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Backfiring through the intake. 1988 Chevrolet P30 Van 454 4BL

1988 454 with Quadrajet carb. 47,000 miles in a motor home.

Only after 10 miles of driving does the engine backfire up the intake under load. Runs great when cold.

I have replaced the fuel filter. Cap and rotor, plugs and plug wires.

I will check the timing this weekend.

Can anyone offer other things to look for?

Can lack of sufficient fuel cause a backfire in the intake? I have a new fuel pump to put on, but how to check for fuel pressure on a carbureted engine? I don’t think it is fuel pressure because after sitting at a light at idle it will pop and sputter trying to gain speed.

If I keep the rpm’s high it will sputter less.

Click and Clack did say to look for vacuum hoses that may have blown off during the backfiring.

How hot is the intake manifold above cylinders 3 and 5 when sputtering and backfiring?

Hard to answer as I don’t have my hand there when driving… but if I take your question and think about what you are trying to get at. That is where the choke pull of is? or maybe some other type of sensor? I know that the exhaust cross over is in between there and 4 and 6. Your thinking maybe this area is getting too hot??
Do expound on your question and I will try to follow.

One more thing. I am not sure that the thermostat is working right. The temp gauge seems on the low side but I have not put a thermometer on it because the radiator has a foot long curved neck to deal with.

Thank you for your help

So old, so timing chain. If the ignition timing is off, the chain may be worn out and may have slipped a few teeth.

It is worth checking for the 5-7 ignition wire cross. It’s easy to put the wires on the wrong plugs.

“Runs great cold”

Yes, I wonder if there is any restriction in the exhaust on either side causing the cross over to heat up causing a lean condition and explosion in the intake as well. Even if there is no restriction the heat sometimes causes problems. There are after market heat shields and thick gasket spacers to eliminate such problems. If there is a restriction a few miles at highway speed will usually get the intake hot enough to determine if it is the problem.

before you adjust your timing remove your cap to some good light and check for microscopoic cracks barely visable, most likely from center to post, or post to post
as the engine warms they will get larger.