Originally Posted by b088ybt422
My engine is a 350. 30 over with stock crank reconditioned rods, and forged 2.4cc dome pistons. I am running dart ss heads with 72cc combustion chambers for right around a 10.1 compression ratio, 1.94/1.5 valves. I have a lunati vodoo 60104 Duration 276/284, Lift .504/.525. I have a perormer rpm intake and a 3k stall conveter. My question is I was running rich with a holley 750 vacum secondary so I tried to switch to a 650 double pumper but when trying to tune the 650 I have as much timing as posible before the engine kicks back aginst the starter and useing the mixture screw I can turn them all the way out and the engine still does not idle properly as I turn them out the idle does get better but when the screws are all the way out still is not good enough. Does this mean a 650 is to small or can I do something to it to make it work?
Everything depends on the jetting, the power valve and the amount of transfer slot that's exposed. Just changing from a 750 to a 650 does not necessarily mean there will be a difference in the air/fuel ratio.
In almost every case, the best bet is to "baseline" the carb to the original, as-delivered specs.
In the case of the 3310 750 vacuum secondary (if that's what it is) the jetting will be 72 primary and a #21 plate, equal to a 75 jet on the secondary side. 65 PV and a 31 squirter.
The 4777-1 through -7 650 DP is jetted 67 primary and 76 or 73 secondary, depending on the exact number. 65 PV and 28 squirter.
The timing curve has a LOT to do w/how the engine transitions from idle on up. What's your timing at idle and total w/o adding the vacuum advance, and are you using a vac. adv., if so what type of vacuum- ported or manifold- are you using?