Back fire and power loss

[Drockw]

Bought a 1987 class a has Chevy 7.4 big block I have replace the plugs rotor rebuilt the carburetor put in a new fuel pump change the fuel filter runs once it gets warm and loses power and backfires won't go over 35 mph once it w
arms up

[campingking]

ignition coil under rotor inside distributor.

[wildmanbaker]

Fishbreath wrote:
How many miles and do you know the maintenance history?
I do agree that getting it on an scope will help to determine the problems. The 454 with high mileage (over 100k) had some issues with the timing chain jumping a tooth.

Its not just the 454, its all SB/BB chevys, fords, and Mopars. Its the funky timing chains that strength and wear the gears. We use to recommend changing the timing sets at about 75K to prevent failure and bending valves/pushrods.

[Fishbreath]

How many miles and do you know the maintenance history?
I do agree that getting it on an scope will help to determine the problems. The 454 with high mileage (over 100k) had some issues with the timing chain jumping a tooth.

[allbrandauto]

if the engine has a vacuum advance on the dist. disconnect the hose and drive it .it may seem a little lazy but if it doesn't backfire anymore its the pick-up coil in the dist.

[Gjac]

I would start by removing the doghouse cover and starting the engine at night, do you see any arcing from the wires to the engine block? The next thing I would do is check to see if that vintage engine has a plastic timing gear, then check the timing. Base timing is 4*BTDC it will run better between 9-12* BTDC. Let us no what you find and you will get get better help rather than a bunch of possible solutions.

[Stim]

I had the same thing with a 1983 Winnie 7.4 that I used to have (sold in 2001) and I can't remember what the problem was. ???
But I do remember it was associated with the distributor.
Could have been with the advance or ignition trigger or trigger wiring.
Should still be HEI distributor, BEST non-computer ignition system ever made!

[ernie1]

If it was a point and condenser setup I'd say definitely the condenser but I think that was way past gone in 87. I then would put a timing light to the distributor and check to see if the vacuum advance and /or the mechanical advance is working. It's a very common problem with the older distributors to have a gummed up mechanical advance.

[Bruce_Brown]

Plugged converter?

[Drockw]

Yes it does have egr and it works fine

[Chum_lee]

Suspect a vacuum leak somewhere in the intake. If it runs fine when somewhat cold and falters when warmed up, its usually a fuel supply issue. (or too much air) Does this engine have EGR?

Chum lee

[rgatijnet1]

For 1987 I assume that you may have a carburetor. Sometimes a backfire will ruin the power valve diaphram. Not a real expensive part and not too difficult to change.

[Drockw]

Thanks will dig deeper

[Executive45]

Hook it up to a diagnostic machine for the correct answer. Too many things that could cause a backfire. And, we don't know if it's backfiring through the carb or exhaust. Those engines were notorious for cracked exhaust manifolds so check that. Backfires are caused by raw fuel getting into the exhaust system before igniting. So, suspect the timing off, plug wires, wrong carb settings, timing off, cracked rotor cap, etc. etc. etc....Dennis

[MountainAir05]

backfire thru carb or exhaust. If exhaust then the spark stuff.