The other day I put on the vacuum gauge, and it only read 5 lbs of vacuum at idle in park. I thought that was odd, so I started adjusting the 4 corner Idle screws until I ended up at around 10 or 11 lbs of vacuum which is more normal for this cam. after revving the engine a few times, I noticed it was back down to 5 or 6 at idle.
When you rev the motor you clean the intake floor of fuel puddles and the motor goes lean until the puddles return.
Depending on cam duration 20* initial sounds low to me. I have no idea what timing control you are using to pull or control timing under boost so this is just a thought. All my small blocks like a lot of initial timing with lots of cam duration. As an example I run my [email protected] 340 with 28* initial.
Take a pencil eraser and block off each idle air bleed one at a time with the motor at idle. Motor should go rich each time and that tells if all four idle circuits are working. You can also have someone hold the brakes and adjust the idle mixture in gear. On a street car I always favor the primary idle mixture more than the rear. Example 1 turn out primary, 3/4 turn secondary. It helps the street manners.
All my cars are NA pump gas and .067 idle air bleeds work best for me at sea level.
If your metering block gaskets are old and you've done multiple jet changes you may want to replace them. you can have a vacuum leak there that is hard to find. I also assume your power valve is good.
