ok so i've been playing with getting the timing right on my 390
here's what i did and found out today
marked my balancer at
tdc
8 btdc
and 30btdc different marks for each location
i checked timing with vacuum advance disconnected
8btdc
i hooked up vacuum advance and checked timing with engine at 3500 rpm
it was wayyyy past 30btdc more like 45-50
so i disconnected vacuum advance and checked timing at 3500 rpm
35-38 btdc

so my vacuum advance is advancing the timing even more than the mechanical advance ?? any help ideas ?

 

Do you have the stock dizzy? 35-38 deg sounds correct for total mechanical advance. You cannot set the timing correctly with the vacuum connected. I don't know if that's too much advance with the vac. Is it running poorly?

Kurt

 

yes stock dizzy , from what i have read should max out to 38 ish with vacuum advance connected . engine seems to run better with no vacuum advance hooked up .

 

No, the vac should be disconnected with a total of 35-38. There are numbered slots at the bottom of the dizzy that correspond to advance. The number times 2 plus the initial should equal ur total. Mine for example is 13L x 2=26+12 initial = 38 deg. I have a pertronix installed in my dizzy, so it runs better with a more advanced intial of 12. Check ur vacuum connection. Does it come from the base of the carb? If it does, you should have no vac at idle.

Kurt

 

+1^^

Total Mech. advance should be 35-38° with vacuum disconnected and plugged. If you check it with the vacuum hooked up 45-50° is a normal number.

 

THANKS SLEEPY , so i guess it's normal ,
i have the vacuum plugged into the timed port on the side of the holley carb
mechanical is set to the 15L so 30 +8 = 38
maybe i'll try the 13L and a little more intial

 

port on bottom of carb has 15 iom ( inches of mercury) at idle
timed port on carb has 0 iom at idle and jumps up to 15 iom at 1000 rpm

 

That's what i run on my 390 and it seems very happy, 13L for 26 degrees mechanical + 12 initial for 38 total, the vacuum advance (ported) kicks in an additional 12 degrees, all in by 3,000rpm.