power valve question

[pl235c]

I have a 750 double pumper on my small block, the car runs great off idle, drives great at part throttle,
has no real issues except the car runs super rich at idle. If I adjust the mixture screws in so it runs clean
it still has throttle response but after you rev it up it will take like 5 seconds to return to an idle.
It does have 4 corner idle. Is this a case of the power valve being open at idle?

 

[SSmonte408]

what size pv is in it? how much vacuum does your motor make at idle? How far out are your idle mixture screws? seems like mine worked best ~1/4 turn out on the 4 corner set-up.

 

[holtjohn]

Can you tell us a little more about the motor? If you have a single plane intake on the motor that could be the issue, you might need to run a four hole spacer to increase the signal to the carb.

 

[pl235c]

victor jr., 1" open spacer, 72 square on jets, 6.5 primary pv, sec. pv is plugged,
mixture screws are 1.5 turns out,
355, solid flat tappet cam 560 lift 290 adv. duration, 9.75:1 comp., sportsman II heads,
3500, 4.10 gear 28" tire 3700# with driver...


turning the mixture screws in until it idles clean results in decent
throttle response but the motor takes a few seconds to return to idle.

 

[holtjohn]

YA you might try the four hole spacer and see if that helps out, it did for me. I would measure the vacuum at idle and choose a power valve that is half of you vacuum if it is under 11, if it is above 11 leave it alone.

 

[pl235c]

when you say signal you mean vacuum, correct? If that is the case
won't it draw even more fuel through the carb? Where should I plumb
the vacuum guage to check my vacuum?
The metering block port has been plugged and the only other port is the
one in the base plate, will this work?

 

[holtjohn]

haha you would think more vacuum would suck more fuel but it is quite the oppisite. see a carb is a controled vamuum leak and it needs vaccum signals for things to work right, like the power valve. The power valve is held shut with vacuum, the 6.5 power vavle is basicly held shut by 6.5 hg of vacuum, when the motor goes WOT there is no vacuum or very close to no vacuum. The power vavle then opens and gives you more fuel, if there isnt a strong signal to the carb, which single plane intake are bad for, the power valve has a tendency to let fuel by , so the carb runs rich. That port should work if it pulls vacuum all the time and isnt the timed vacuum port. you get me

 

[slimbo]

the power valve feeds the wells that lead to the boosters. the only time fuel can be added by the power valve is when the boosters are flowing. the power valve cannot add fuel at idle. seee if your dist advace is sticking and making the idle stay up for a few seconds. or possibly a vac leak. and the proper way to choose a power valve is about 2 inches less vac than the car idling in gear. my car idles at around 9 inches in gear so i run a 6.5 valve

 

[pl235c]

I tried to set the carb today, when it saw the most vacuum (about 9 hg) it was real lazy off idle and ran really weird, it idled great, but in gear just giving it a little throttle it was hesitant and tried to die, so I put it back the way I had it, and it idled at about 6 hg but just like before ran very rich, but had great manners all the way around.

slimbo if the mech advance was sticking wouldnt it do it all the time?
i'm totally clueless to all this crap as you guys probably can tell.
just help me please.

I just remembered when I put the carb together I out in pv blowout
protection and followed the instructions so the check ball would allow
flow into the plenum, (not up like a backfire) is it possible this is plugged
causing the power valve to stay open?

 

[holtjohn]

With the car running look down the boosters do you see any fuel driping out at idle?