Holley power valve

[Paul G]

I rebuilt my 750 CFM, 4160, 80508-1 Holley a couple months back and it hasn't been right since. I Put a kit #37-754 in it because my needle/seat assy were leaking past and flooding out the carb. Should have just put new needle/seats in it and maybe I wouldn't be having these problems.

The motor stumbles badly from "off idle" to "full throttle", basically when I floor it. Slowly press the pedal and all is well. Hold the brakes to get past the stumble and all is well. Floor it at speed and all is well.

I Adjusted the accel pump arm like the instructions say, .015 at full throttle, no play between the arm and lever. Get a good squirt right off the throttle stop. This had no change in the stumble. Usually if this adjustment is off I will get a slight hesitation. Not the stumble I am getting.

The kit came with a blue 50cc accel pump. I put my old 30cc pump back in, no change. Today I pulled out the power valve, it is a 65 just like my old one. The only difference is the old one has a lot more resistance to open it than the new one has. I can suck (through a piece of hose) the new valve open easily, can just push it open easily, I have to suck harder to get the old one to open, and it takes a bit more push to move it open.

Is the new valve okay or trash?

BTW, no backfires have occurred, ever. And one time I noticed in my mirror black smoke during the stumble.

[R2]

Maybe try a few different,,,bigger squirters... / shooters.... for a longer pump shot ?

[Paul G]

The accel pump circuit was the problem.

I took my old power valve and my new one to Lopers in Phoenix. They are one of the few speed shops left. Only two in all of Phoenix. We determined that my old power valve had been hacked. The round nut had been turned in increasing the spring tension and making it have very short plunger travel. I picked up an 85 PV instead of the stock 65. An 85 more closely felt like the hacked 65 I have. With 10" vac in drive an 85 is a better choice for a performance motor. 

Put it back together with the 85 PV and no difference in tip in performance. Still had that nasty bog, but no backfire. Reading up some more I started to find that a lean, not enough fuel, condition is accompanied by a backfire. I was not getting any backfire so I went the other way. I bent the throttle arm that pushes on the accel pump arm. The arm had been bent before, marks from a pliers were on it. Bent the arm up a little shortening the accel shot. Voila! That was it. Moving in the right direction. I moved the black plastic accel pump cam to position 2. That made it a little better yet. I want to try a smaller squirter. It has a 25 which should be stock. But who knows, that may have been drilled out, or just too big.
 

[charger_fan_4ever]

You runnning a 4150 style mechanical holley ?

with only 10in of vac @ idle i'd go with like a 4.5 powervalve. A 6.5 powervalve is std for stockish engines with like 15inches of vacuul at idle. To me it sounds like when you tromp it the vacuum drops and your PV is opening almost instantly.

[doctor4766]

Holley themselves have some good info on power valves.
Check this one out
http://www.youtube.com/watch?v=6vKlAnKdcB4

[Paul G]

You runnning a 4150 style mechanical holley ?

with only 10in of vac @ idle i'd go with like a 4.5 powervalve. A 6.5 powervalve is std for stockish engines with like 15inches of vacuul at idle. To me it sounds like when you tromp it the vacuum drops and your PV is opening almost instantly.

It's a 4160, vacuum secondary. I dont know the diff between them really. We picked a 85 because it will open sooner more like the hacked 65 that was in it. There are two schools of thought on PV's. One is, for a stockish motor, to use a PV half the vacuum at idle. The other is for a performance motor, to use a PV 2 steps lower than the vacuum at idle. Both vacuum numbers taken in drive, if automatic trans.

I didnt really feel much difference between the 65 and the 85. Maybe on a dyno you would see the numbers, or when I get the accel pump straightened out I might be able to tell a bigger difference.

[charger_fan_4ever]

i had a 425hp small block ford and with 1 cam it was making 7 inches of vacuum, so i went with a 4.5 PV. I swapped cams and ended up with 15 inches of vacuum with the new cam and more bottom end. Still had the same 4.5 PV in the same  750 proform carb. I had to jet down 3 jet sizes, but never touched the PV and no noticeable difference. LAuched it on slicks at the strip and no hesitiation.

I'm not too familiar with the 4160s. Basically the 4150s are mechanical as once the throttle is 1/2 open the linkage opens the secondaries. No input from vacuum.