I had the car out for a long cruise last night and when I got off the highway It wouldnt hold an idle. It was the first cruise after I lashed the valves, they were on the loose side. When I got home I immediatly put a vacuum gauge on it and got 6 to 6.5 inches. my holley has a 6.5 pv. Is this why it wouldnt idle well. Do I need a different size pv and what do you guys recommend. Will a 3.5 pv cause a lean cruise A/F ratio.

6 to 6.5" of Manifold vacuum?? :eek:
Give us some details on the engine combo.
Where did you connect the vacuum gauge?
If that's the case, a 6.5 power valve would be open all the time, causing an extremely rich idle.
Did you check the float levels?
I high float (usually on the secondary side) will cause idle problems also...

Its a 540 with solid roller 255/264 at .050 680/685 lift on a 110lsa, heads are alum merlins 345 cc ports/ ported slightly 9.6:1, single plain merlin 4150 opening and 1000 cfm holley. I know I dont have any vac leaks. The base plate has two small vacuum ports side by side on the pasenger side of the carb, I measured it from the closer one to the firewall, full operating temp after a 10mile highway cruise. Does the 4 corner idle screws make a big difference in the vacuum measurement, I believe they are 1.5 tuns out all around.

Thanks Rich

An open PV won't cause a rich idle problem, but a ruptured one will. A stuck float will do it, too. The 4 corner idle system shouldn't make much, if any, difference in vacuum. That's a big cam and could cause a low vacuum reading. The 4 corner should help with getting the idle right. Try setting them to .75 turns out to start with.

I would go ahead and change the powervalve to the 3.5 that will eleminate one problem.

The powervalve may cause a rich condition at cruize speed but won't hurt wide open throttle or idle if it isn't ruptured. Get the idle cleaned up then take a vacume reading at cruize speed while driving and pick a powervalve just below that reading and I'll bet it'll be real close to perfect. JMO

I also have street 540 with a slightly bigger cam. I have about 7 inches vacumn at idle so I put in a 3.5 power valve. Works fine. I would put in a 3.5 if you only have 6.5 inches of vacumn, before you do anything else.