I found the following on the net, but I don't recall where so I can't give credit.
Anyone have argument with the contents?

"To properly size a power valve, take a vacuum reading at idle and if it is above 12" for a standard transmission a 6.5" will be safe to use. For automatic transmissions take a vacuum reading in gear at idle and if the vacuum is below 12" divide that in half for proper size. Example 9" of vacuum in gear at idle will require a 4.5" power valve."

JamesC

 

Looks like Holley's standard instructions from the 60s, paraphrased....

Some myths JUST WON'T DIE.

I strongly disagree.

Imagine the following scenario:

61 Chevy 283; 30-30 solid cam; stock Powerglide converter; 3.08 gears; lop-eared high school kid with NO tuning skillz whatsoever.

THAT'S what those instructions are for. Yes the car will run; no it won't die outright when you give it gas; but that's about as far as it goes.

Follow the Holley tuning sticky that Apeiron built out of a few of my posts on the subject at the top of this forum. You'll find that the car will run about a hundred times better and use about half as much gas as with the "stock" Holley setup, and that the PV won't come out ANYWHERE NEAR that. Specifically, it will come out ALOT higher.

From what I can recall, you have an engine that should be making around 16 - 18" of vacuum; you'll end up with a 105 or 125 PV.

 

Interestingly enough, my 350 HO Deluxe, which is readied for an automatic transmission, was, of course, equipped with a flex plate and a 55 power valve.

JamesC

 

It's been proven near false too many times over. Near false assumes the person tuning knows what the PV is supposed to do... and has a combo well matched.

I fully agree with sofa here… My lil 355 idles with 10.5 - 11" vacuum yet I run a 10.5 PV. Can't be.. say it isn't so. Yet she runs really well and last summer knocked down 15-18 mpg regularly with a 750DP. Set the PV to cruise vacuum….

Holley recommends them based on worst case scenario… wrong combo… wrong hands tuning.

 

I always liked some of the information in this thread

https://www.thirdgen.org/forums/carb...ve-choice.html

goes along well with the sticky and clears some things up. Did for me anyways.

 

After reading this and the sticky, i opened up my carb and of course there was a 65 PV in it, which would explain the looooong bog (car might actually stall if I'm not careful). I will buy new jets as described (maybe just get an entire set) but I will also be replacing the PV with a higher one, just not sure where to start. I think I will buy 3 different sizes to tune it right, the 105 and 125 as mentioned but should I buy 1 higher or 1 lower numbered? My car is an auto and has about 14" vacuum at idle. What might you suggest? The way I figure it is guess at the right one and buy 1 step higher and 1 lower. Any suggestions??

 

Forget idle... what is vacuum at typical cruise? Try one 2" or so below that.

 

Idle vacuum is a poor way to choose a power valve in general. I've only found ONE way to do it right. Hook up a vacuum gague where you can see it and take a drive. Drive it like you normally would. Observe what kind of vacuum levels you see pulling away from a stop, cruising constant speed, etc. The only vacuum levels that don't matter are when you're coasting with the throttle closed- those will always be higher than any power valve.

Remember that a power valve's purpose is to give extra fuel enrichment ONLY under load when richer mixtures are called for. Steady-state cruising, obviously you want it closed. Even under modest acceleration you want it closed (you don't want it opening every time you pull away from a stop sign). You only want it to open when you are calling for more serious grunt.

I've found that the stock 6.5 PV is pretty good for this in more situations than most people think. If you're moving more than 2" in either direction you're probably chasing an issue better addressed with jetting or air bleeds than with a power valve.

 

Hooked up the gauge and had it in the car with me. In gear idle was about 11.5-12", pulling away from a stop normally was about 10", at cruise it showed a low of around 11" (rarely) at slower speeds in OD up to about 16". I'm thinking a 95 now, much higher and it might be opening at idle.
Also, i mentioned in the above post about the long lag, well I did hit it pretty good from a stop to see what would happen and it nearly died and the vacuum of course dropped to nearly 0. What else might cause this? BTW it is a 650 DP and the engine was at operating temp.

 

Hard to say...

But common sense says, take care of what you ALREADY KNOW is wrong (i.e. put in the 95 or thereabouts), and THEN go from there. Might fix the whole deal completely.

 

Nailing it from a stop and having it nose-over is usually an accelerator pump issue.

And I wouldn't go quite as high as a 9.5 PV given what you observed. Maybe an 8.0 or thereabouts. You want it significantly under the vacuum levels you observe in typical driving. Not radically low, but don't just say "it's 10 pulling from a stop so stick a 9.5 in it". Give it a little more breathing space than that.

This gets back to my comment about most setups being within ~2" of the stock 6.5 PV. If it's a N/A combo that's the range you're going to want to be in. If you need to go outside it, you probably have issues better addressed with something other than a PV change.

 

That or the throttle blades are not adjusted properly. To get my car to idle at the right RPM my throttle blades had the slit exposed by 0.130" which causes really bad stumble from a stop or closed throttle. This was my initial set-up, had to go back and adjust it to where the slit isn't exposed more than 0.040". Sometimes this has to be fixed by adjusting the secondary throttle stop and/or by drilling some small holes in the primary throttle blades to allow more air to pass.

 

Sorry I didn't explain very well my thoughts, it was more in regard to Damon's post regarding having to move more than 2" in any direction and if he thought it could be a different issue what that might be. I am going to change out the PV, probably get an 80 or 90 (agree or disagree with those choices?) and see if the problem gets better or goes away completely. I know enough to know not to change more than 1 thing at a time so I will try that first.

 

Those would be reasonable starting points, if they were available... but they're not. For some reason Holley decided to make em all at the half-inch points, so they're 85, 95, 65, etc.

But yeah, what you're proposing, is ballpark-ish, so go ahead and start there. Get a range of em though on either side of wherever you start (like, have a 75, 85, 95, & 105 on hand) so you can try diffferent ones and see which is BEST. Keep trying em until NOTHING you do makes it any better.... i.e., get it THE BEST IT CAN BE, not just "better" than it is now.

 

best way is to invest in a vacuum and a wideband AFR gages, makes carburetor tuning a lots of fun, i use a 10.5 i had a 8.5 but wasn't oppening for some reason, having it open early makes a great transition to the secondary's but you have to lean out the primary cirsuit or it will be too rich too early, basycally it will cruise at 14.7:1, mid throttle 13:1, full throttle 12:1 + -