild cam/4 spd/20 ft. paint (looks perfect from 20 ft.), but it sure runs nice.

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




Back fire

Posted: 10/17/05 6:33pm Message 6 of 8
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CUYAHOGA FALLS, OH - USA
Joined: 12/2/2003
Posts: 6424
Vette(s): 1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

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?

kstyer38642.774224537


Back fire

Posted: 10/18/05 8:29pm Message 7 of 8
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Rocklin, CA - USA
Joined: 4/2/2005
Posts: 484
Vette(s): 1970 T-Top350/350hp numbers matching 4 speed. It's a mess and 1970 Convertible 4 speed.
It does it when I let off the gas to shift. After about four minutes of driving it goes away. Its not one loud pop but couple off pooping nosies.


Back fire

Posted: 10/19/05 6:56pm Message 8 of 8
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CUYAHOGA FALLS, OH - USA
Joined: 12/2/2003
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Vette(s): 1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!

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.




in Forum: C3 Engines


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