Exhaust backfire/popping
 

Hello guys,

I've been having this problem for a while now, and I just can't get it figured out.

I have a 71 C20 with a stock 350, and it backfires or pops out of the exhaust when it hits around 2500-3000rpm. It pops erratically, anything between 1-5 seconds. It used to be worse (like popping during idle) but after changing the sparkplugs, it only does it in higher RPMs as I mentioned. It no longer backfires/pops during idle.

When I let go of the gas when I shift gears, it backfires/pops/gurgles a lot too.

However, once the truck has been running for several minutes and I get driving, the popping seems to be less prevalent, but still occurs on occasion. And it obviously still happens during gear shifts when I lift.

I know there could be 100 reasons for why this is happening, but I'm just wondering if anyone has experienced something like this before and could give some pointers from personal experience.

I think a key clue is that it only happens once the RPMs begins to pick up past 2500, and almost never happens at anything lower.

Any help is appreciated, thanks!

-David

Re: Exhaust backfire/popping
 

I had a similar issue a while back when I first purchased my truck, it was caused by back pressure, a bolt from the headers to the exhaust was missing, replaced the bolt and symptoms disappeared.

Re: Exhaust backfire/popping
 

My first thought would possibly be the points having a weak spring? Unless you've done an HEI swap.

Re: Exhaust backfire/popping
 

Thats is what mine was doing...I found a worn off cam lobe eventually.

Hope its something simpler for you.
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Re: Exhaust backfire/popping
 

That's what I was thinking too.

Re: Exhaust backfire/popping
 

I had a similiar issue with my truck and it was caused by backpressure. The donut from the exhaust manifold to my exhaust. Once i sealed that nice and tight it went away.

Re: Exhaust backfire/popping
 

Check your distributor cap for cracks on the inside. A crack will cause spark to jump causing a misfire.

Good Luck.

Re: Exhaust backfire/popping
 

I recently just changed the donut gasket around the exhaust manifold flange so it doesn't leak there anymore. But the popping is still there, although it lessened.

So I checked the exhaust for other leaks by blowing a Shopvac through the pipe, and discovered quite a few places where it's leaking.

(i've bolded my questions)

First, the one that concerns me the most, is a leak coming from near spark plug #6. The exhaust manifold flange bends near there, but it's not coming from the flange. Nor is it coming from the plug, since it's new and very tight. It feels like it's coming from near a bolt next to plug #6 that connects the cylinder head to the block. But I'm really not sure, because the manifold blocks my view. When the motor runs, I can feel exhaust gases pumping out of there. At idle, I can feel it pulsating at just around the idle RPM, which means it probably comes from an individual cylinder, before the manifold.

Does this sound like a cracked cylinder head? Or is it something else?

The second exhaust leak comes from the muffler to exhaust. The connection is held together by clamps. I've tightened the clamps down extremely tight but it still leaks like crazy. How do you get rid of those leaks where two pipes stick into each other with no gasket?

And my final question- is it okay to drive the truck with these exhaust leaks? Other than the smell, will it harm the motor since it's popping/backfiring?

Re: Exhaust backfire/popping
 

It's ok to drive with exhaust leaks so long as you aren't gassing yourself in the cab. I don't have an answer to why the popping is occuring, but I can answer your question about how to get rid of leaks between stuck-together pipes - weld them together.

Re: Exhaust backfire/popping
 

Could this be an ignition timing issue? Sometimes when I turn the key off, it would run roughly for an additional second or so. And when I step on the gas, there is a little more than half a second of lag before I get any power. This is the first carbureted vehicle I've owned, so I'm not sure if these symptoms are normal or not...

Re: Exhaust backfire/popping
 

Little chevy heads are bad about cracking between the valves. Even the good "double humps". A crack here causes a random "spit" at idle out the exhaust.

Your problem sounds like an exhaust leak. Weld the pipes together. Clamps suck!

The continued run with key OFF is most likely crappy gas. Ethanol is super bad for carbed engines. Run premium only or buy your gas from a Farm Co-op. Farm gas does not have ethanol. It is legal... at least in Texas to run on the street. Or buy fuel additive that have LEAD in it. make sure it says "Not for O2 sensors".

Re: Exhaust backfire/popping
 

Could be, check to make sure the vacuum and mechanical advances are working correctly. After 30 or 40 years, sometimes things need a little help.