Thanks for the suggestions. I have tried many different timings, adjusting the idle mixture each time. If I move the vac line to the ported source, and set the timing to 10 degrees, which in this case is both static and idle, the car does not hesitate as bad when rolling into the throttle. When it does stumble, it fires back throught the carb. Next I will try using a more agressive acceleration pump cam to try to get the mixture a little richer under acceleration. Most of the posts I read on this site say the vac should be hooked up to the manifold source, but that provides 20 degrees at idle, and the hesitation is horrible. I am curious what changes any of you have made when installing a Holley 4150 onto a 275HP 327 to the stock Holley setup. Did you change the primary jets, accelerator pump cam or orifice from the ones that came stock with the carb from Holley?