10.5 power valve ???
According to spec sheet, it should have 70's primary ....
That's a starting point. Doesn't matter now that you're tuning it to your engine and car. Its good to keep in mind so that when you start tuning the secondaries, its likely you'll be able to lean them the same amount as the primaries.
The folks above gave good advice on the Primary main jet - make that as lean as it will tolerate when cruising at mid rpm, highway speeds. Something like 3000 rpm, 60-65 mph. It will surge and die when its too lean at moderately light load. If that happens, slow down , drive home at 55 and jet up. :)
Powervalve opening point is to provide extra fuel into the main circuit only when the load gets too high. Whether this is at 10" of vacuum, or 6" will depend on your combo. If its a flat spot or a little doggy when your rolling along at a good clip and squeeze down to 10", then it definately try the 10.5 PV. If that's not it, then go back cause its wasting fuel and washing down the cylinder walls.
A couple other possibilities you can test to eliminate.
1. At 40% throttle, the secondaries might be cracking open. Test by disconnecting. If it's the secondaries at their opening point, then adjust the secondary idle mix (idle feed restriction and idle airbleed). If it's unstable, relocate the idle feed restriction to the lower position. see
Idle feed restrictions and placment for more information and direct to
pictures here
2. Primary air correction issue. If its not the secondaries, and its not the transfer/accelerator pump, then it could be too much air correction. Air correction should gradually bring in the main system and then keep the AFR steady at any given throttle postion. But too much air can do strange unpredictable things. To be in the ballpark, follow what the old Holley's did...
Detailed explanation here:
http://racingfuelsystems.wonko3.myfunforum.org/Emulsion_Tuning_about42.html
From there you can just poke around the forum with the search to find a starting point for your emulsion and main air bleed. It will be something like one .027" dia. e-bleed near fuel level and another at or near the bottom. Then use the main air bleed to get the AFR to flat line at the top.