Carb backfire and Timing mark off 79 Chevy 350

[jcjoggerst]

I just bought a 79 Chevy C20 with a 350. It runs very rough and seems to be backfiring through the carb. (That's what it sounds like but no flame). Runs just as rough at idle or higher RPMs. The kid I bought it from said it just gradually started getting worse.

Well I checked timing first, and the timing mark is way off. It's about 2-3 inches BTDC. However if I turn the distributor to make the line closer it runs worse and eventually dies before I can get it to the timing mark. I've read how the harmonic balancer rubber part can slip so I just put the timing back and moved on. But now here's the interesting part. I brought #1 cylinder to TDC and the timing mark is straight up. And the distributor rotor points at the post going to the #1 cylinder! How can that be?!?!?! Why does the timing light fire when so far before TDC??

The distributor cap and rotor look in good shape as well as the plug wires. The mechanical advance mechanism looks to be in good shape not worn or loose. I made sure the wires are separated to eliminate any crossfire - he had them zip tied but when I separated them it made no difference. I've been thinking it could be a fuel problem, but with the timing way off it makes me think there's an electrical / timing issue. Oh also the timing mark does not move with vacuum advance attached or not. The fuel pump is getting fuel to the carb fine.

Any thoughts / suggestions would be MUCH appreciated!

[bake74]

     Try using a vacuum gauge to set your timing, go to you tube and look at a couple of different video's on how to do it.
     It does sound like your harmonic balancer slipped and no longer lined up.

[jcjoggerst]

Ok I'll do this later today thanks for the suggestion. Anyone have any idea if it could be from the ignition coil secondary windings? The test in my Haynes for that failed with infinite resistance. Rotor button to ground terminal was in spec, and the primary coil had the proper resistance, but rotor button to BAT was infinite. I took the coil out to make sure it wasn't just the end connector. I can't tell from googling what effect that could have exactly? The manual says only to replace the coil if BOTH primary and secondary fail, but I think I'll run over and grab a new coil anyway.

[Jason S]

While running under current conditions, is the engine idling at a fairly high rpm ( >1,000 rpm)?

[jcjoggerst]

I don't have a tach, but no it seems to be idling very low (<1000 RPMs). Why do you ask? I've tried swapping coil, module, rotor, cap. No help so far. Now I'm thinking carb or valve issue

[Blazin]

Gradually got worse, sounds like a bad cam to me. Only other time backfire/ popping through the carb is if the dist. is 180 out, but would be accompanied by fire.
Pull the valve covers off and run the engine, All the rockers should have a good amount of travel, and be the same amount of travel.
I would bet when you increase the RPMs the popping increases as well? Under a load / driving the truck probably pops worse?

[jcjoggerst]

Gradually got worse, sounds like a bad cam to me. Only other time backfire/ popping through the carb is if the dist. is 180 out, but would be accompanied by fire.
Pull the valve covers off and run the engine, All the rockers should have a good amount of travel, and be the same amount of travel.
I would bet when you increase the RPMs the popping increases as well? Under a load / driving the truck probably pops worse?

Oh no I hope not! Cam is more than I have tools or time for! But yes popping increases with higher RPMs and/or driving. Thanks for the reply I'll check the rockers.

[Blazin]

If the balancer slipped, you could still adjust / turn the distributor and make it run better. I know you need the timing mark to time a motor, but you can get it close, running the best by ear without a mark, or light.

[jcjoggerst]

If the balancer slipped, you could still adjust / turn the distributor and make it run better. I know you need the timing mark to time a motor, but you can get it close, running the best by ear without a mark, or light.

Yea at this point I don't even care about the timing mark. I found the place it runs best (by ear and vacuum) but "best" is a stretch. It still runs like crap in that spot.

[bd]

Gradually got worse, sounds like a bad cam to me. Only other time backfire/ popping through the carb is if the dist. is 180 out, but would be accompanied by fire.
Pull the valve covers off and run the engine, All the rockers should have a good amount of travel, and be the same amount of travel.
I would bet when you increase the RPMs the popping increases as well? Under a load / driving the truck probably pops worse?

X2

Flat exhaust lobe especially will cause "pop-pop-pop-pop..." through the intake.  The quicker/further the throttle opening, the worse the symptoms.

[79gmc15]

What about the timing be off at the crank? Would that cause this?

[bd]

What about the timing be off at the crank? Would that cause this?

See the following...

...Only other time backfire/ popping through the carb is if the dist. is 180 out, but would be accompanied by fire...

If the balancer slipped, you could still adjust / turn the distributor and make it run better. I know you need the timing mark to time a motor, but you can get it close, running the best by ear without a mark, or light.

The symptoms, as they are described by jcjoggerst, are too severe for the problem to be caused by ignition timing, alone.  The cam isn't the only possibility, but it is a good bet that should be investigated.

[jcjoggerst]

Gradually got worse, sounds like a bad cam to me. Only other time backfire/ popping through the carb is if the dist. is 180 out, but would be accompanied by fire.
Pull the valve covers off and run the engine, All the rockers should have a good amount of travel, and be the same amount of travel.
I would bet when you increase the RPMs the popping increases as well? Under a load / driving the truck probably pops worse?

X2

Flat exhaust lobe especially will cause "pop-pop-pop-pop..." through the intake.  The quicker/further the throttle opening, the worse the symptoms.

Hmmm well I don't think I would describe it as "pop-pop-pop-pop...". It's more intermittent - not regular. At any rate thanks all for the replies I'm gonna check out the rockers travel as suggested above, then most likely take off the heads while i'm at it...

[jcjoggerst]

OK took off valve covers - the #4 exhaust push rod is not coming up at all. So exhaust valve not opening and causing my carb backfire. Pulled the pushrod out and it's fine. That's as far as I've gotten though. I'm hoping its just a lifter issue and not the cam. All the other valves seem fine though.

Anyone have tips on pulling out / replacing the lifter? I'm hoping I can just take the intake manifold off and not have to remove the head.

[454Man]

Might aswell slap a new cam/lifters if your gonna that far. Rockers, valves, push rods, and lifters not moving = flat cam lobe.  hope this helps and good luck!