340 popping through carb at low RPM.

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Matrick69

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I asked this question in my project thread and it hasn't gotten a whole lot of attention. My 69 Barracuda 340 4spd has an issue of popping through the carb during acceleration at low RPM. Most notably when taking off from a stop sign. If I baby it and accelerate slowly, it wont do it but every normal attempt it pops. If I leave hard with more RPM it works fine. Engine idles perfect and runs great rolling down the highway. Problem seems to be worse once it warms up. I have messed with the timing a little in each direction with no change. All my ignition components are new as well as the carb.

Ive read similar posts and many suggest a worn/wiped cam. If this was the case, I would assume there would be some valve train noise. My engine is noise free.

Thanks in advance for the help guys! This is my first Mopar and will definitely be learning along the way.
 
Many possibilities, I will start. Carb accelerator pump not giving a full shot. Look down the venturi and make sure you are getting a full squirt. Then we can talk about timing and cam timing, vacuum advance, points etc.
 
Describe the pop. Does it pop with every fire of the cylinder? In other words, very rhythmic? If it does, then SOMETHING is holding an exhaust valve shut. Whether it's a wiped lobe, bent pushrod, broken rocker......whatever. The engine actually will not skip. It simply blows the exhaust back up through the intake manifold and is the noise you're hearing.
 
Eddy carb...You are short on pump shot or need stiffer step up spring to enrichen more if it's off the line.

....but cruise would intake valve cracked or open....exh lobe flat.... very rare to have the timing that far advanced to emulate that...but you never know.
If an exh valve were hung open it would pop ...pop... pop.. out the tail pipe.
 
Pull both valve covers and disable the ignition and have some one crank the engine and watch the valve train make sure all the rockers move up and down equally
 
When I first got it running, it would pop from the exhaust at idle once warmed up. After I got the timing sqaured away, that issue is gone. Once and a while it will pop out the exhaust as it idles down at a stop light. Usually only one or two pops and it quits.
 
When I first got it running, it would pop from the exhaust at idle once warmed up. After I got the timing sqaured away, that issue is gone. Once and a while it will pop out the exhaust as it idles down at a stop light. Usually only one or two pops and it quits.
You said you have the TIMING SQAURED AWAY.
What is the initial timing at idle?
Do you know how much total centrifugal advance the distributor has?
Are you running a vacuum advance?

There is a good possibility that the carb is lean on the primary side.
Have you tried tuning the carb per the Edelbrock tuning guide?
 
The timing was set when I got the engine and had a slight ping under acceleration. I backed the timing off slightly and the ping and exhaust pop went away at idle. I have not put a light on it yet. The distributor is not vacuum advance and I have not done any carb adjustment other than the procedure for the two front screws.
 
Please see the question in post 6. Describe the pop please. Does it go pop pop pop pop or is it pop here pop there run good a minute and pop here and there again?
 
Please see the question in post 6. Describe the pop please. Does it go pop pop pop pop or is it pop here pop there run good a minute and pop here and there again?
It definitely not a pop pop pop. More like one or two pops on acceleration and then clears up once it's off that 1000-1200 rpm range. It will also do it randomly on the highway if the motor slows down going up a hill in 4th gear. If I speed up to keep the rpm up, it's fine
 
It definitely not a pop pop pop. More like one or two pops on acceleration and then clears up once it's off that 1000-1200 rpm range. It will also do it randomly on the highway if the motor slows down going up a hill in 4th gear. If I speed up to keep the rpm up, it's fine

See? That was a very important question that you ignored. Had I not insisted on you answering it, you would not have known that the camshaft and valve train has an over 90% chance now of NOT being your problem. See how easy that was?

I'll be damned if it almost doesn't sound like a couple of plug wires crossed up. 18436572 is the firing order, and that's rotating clockwise. Double check your firing order and let us know. And before you say it, don't say "I know it's right", go check it. I cannot tell you how many times I JUST KNEW I had it right and it wasn't. LOL
 
I've had the same problem with a loose coil wire. Again it could be several things but another thing to look for...
 
See? That was a very important question that you ignored. Had I not insisted on you answering it, you would not have known that the camshaft and valve train has an over 90% chance now of NOT being your problem. See how easy that was?

I'll be damned if it almost doesn't sound like a couple of plug wires crossed up. 18436572 is the firing order, and that's rotating clockwise. Double check your firing order and let us know. And before you say it, don't say "I know it's right", go check it. I cannot tell you how many times I JUST KNEW I had it right and it wasn't. LOL
Sorry, I wasn't trying to ignore you. In the middle of a yard project with my wife and trying to get to the bottom of this. I took the air cleaner off and checked the wires again and they are correct. It has a unilite distributor in it and I've read multiple horror stories about them. Wondering if it could be part of the problem.
 
Sorry, I wasn't trying to ignore you. In the middle of a yard project with my wife and trying to get to the bottom of this. I took the air cleaner off and checked the wires again and they are correct. It has a unilite distributor in it and I've read multiple horror stories about them. Wondering if it could be part of the problem.

Anything's possible I suppose. I've always had good service with Mallory products.
 
Check the inside of the distributor cap for carbon tracking or a loose distributor shaft moving around.
 
One more tip. Open the hood running at night. Look for arcing around the plugs and wires and at the cap. If it's an automatic, get someone to sit in it hold the brake hard and apply the parking brake. Drop it in drive and gently run the RPM up just a little to load it up some and look for arcs. Sometimes it takes a little load for it to show up.
 
I had a Unilite on a car that had a high speed miss pulling up hills. Searched for that for years until I replaced the whole distributor and the miss disappeared. Have not been a fan of Unilite since.
 
I made a short video tonight while we were cruising around and will post it once I figure out how. Does it have to be a link to a you tube video?
 
Describe the pop. Does it pop with every fire of the cylinder? In other words, very rhythmic? If it does, then SOMETHING is holding an exhaust valve shut. Whether it's a wiped lobe, bent pushrod, broken rocker......whatever. The engine actually will not skip. It simply blows the exhaust back up through the intake manifold and is the noise you're hearing.
Burnt valve------------
 
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