Popping under quick throttle opening

That sounds nasty. Hook up a vacuum gauge to a manifold vacuum port. A vacuum gauge is very handy in trouble shooting. It really sounds like something banging around as there doesn't seem to be a stumble when you blip the engine, just that knocking sound.

Hmmm, I suppose it could be some nasty rod knock.

I'd be tempted to look into other areas of your driveline. Could be that the driveline is shifting under load. Driveshaft/u-joint comming into contact with exhaust, etc.

Definately inspect the motor mounts. When my motor mount let go, that's how I knew... the fan started to come in contact with the fan shroud when the engine was under load.

2nd time a mount let go, I wasn't so lucky

sounds like valve knocking on a piston to me!

hardsy said:

sounds like valve knocking on a piston to me!

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If that was the case, I'd expect it to make that sound all of the time. Not just when the engine is under load.

Ehhhhh, dunno. More load = more cylinder pressure. Something that definitely require no more running till it's figured out.

I still say it's a broken piston . . .

zookeeper said:

Holy crap!!! I never watched your vid, but forget everything I said earlier, you have something very unhappy inside the motor. Start by pulling the plugs and I'd bet you find something broken in at least one cylinder.

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jcode68 said:

That sounds really bad, definately stop driving it until you identify the problem. If it is backfiring through the carb, it might be possible that you have the firing order wrong. You should verify the firing order based on the cam you have installed. Eliminating the simple things first is the best place to start. Did you just complete an engine rebuild or something?

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10secgoal said:

WOW, stop running it before you do more damage. Sounds like a bent or sticky valve to me. But either way, you got some work ahead of you.

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stonecoldtx said:

That doesn't sound like "popping" to me, it sounds more like a broken piston, because it only occurs under a load . . .

You might want to take a compression test, and maybe try to put it under a load to see if you can determine which cylinder is making the noise.

You may be able to accomplish this by having your emergency brake on with the back end raised, and then have someone rev the engine a bit and let out on the clutch while you or someone else is under the hood pulling plug wires.

If you can make the engine noise occur under those conditions, then it should stop when you pull the plug wire of the bad cylinder.

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stonecoldtx said:

I still say it's a broken piston . . .

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don't ignore these guys...Most of em have years in the business....i think you're hoping its going to be a simple fix...but i don't think it is. Take the hour to pull the heads off the car and take a look. You'll be able to see if its a sticky valve and what kind of damage has been done.



if it were a pump shot problem, you'd get a sputter and stall...or better yet a backfire (if its too small) because it would be running to lean under load. this sounds like a hard core knock. Whether it be from pining or (most likely) that someone inside your motor isn't a happy camper.

you say its not as bad in person...I say it ALWAYS sounds better in video. video mutes out stuff you wouldn't hear otherwise.


do yourself a favor, pull the heads, and find out.

The knock gets faster with the rpm's, definately not a lean pop it's as it's consistent with engine speed and doesn't do it at the hit of the throttle.

I totally agree with Jlang pull the heads and have a look, revving the crap out of it aint gonna fix it!

351Powered said:

Spent the day talking to several other guys about my problem and they seemed to think it was the accelerator pump or to try using a bigger discharge nozzle, since the one on the 4160 is smaller than what would have been on the stock carb. (Much as you suggested Zoo Keeper) I'll post back after these two things get checked out. If I get a larger nozzle, how much bigger should I go? The 4160 has a .031 should I pick the size the original 750cfm carb would had?

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Ok, I'm going to give you the benefit of a doubt. Maybe, the video sound is misleading on the severity of the noise. You are the only one on this forum who has actually heard it in real life, so let's just say it is a pop caused by a super lean carb. It could either be a vacuum leak or a lean accel pump causing the pop. You need to make sure it is NOT a vacuum leak before you start tuning it. To do that, try hooking up a hand-held vacuum gauge to a manifold vacuum port. If your car has a stock or mild cam, you should see a steady reading at idle. The idle should not fluctuate, nor should the gauge reading. If you get a fluctuating needle, or the car will not idle on it's own, then replace either the base gasket or the manifold gasket or both to fix the vacuum leak. Once the engine has a good idle, and a steady vacuum reading, you can start to tune the carb. First make sure the accel pump circuit is working as it should, regardless of a small pump shot. Witht he engine not running, and the air cleaner off, look down in the carb and work the throttle by hand, and you need to see a shot of gas coming from each squirter. If you do not, then replace the accel pump umbrella and spring and clean all the passages in the metering block, as well as blow out the squirter nozzle. Then recheck it for an accel pump discharge (once the carb bowls have gas in them again). If you now have gas where you didn't before, drive the car again and evaluate any change.
If the carb still pops, it's time to start tuning. You can either tune the nozzles or the cams or both, but only make one change at a time. I'd start by making sure the linkage is tight on the cam. The cam is located on the throttle shaft near the throttle linkage, and all Holleys have two positions. They are marked "1" and "2". If the screw is in the "1" location, remove it, move the cam slightly until the "2" hole lines up, reinstall the screw, then test drive. You can also swap different cams altogether, and your local parts guy or Summit can help you pick a larger accel pump cam. If you want to swap squirters, then first note the squirter you have by removing it, then noting the number on the side. If I remember right, they are numbered from (I think) 25 to 51. The smaller numbers give the longest shot of gas time-wise, but the smallest amount. Going to a larger number increases the actual amount of gas, but decreases the time of the shot. If it's too lean, go up 2 to 3 sizes and then test drive. If you are doing this on the car, be very careful since there's a tiny washer under the squirter, and dropping it into the motor will make you say lots of bad words. Also, don't rule out timing, check your timing curve before you start to ensure your initial timing is right, and to make sure the advance is working correctly. Hope this helps. Jim