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Discussion Starter · #1 ·
I had a few minutes this morning to play with the carb some more. I checked the primary fuel level. It was even with the bottom of the sight glass. I adjusted the secondary fuel level to be even with the bottom of that sight glass.

No change on the surging deal.

I unhooked the vacuum advance and plugged that port. It is currently hooked up to the carburator, on the passenger side...not at the base, but higher up. The engine was still surging.

I went around and squeezed off every vacuum line coming off the carb and intake one at a time to check and see if I had a vacuum leak somewhere. Nothing made any difference except the PCV line. I squeezed it off and the engine chugged down and died. I restarted the engine and pulled the PCV valve out of the grommet on the valve cover and the idle picked up and the surge went away.

I guess I think what the surge is related to is engine vacuum cycling during some sort of fight between the PCV valve and the power valve. I've got a stock 4160 power valve installed. I believe that opens once engine vacuum drops below 6.5 inches of mercury. Once the power valve opens, I'm theorizing that the idle speed surges up, increasing vacuum and closing the power valve.

I guess I'm thinking step one is replace the PCV valve.
Step 2 is replace the power valve with the next lowest numerical valve. I THINK I have a 6.5 inch valve. I THINK that would mean installing a 4.5 inch valve.

Does that sound about right?


EDITED PORTION...Hang on hang on hang on. Looking at the Holley Technical information page, they show 2 Power Valves...A primary AND A SECONDARY. Unfortunately, they have one drawing for the 4150/4160. Does my 4160 have a SECONDARY POWER VALVE??? If so...THAT would explain why this problem went away with the secondary float adjusted all the way in????

1,106 Posts
Get a vacuum gauge. The power valve setting should be half of your idle vacuum. IE at idle your vacuum is 15 inches then you power valve should be 7.5. Also, there are economy power valves with 2 stages and performance power valves. The economy PV is an easy way to up your MPG but will lose some power at the transition. Oh, there is something else that I just can't remember right now, that can blow out if the carb is not tuned correctly. Maybe someone else will bring it up.

Good luck
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