OK, engine has been running fantastic but...
after a long run and leaving it over night, start the next day and lots of black soot on the ground. Doesnt smell rich when I drive it and plugs look great. sooo...
I started doing a search and checked my vacuum and sure enough in drive at idle I have 11" vacuum. Check the power valve and its a 6.5. ahhh easy fix,I'll get a 5.5 power valve. Well actually I got a 5.5 "high flow" power valve it turns out. When I replaced the valve the 6.5 valve actually had two of the round gaskets fitted. I fitted the 5.5 valve with one gasket as their is only one supplied in the pack.
Well...
Now its running very very rich and backfiring at the pipes after quick flicks of the throttle and vaccum gauge is quiviring like crazy +or- 2 indicating a rich or lean condition.
I assume it must be the "high flow" power valve but cannot find any info on this type of valve.
so..
Is a standard 5.5 the go?
Can anyone shed some light on these valves?

Thankyou,

New crate 350/375hp Vortec
New 3310 Holley
10.1 Compression, 98 Octane
36 degrees Total

I would think so, has all the earmarks of too rich from that high flow.

That motor should be fine on just standard flow 5 psi setup, that's what I run and have no trouble getting gas up in there.

Thanks Simon, but I'm not talking about the psi from the fuel pump to the carb. The power valve is in the carb.
I've ordered a 5.5 power valve so will see how it goes.

The standard power valve is good for almost all cases. If the power valve channel restrictions (PVCRs) are over .090 or so you need the high flow valve so the valve itself doesn't become a restriction. Don't set the power valve based on idle vacuum. Holley's recommendation is just a ballpark guess. Here's a decent article showing how to select the correct valve. http://community-2.webtv.net/MATTGRU/pv/ The high flow valve is very delicate and easily distorted since it's missing a lot of material. (Thats' how they get the high flow part.) Sounds like the valve wasn't seated and was leaking. Soot on the floor when starting is probably choke adjustment or valve seals. If the power valve opens too early you waste gas. You want the highest number valve that will still allow the motor to run right at heavy throttle. Open at idle doesn't matter. Giving it some throttle will raise the vacuum and the valve will be closed before the main circuit starts. Drive around with a vacuum gage and you'll see.

What kind of single gasket? Not the hard plastic one I hope. Very difficult to seal them up. Very easy to get a fuel leak past that gasket (especially if the gasket isn't centered on the power valve's gasket flange) which will drip liquid fuel straight into the intake manifold and drive the idle bonkers.

Agreed with above that a high flow valve isn't required unless the PVCRs have been opened way up. The valve itself isn't normally a restriction to flow unless the PVCRs have been drilled out a lot bigger than stock.

Hi Damon, Thanks for the help. Definitely two round paper gaskets were on the factory power valve so I replaced with just one as provided. I was careful to seat the power valve when installing as I did notice the how much movement was available. Will still try the 5.5" standard when it shows up. Thanks