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Old 08-23-2010, 01:50 PM
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Is my carb to small

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?

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Old 08-23-2010, 02:12 PM
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I think the 750 was the better choice but both are not tuned. What is your idle timing and manifold vacuum?

The vac secondary would have given you less carb at street driving and then more when the secondaries were needed.
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Old 08-23-2010, 02:32 PM
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Quote:
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?
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