Help, 6-71 blown 350 problems

[Bob Hollinshead]

I just picked up an old school Nova with a 6 3/4-1 compression 350 chev. It has a Dyer's 6-71 blower that's 6% overdriven and an old Comp solid lifter cam 12-405-5 grind # CS 290AS-14
255@.050 .540 lift intake 265@.050 .563 lift exhaust on a 114 LSA
Ignition is a dual point mechanical advance only distributor.
Two Holley 1850 600VS carbs

This is an old build that ran good for many years but now she has a low rpm backfire, but the midrange and top end seem fair. The valve lash seems stable so I think the cam is OK. I just bought it this way so I need to dig in and find out what's wrong with her and hope you guys can give me some thoughts on what to look for. I've got a new unilite distributor that I plan to install and will freshen the carbs. In the Dyer's paperwork I got with the car it says possible causes for a low rpm backfire to be:
Vacume leak/blown blower gaskets
lean idle mixture
vacume leak on blower between end plates and case
carb base gaskets
ruptured power valves
leaking intake gaskets

Can you think of anything else to look for?
Blowers are new to me, any reason why I should keep the powervalves?
If I pull the powervalves and jet it up would that be a good move?
This is in a 72 Nova, th400, 4.10 gears--A real old school car with chrome ladder bars, ET mags, candy red, the whole 70's vibe...

Thanks for your help, Bob

One more question The blower front gear case has an oil line plumbed from the front of the block and a drain tube that goes to the right valve cover. In the feed line there is an adjustable valve that I assume is to regulate oil flow- any idea how this should be adjusted?

[rookie]

Low initial timing, dirty air bleeds in the carbs, if it’s been sitting awhile it may be just a major tune and carb rebuild.

The power valves will keep it from loading up at part throttle like around the pits and staging the car, but if its race only you don’t really need them.

If the car has been moved to a state with different elevation or temp and has not been retuned for that climate it could cause a lean back fire

I would upgrade the ignition and rebuild the carbs and then go from there.