What can cause a backfire?

[coachdtierney54]

I live on a mountain. Whenever I coast down the mountain I hear several popping noises coming from the exhaust. Once I slow down to about 35-40 mph, it stops. It's an 85 C30 with a 454. I have tried to retune the carb thinking it may be running too rich, but that did not help. Could it be a timing issue? Any ideas?

Finally, thanks to all that have given me some great advice over the last few months! You guys are amazing!

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[74 Shortbed]

You have an exhaust leak somewhere.

 

[coachdtierney54]

I recently replaced the header gaskets and I do not hear the usual hissing from them.

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[74 Shortbed]

May not be at the header gaskets, could be anywhere and very slight and not heard in the cab, need to run it on stands and get underneath and listen for it.

 

[smoothandlow84]

Could also be a bad plug or plug wire...an intermittent electrical issue. Cross plug wires will also cause a timing/backfire as well. (Don't ask me how I know that....)

 

[coachdtierney54]

Plugs and wires were changed when I replaced the header gaskets

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[Cuba]

Lean condition with fuel can cause exhaust backfire. In my cases, I've experienced that usually in acceleration though.

Also timing issues such as too retarded.

Another way to check for exhaust leaks is to jack it up, run it, and with a buddy... cover the tailpipe intermittently with a rag. This obviously pushes exhaust through the leaks, and sometimes easier to find. Look for soot on the exhaust, this is where it'll be leaking.

Those are just a few issues that can cause the exhaust to backfire ... the intake can backfire too.

 

[Honky Kong jr]

Valve float when using the engine as a brake.

 

[1987 GMC Jimmy]

Or you can stick a vacuum cleaner wand in the tailpipe and put a rag around it. You just can't leave it in there for long, or you'll overheat the motor.

 

[87scotty]

i always though that was a good sound when engine braking a little cackle in the exhaust

 

[Honky Kong jr]

i always though that was a good sound when engine braking a little cackle in the exhaust

Plays hell on mufflers tho.

 

[74 Shortbed]

Plays hell on mufflers tho.

Yeah, like it can blow the muffler right off the vehicle, lol..

 

[theblindchicken]

Yeah, like it can blow the muffler right off the vehicle, lol..

Then you'll just increase the efficiency of the engine and drop a few lbs. Sounds like a win-win to me.

 

[74 Shortbed]

Then you'll just increase the efficiency of the engine and drop a few lbs. Sounds like a win-win to me.

Good point..

 

[coachdtierney54]

Drove it a lot today, it's getting worse.

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