Hard acceleration issue

[Coach]

Thought I would try again and post this a 2nd time to see if anyone has any thoughts. Upon hard acceleration from a complete stop or if almost at a complete stop and then hard acceleration, I am getting a loud popping sound coming from right under the floor, right where the floor shifter is. It is an intermittent problem and has happened only 3-4 times in the last month. Once it happens, I let off the gas immediately and it goes away and I can continue driving with no issues. Not sure if this could be a transmission mount, u-joint, driveshaft, or an actual transmission problem (700r4 - nothing but problems with that tranny, eventually I will be going to a TH350). Thanks for any help/advice.

[airdale94]

If you have any joints in your new exhaust around the area in which you think your noise is coming from, I would suspect your exhaust/timing. New headers and performance exhaust can alter tuning and a leak at a flange will make a "pop". Can be deceiving as to what the sound would be expected.

[Longhorn 70]

Lean or intermittent accelerator pump? I have seen this in the past...it was loose accelerator pump linkage on a carter 2bbl. I have also seen sticking check balls in the pump bore that doesn't allow the pump to squirt fully into the carb part of the gas squirts back into the bowl on a Q-jet. You may just be hearing it come up through the floor due to when/where it happens.

[Coach]

Thanks for the reply gentlemen. Hey Longhorn, I think you may have hit it on the head. After reading your reply, I starting looking up accelerator pumps on the internet and there are a couple YouTube videos made by Holley specifically regarding this issue (my carb is a brand new Holley 670 Street Avenger that was installed with my new crate 350/290 in October). The sound I was trying to describe sounds exactly like the sound of the pop when the carb backfired and chokeplate snapping closed. When it first happened, I thought it was the TV cable ratching loose, like if the cable was binding and too tight and popped. She is just about due for the 500 mile oil change, so I am going to have this issue checked along with the timing. One last question, could the timing be off due to the vacuum advance being hooked up? Chevy recommends that with this engine and HEI distributor I also put on brand new with the engine, carb and wires that the vacuum advance is supposed to be left disconnected and the engine is designed to operate using only the centrifugal advance to achieve the correct timing. My mechanic hooked up the vacuum advance about a month or so ago, which gave me better acceleration in first gear, but that is also when this new issue started.

[Longhorn 70]

the HEIs do have a LOT of advance available with vacuum advance. There are limiters you can put in yours to keep it from going too far.

I have my vacuum advance hooked up to manifold vacuum. When I mash it, the vacuum goes low for a few seconds then comes back up. when I cruise, I make sure I have NO MORE than 38ºBTDC combined between mechanical and vacuum. I had to drop mine to 36º, maybe my balancer is a bit off.