Backfiring Through Carburetor

[Achester27]

Hello all,

I just have a quick question that has been confusing me for a good while now. My car constantly backfires through the carb when I give it a quick shot of gas or in the higher RPM's. Its not really a backfire though, more like a muffled popping sound, I guess backfiring is the easiest way to describe it.

My Engine specs are:
-Chevy 350
-Holly 600 carb
-Mallory unilite distributor


Everybody has been telling me it is a timing issue, but i have fiddled enough with timing it for a while now and have seen no results.

Please help

[RahX]

Have you put a cam in it? Timing is a likely culprit since you put a distributor in it. Is it a vac advance?

[Achester27]

Mechanical, you think a lobe is bad on my cam?

oh its a chevy 350 from '74 if that helps at all.

[MagicRat]

Put vacuum gauge on it, and have a look at the manifold vac at idle. You may have a leaking intake valve and/or a worn exhaust valve lobe which will cause the needle to fluctuate.

[Achester27]

Well the thing is that I took it apart. I currently have the intake off and am planning to bring it down to the block. Is there anything I should look out for that would tell me why its doing that?

Only reason im taking it apart is my gasket set was on sale and i needed to fix the notorious oilpan leak.

[RahX]

Oil pans are best sealed with black silicone So the engine is fairly virgin aside from some minor updates? I'd definitely look at the cam while you're close. The lobes should be pretty egg shaped. The rear lobes on the cam tend to wear the most for some reason.

[Achester27]

Alright I'll look out for that.

Yeah, my engine is pretty plain. I also have aluminum heads.
You think it would be a stuck lifter or something thats causing it if it isn't that cam?

[Achester27]

Update:

I took all my lifters out today and found the culprit. The very last lifter I pulled out had completely worn down on the bottom. I'm taking a look at the cam tomorrow, but I'm pretty sure that will be worn away as well.

Anybody know where i could pick up a good deal on cam/lifter combos for a chevy 350? I'm not talking racing application, just street performer cams.

[RahX]

Summit all day long. Might as well step up to a set of 1.6:1 rockers and some good pushrods while you're at it.

[Achester27]

Another update:

I took the cam out and sure enough, it was worn on the lobe as well. This was for the exhaust valve too, so all this time my cylinder was just shooting exhaust back through the carb, which would explain the backfiring.

Anyway I picked up a cam from jegs, I found one with a good rpm range for street use and good lift that my heads can handle.

Thanks for the help!