How difficult is ito to replace the power valve in Holley carburetors. I'm pretty sure mine has blown, and is in need of replacement. If I have a long duration cam, I would want to go to a numerically lower power valve?

 

No more difficult than changing jets. Remove bowl, remove metering block, unscrew old PV, screw in new, reverse for assembly.

As for selecting a PV, I posted this back on December 19:

The Holley web site ( http://www.holley.com/HiOctn/TechSe...nfo/CI-121.html ) says:
"A competition or race engine which has installed a long duration, high overlap cam will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should have a vacuum opening point about 2" Hg below the intake manifold vacuum reading taken.

"A stock engine, or one that is only mildly built for street use, will have high manifold vacuum (17" to 21" Hg) at idle speeds. To determine the correct power valve, the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed. Holley has a 6.5" Hg power valve, P/N 125-65, which usually works out well for most driving situations."

You should probably use the 2nd paragraph to select yours.

I have a 7.5 in mine now, it has a very slight off-idle flat spot still. Speed shop didn't have an 8.5, will be looking for one of those. The carb came with a 6.5, and worked fine until I jetted it down for cruise. That created the flat spot.

 

I think I'm gonna have to use the top paragraph. My cam is actually the largest hydraulic cam that Melling makes for smallblocks. I have practically NO vacuum at idle. Am running a vacuum reserve canister for my brakes.

I went out and bought a 6.5 to replace the stock one, and can't decide whether to stick it in there till I can get my hands on a vacuum guage.

 

5.7 nailed it.

Off topic sort of, do you have the specs for that cam? The largest melling cam I can find is a 22210.

Thanks!

 

6.5 should be stock.

Midnite, email me with your address. I'll mail you a 4.5. No charge. I bought it when people were saying "Get a PV that's rated at 1/2 your idle vacuum." Flat spot off of idle was so big, it was dangerous to drive on the street. With a wild cam, it would be fine. Whether it's what you should use or not, it's yours if you want it.

Oh, vacuum gage is a very basic tuning tool. No tool box is complete without it.

 

SSC

The check out the different cams in the melling high performance catalog online. Here is a link.

 

Thanks! Guess you gotta now where to look.
So that 22210 is a lt1 cam.

SSC