Backfiring through carb and getting worse, even at idle

[rumblebox]

My 81 with a 454 and smog has started to backfire horribly when I give it any gas at all. Now it even does it idling when the timing is set correctly. This happens regardless if the EFE flap in the manifold is open or closed. The left exhaust pipe is significantly louder than the right one, and I figured one of the muffler's was just worn out, but now I'm thinking there's probably a restriction from dirt daubers or something. What do you guys think? I mean, the truck literally had 50 or 60 dirt dauber nests throughout the whole thing when I picked it up.

The gas is pretty fresh, 5psi of fuel pressure, 2 different fresh carbs.

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[HotRodPC]

Do you have a vac gauge? If so, post the results and actions of what the needle is doing. You'd really think dirt daubers clods would heat up and get blown out of the exhaust. So this thing sat for a long time? How about a stuck open valve? Burnt valve? And it's only backfiring through the carb? Not the exhaust too? If it's burping and farting, could likely be timing chain has slipped a tooth or 2. And if the timing chain has alot of wear and you're timing the ignition off a light, then of course your light will be inaccurate cus the slop in the chain is going to be opening the valves at a different time in relation to the timing mark on the balancer. Soooo, you can always retard the ignition timing a few degrees and see if the backfire goes away. If it's an ignition timing issue, and only backfiring through the carb, timing is to far advance and the spark is igniting fuel before it gets into the combustion chamber.

 

[rumblebox]

Do you have a vac gauge? If so, post the results and actions of what the needle is doing. You'd really think dirt daubers clods would heat up and get blown out of the exhaust. So this thing sat for a long time? How about a stuck open valve? Burnt valve? And it's only backfiring through the carb? Not the exhaust too? If it's burping and farting, could likely be timing chain has slipped a tooth or 2. And if the timing chain has alot of wear and you're timing the ignition off a light, then of course your light will be inaccurate cus the slop in the chain is going to be opening the valves at a different time in relation to the timing mark on the balancer. Soooo, you can always retard the timing a few degrees and see if the backfire goes away.

I retarded it enough to where it wouldn't even idle, and it still did it. It only has 91k miles. I'm gonna pick up a vac gauge this week. I'm also going to disconnect the exhaust in front of the mufflers and see what happens.

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[rumblebox]

Intake only

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[HotRodPC]

yeah, only 91K, it shouldn't be a burnt valve. By 74 it should have been OK for unleaded fuel otherwise I'd suggest maybe it's possible for recessed valve or valves too. I think by 72 heads were safe for unleaded fuel.

 

[custodian]

Check your ignition module if you have electronic ignition.

 

[highdesertrange]

popping though the intake, valve train issue. highdesertranger

 

[custodian]

Not always a valve issue.

I had a built 283 that I put a stock GM electronic distributor in and was driving fine until I got up to go to work one morning and it started popping through the carb. The more I drove it the worst it got.

I also thought timing until I was told to check my ignition module. Replaced it with one I new was good and solved my problem.

 

[rumblebox]

So I figured it out immediately after removing a valve cover a couple weeks ago. Someone had over-tightened one of the rockers, and it completely wiped a lobe off the cam. So the exhaust valve opens almost not at all. My first thought was a bent valve causing the lifter to collapse, but the push rod did the same thing when I removed the rocker.

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[rumblebox]

https://youtu.be/c3Ng6_jJxGg

https://youtu.be/DP5v8rWhZ7s
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[HotRodPC]

So I figured it out immediately after removing a valve cover a couple weeks ago. Someone had over-tightened one of the rockers, and it completely wiped a lobe off the cam. So the exhaust valve opens almost not at all. My first thought was a bent valve causing the lifter to collapse, but the push rod did the same thing when I removed the rocker.

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That sucks ass. Good excuse to step up an RV cam though. Just be sure to replace all lifters too.

 

[rumblebox]

That sucks ass. Good excuse to step up an RV cam though. Just be sure to replace all lifters too.

Im just installing the cam and lifters from my 74, since im about to rebuild that engine anyway.

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