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holley tuning

3K views 3 replies 3 participants last post by  onovakind67 
#1 ·
street driven 412 sbc,thm 400, 292 comp cam, msd hei, hurricane high rise intake, new holley 750 vac sec, new accel shorty plugs... replaced 850 speed demon with 2'' spacer due to richness. question : motor ran great with 850 on it, cant seem to get holley to steady idle and it seems like it surges at idle, also you have to feather pedal at startup for awhile before it will idle, idle mix scews at 1 turn and i have turned them in and out 1/4 with no change, vac. 8"-10", idles 850-950
 
#2 ·
292 is too big a cam in my opinion, but that isn't your problem. You have a vacuum leak somewhere.

Every carb that leaves the factory, no matter who makes it, is set up to mix the correct amount of gasoline with the amount of air that will flow through the carb if you had 28 mm of Hg vacuum (which you do not). You shouldn't have to change anything once you bolt the carb onto the car. If you think it needs to be tweaked instead ask yourself what else besides a carb problem could be causing this?

I suspect you have a vacuum leak beneath your intake manifold, caused by a gasket that slipped when you installed the manifold. If you pull the intake and discover oil in a head intake runner you will know I was correct. Because your motor has a suspected leak agravated by a lot of cam overlap, your carb is having a difficult time metering the correct amount of fuel to the amount of air; especially at idle.

Big Dave
 
#3 ·
Thanks for the response, I forgot to mention that a engine machine shop assembled the motor in June and figured the intake gasket would be right. how much vac. should I have anyway?
What do you think about the old 850 demon? The plugs where black after just afew drives around town.
 
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