Power valve. Holley carb.. How do you tell if its bad??

Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 07:45 PM

How can you tell if the power vavle is bad on a Holley 750 DP? Can you suck or blow on it and tell? I dont want to replace it if its good, but I don't know how to tell. The car is running bad most of the time,((After I changed the jets in the carb, but that's another story))and was told that the P/V might have gone south.. scott

Posted By: OzHemi

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 07:50 PM

Yep, you can suck on it, see if it moves in and stays. If you can keep sucking and it is leaking air, it's bad.

The car running really rich all of the time (all of a sudden) is what makes me usually think the power valve went.

They could go hard and not move on carbs that have been sitting around a long time (had that happen once too) but usually it seems they blow out.

Posted By: 70AARcuda

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 07:52 PM

you can actually suck on it and feel the little plunger move in the back...

someone makes an adapter that you can screw the power valve into and check it will a vacuum pump...

or you can just replace it...

Posted By: Mr.Yuck

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 07:57 PM

Quote:

you can actually suck on it and feel the little plunger move in the back...

someone makes an adapter that you can screw the power valve into and check it will a vacuum pump...

or you can just replace it...

yeah that

Posted By: Dodgem

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:05 PM

I suck on them create a good vacuum with spring pulled closed and stick them to my tongue if they hang on good like a coke bottle they are good if they drop off or can get no seal at all they are bad. LOL!!

Posted By: scotts66charger

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:34 PM

ok, sounds good. Another question. I moved from 4000' high (High Desert) to 250' high ( lowlands).. Car ran fine up high, but runs like crap now. I was told here to go to smaller jets, as it was to rich. I did this (went from 73 front to 70). I have the carb apart now, and was wondering, if i should go to a lower # on the power valve as well. the # on the front says (7 on one side and 4 on the other side. Is this a 74?.) so anyway, do I want to go with a lower # on the power valve, say 65? Thanx. scott

Posted By: 70AARcuda

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:39 PM

power valve numbers are based on vacuum inside the engine...

use a vacuum guage and check your vaccum ilding in gear..

you should actually have higher vacuum reading at sea level..

Posted By: Crizila

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:49 PM

Quote:

How can you tell if the power vavle is bad on a Holley 750 DP? Can you suck or blow on it and tell? I dont want to replace it if its good, but I don't know how to tell. The car is running bad most of the time,((After I changed the jets in the carb, but that's another story))and was told that the P/V might have gone south.. scott

I made an adapter with an old spark plug boot. The large end of the boot fits over the diaphram side of the power valve ( hold it against the metal portion ). Ran a vacuum hose from the small end of the boot to my Mityvac. You can not only see if it works, but you can verify the opening vacuum.

Posted By: Dodgem

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:54 PM

Quote:

ok, sounds good. Another question. I moved from 4000' high (High Desert) to 250' high ( lowlands).. Car ran fine up high, but runs like crap now. I was told here to go to smaller jets, as it was to rich. I did this (went from 73 front to 70). I have the carb apart now, and was wondering, if i should go to a lower # on the power valve as well. the # on the front says (7 on one side and 4 on the other side. Is this a 74?.) so anyway, do I want to go with a lower # on the power valve, say 65? Thanx. scott

Richen for closer to sea level ! LOL! what carb you running.

Posted By: Dodgem

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 08:56 PM

I would think a 750 DP would want 72 or 73 front with a power valve (try a 5.5 or 65) and 80's to 82's rear no PV.

Posted By: scotts66charger

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 10:22 PM

Im running a Holley 750 DP.. I changed the front jets to 70's and it runs way better low end, but when i get up around 80 MPH, it seems to peter out. I cant tell if it's running out of gas ( with the smaller jets in the rear,73's) or if its still to big(jets)in the rear.Do you think I should go bigger or smaller in the rear?

Posted By: VanishPt

Re: Power valve. Holley carb.. How do you tell if its bad?? - 05/13/09 11:04 PM

if the pv is blown, turning the idle mixture screws in all the way will not kill the engine.