Slant Six Forum :: Bad hesitation

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Author:	slantrat [ Wed Oct 05, 2011 8:28 pm ]
Post subject:	Bad hesitation
I have a 225 slant. A mild build, .030 over bore, the head is maxed out. Shaved .125 pocket ported and blended on the intake and exhaust with 187/150 valves. I converted to the GM HEI and ACCEL coil. Currently running a new 390 cfm holley. It has a new rebuilt 904 tranny with no stall, 4:11 posi rear sitting in a 1928 body with a custom z'd and channelled frame, fairly light. The problem that I am having is a bad hesitation until 4000 rpm's. It will idle fine and runs great when the rpm's come up. Off idle to 4000 it stumbles all over the place. It does this with the stock head. It does it with my Weber. It does it with a 500cfm carb built for race. I've spun the distributer from 20 degrees adv. to 20 degrees retard. Can anyone point me in the right direction?

Author:	wjajr [ Thu Oct 06, 2011 6:09 am ]
Post subject:
What jets are installed in the 4160 Holley 390? What accelerator pump, cam, screw position, and squirter used? What do you mean by; â€œno stallâ€, would that be a stock converter? What type of intake & exhaust manifold are you using? Two conditions come to mind that may promote stumbles from three different carburetors. First is a big vacuum leak below carburetorâ€™s base, or in plumbing to vacuum operated devices such as power brakes etc.. For the Holley; adjusting idle speed and throttle opening too far so that transition circuits are taped into will cause a bad stumble at all throttle openings until secondaryâ€™s open. With the 390, throttle plates have to be closed down so that only about 0.026â€ of transition circuit slot is exposed below throttle plates at idle, and secondary throttle plateâ€™s opening need to be properly adjusted as well because they also have a non fuel adjustable idle circuit. In other words, it is adjusted by presetting throttle plate opening to allow correct amount of air flow at idle. This keeps fuel in secondary fuel bowl fresh. Second is rich condition caused by too much jet, and or wrong power valve that doesnâ€™t clear up until engine develops enough rpm, or rather air flow, to handle the rich mixture. There is more to tuning a 390, but where other carburetors caused same drivability problems, lets check the basics first; vacuum problems, A-F mixture, accelerator shot, and proper timing.

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