Popping under quick throttle opening

ForceFed70 · Apr 19, 2009

That is certainly not a carb/tuning problem.

The way you describe the sound/symptoms it leads people to think accelerator pump, but when you watch the video...It's clear that this is something else.

351Powered · Apr 21, 2009

1) Thanks for the replies everyone. I have gotten several others to listen to the car in person and it is a popping coming out of the carb

2) I hooked up a vac gauge to the manifold intake and I AM NOT getting a steady reading. The needle is hopping around between 15 and 20. I will try replacing the gaskets as you suggested Jim.

If all else fails, guess I should start doing some research on pulling the heads. I was a little hesitant to do so due to the fact I haven't done this before, but if that's what needs to be done, so be it. Thanks for the tips guys. ill keep you posted.

hardsy · Apr 21, 2009

possibly an exhaust valve no opening properly causing the compressed gasses to exit back through the intake track. bent pushrod maybe?

zookeeper · Apr 21, 2009

Before you pull anything apart, try pulling the plugs for inspection. If it is a valve, or in intake leak, one plug will look bad. Your compression numbers would indicate to me, that it's not an intake valve though. If it were, that cylinder would be much lower than the 145. Based purely on that, I'd be inclined to think the motor may be OK, but that it's more an intake leak, or possibly an ignition timing problem. You say it's OK if you accelerate slowly, which could also be a sign the vacuum advance isn't working. Perhaps it has a leaky diaphragm, which would also give you an erratic reading.

gtss · Apr 21, 2009

I have to agree with some of the previous posts
" possibly an exhaust valve no opening properly causing the compressed gasses to exit back through the intake track. bent pushrod maybe "

and pull the plugs to take a further look

a broken valve spring will cause strange noises, and often won't show up during a compression test due to the lower rpm

gtss

2nd Mustang · Apr 21, 2009

Google is a wonderful thing:

How to Use and Interpret a Vacuum Gauge

Read scenarios 5, 6,7,8,9, etc.

jcode68 · Apr 22, 2009

that link is awesome! New contribution to the toolbox

ron67fb · Apr 25, 2009

Work on this over the weekend? Sounds like an engine brake on a truck, which works by keeping the exhaust valve closed. Maybe broken/weak exhaust spring and/or pushrod, collapsed lifter or flat lobe on the cam. Another possibility is an intake rocker adjusted too tight. Re-adjust all the rockers and see what that does?

zookeeper · Apr 25, 2009

That's a good assesment, and that's exact;y what it sounds like, but he says it runs OK as long as he accelerates slowly. Wouldn't valve related problems show up no matter how slowly he accelerates?

ron67fb · Apr 26, 2009

Dunno, maybe an exhaust lifter starts to pump up as the revs go up? Slowly revving the engine could give the lifter enough time to get oil into it. The video is too short to tell, but it sounds like the noise gets quieter after each successive rev.

2+2GT · Apr 26, 2009

Have you checked the distributor advance? If someone as messed with the vacuum advance, and it's popping to full advance when you dump the throttle, it would do as you describe, but would be OK if you opened the throttle slowly, as you also describe.

351Powered · Apr 30, 2009

So, it turns out I am the proud owner of a wiped cam. Interestingly enough, the heads have been machined recently and adjustable rockers were put in. Now to get a new cam and figure out why it got worn in the first place...

2nd Mustang · Apr 30, 2009

351Powered said:
So, it turns out I am the proud owner of a wiped cam. Interestingly enough, the heads have been machined recently and adjustable rockers were put in. Now to get a new cam and figure out why it got worn in the first place...

Pushrods?

Hack · May 1, 2009

Make sure you replace all the bearings and clean the motor thoroughly before reassembling.

chucks1967 · May 1, 2009

You are absolutely sure its "popping" through the carb? It sounds like I problem I had. Sounded just like that. It actually turned out to be the flywheel bolts were loose. its sounded awful just like a rod, but it wasn't a consistent knock or pop sound. Get a stethoscope or a long screw driver. put the end of the stethoscope against the block around the front side and back and you will be able to tell where it is coming from.

stonecoldtx · May 1, 2009

chucks1967 said:
You are absolutely sure its "popping" through the carb? It sounds like I problem I had. Sounded just like that. It actually turned out to be the flywheel bolts were loose. its sounded awful just like a rod, but it wasn't a consistent knock or pop sound. Get a stethoscope or a long screw driver. put the end of the stethoscope against the block around the front side and back and you will be able to tell where it is coming from.

I think if you look a couple of posts up, you'll see he posted that he found that the engine had a flat camshaft . . .

chucks1967 · May 1, 2009

ahh. you are correct. my bad.

Popping under quick throttle opening

ForceFed70

That's why they call it "dope"

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