Popping sound thru the carburetor!!

[Pac06]

New to the forum, first post. On a 72 Chevy with a 350 engine and 2 barrel carb. Just finished changing tranny fluid when I was test driving truck and went to make sure kickdown worked by punching the accelerator, popping sound started. Now any time I accelerate suddenly, popping thru carb!! Also I noticed a ticking sound from one of the valve covers. Possibly related??? Any help would be greatly appreciated. Thanks!!!

[LONGHAIR]

Related? ........probably. It sounds like you have a bent pushrod, probably on an exhaust valve. The valve is not opening when it should, so the exhaust is coming back into the intake, when that valve opens. That would account for the tick also. The same could happen with a flat lobe on the cam, but it probably wouldn't tick.

[68gmsee]

Welcome to the site...

It's possible you have a bent rod, but the symptoms could also be a stuck lifter. Read the thread below and look at post #6. I believe it was resolved with "seafoam."

It's also possible that you may have dislodged a vacuum hose or it may just happend to have a flat cam. Other possibility is a bad intake valve not seating properly. This lets fuel back into the intake manifold and pops...

oops forgot the thread:

http://67-72chevytrucks.com/vboard/s...hlight=ticking

[LONGHAIR]

A sticky lifter really shouldn't cause a pop in the carburetor though?

[70inline6]

First off, Welcome!

Probably just a coincidence. The popping sound you describe under full acceleration is the engine's response to a lean fuel mixture. It could be a vacuum hose fell off, but more probably it's always been that way and you just noticed it when you did your kick-down test. Turn the fuel mixture screws on the carb 1/4 turn counter-clockwise to richen the mixture a bit and test. Repeat if necessary.

The ticking sound is probably just crud in a lifter that was dislodged from somewhere else. Do the Seafoam rinse as 68gmsee suggested and drive it.

If the tic becomes a TAP or KNOCK, you have bigger problems like the pushrod or connecting rod Longhair and 68gmsee alluded to. Probably nothing more serious than fuel mixture and a little crud though.

[Southpa]

The last time I had backfiring out the intake, under load, was when the center pole of my HEI distributor cap broke. I'm surprised the truck got me home, must've been a real light show inside the cap.

[Pac06]

A little more info the ticking sound will disappear in idle for a couple of minutes then start up again!! Where can I get "seafoam"? Thanks again.

[beautimus]

i gotta agree with longhair on this one, if the pop sounds like a high pitched tapping sound in the carb under acceleration, then you have a bent/broken push rod, or possibly a rocker arm with the bottom busted out of it. pull the valve cover and see. valve cover gaskets are cheap.

[68gmsee]

Originally Posted by LONGHAIR

A sticky lifter really shouldn't cause a pop in the carburetor though?

True... In my post I was referring to the ticking sound re: the sticky lifter.
The rest of the response was probable causes of popping sound from the carb.

Loud popping or backfiring through the carb is usually caused by unburned fuel being ingnited in the intake manifold and being forced back out the carburetor (no where else to go). Sometimes valves not seating properly or at the proper time will cause it.

[Dusty_case]

allmost any autoparts stroe has seafoam over the counter

[Pac06]

Thanks guys, I did find a bent push rod!! I went and replaced it and it ran great, but on first hard acceleration guess what came back, same symptoms. Pulled valve cover off, same push rod bent again. Is the possible problem a bad lifter as some have suggested, or should I go a different route?? Thanks again!!

[68gmsee]

Originally Posted by Pac06

Is the possible problem a bad lifter as some have suggested, or should I go a different route?? Thanks again!!

STOP THE HARD ACCELERATION!!!

Seriously, I think the truck is trying to tell you something. Unless you absolutely, positively have to speed off, don't stomp on the gas. I would gradually increase speed. Unless you have a new engine especially built for racing, these trucks were not designed for that type of sudden stress.

But it could be the lifter is stuck in the down position, essentially making it a solid lifter. Does the adj nut go farther down than the others?

[LONGHAIR]

How did you adjust the valve with this new push rod?
Something is definately wrong, here. Something is binding and so-far the push rod is the weak link. (which is a good thing)
It is possible that the valve stem is bent and binding/sticking. Did the rocker arm and ball look ok when it was apart? Like 68gmsee asked, are the nuts all at "approximately" at the same height? That lifter could be collapsed, essentially making it solid, which may show by the nut being farther down the stud. It could also be too far down because of simply over-tightening when installing.
A new push rod and a "do over" are in order here, along with some inspection.

[Pac06]

I think I got it. Found lifter problem, the little u-shaped spring inside lifter was not seated in on one side. Also top of valve was mushroomed, not straight at all. Underneath rocker arm, wear pattern was off to one side and not in the middle. Thank you guys for all the help. Now I'm going to ask for some more info, when the head goes back on, what is torque value for head bolts and what kind of pattern to tightened them down. Also to tighten down rocker arms is there a procedure or just tightining down on each nut to certain torque spec. Thanks again guys!!!