I think you're describing 440source stealth heads aren't you? =)
I've been disappointed with every set of 516's I've used, including 2 sets that had reasonable 1.74 work and some porting.
I'm really going to try to never use them again lol
I'll politely disagree. 906's have a distinct flow advantage .300-.450" lift even in stock form.
Many 516's are over 80cc.
Some 906's can be 90cc, but some are 87-88.
The CR becomes small.
906 is a better flowing design with larger exhaust valve and better ports.
It is open chamber where 516 is closed but the actual difference in compression ratio is usually much smaller than people think
Fwiw I have what should be bolt on ready 906 heads that I'd sell for about what those cores are. Unclear if you meant they are ready to run or rebuildable cores. Cores have about $5 value honestly.
Sounds like you have vacuum advance hooked to manifold vacuum and not ported? Is that on purpose? Or were you not talking at idle?
Too much timing even at light throttle can cause lean surge/miss