I've been chasing a weird problem, I have a Holley 650 vac. secondary, on a 350 sbc. When I remove the manifold vacuum line on the carb, the idle goes up, also when I step on the brake the idle will change but not as much. So far I have checked and adjusted the float levels, and changed to a lower # power valve (vacuum at idle in gear 7in, 3.5 PV)Thanks Vince
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Re: carb problem
could it be a problem in the brake booster that's letting air leak during idle? what happens if you block off the port that's going to the brakes? idle goes up or down?
-scottwww.realtuners.com - catch the RealTuners Radio Podcast on Youtube, Facebook, iTunes, and anywhere else podcasts are distributed!
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Re: carb problem
It's not a problem, it's how they work. When you remove the vacuum hose, you are letting more air into the engine, so of course the idle speed goes up. When you press the brakes, the booster lets air into the engine because it has to empty the air out of vacuum reservoir, which means it has to let the engine suck the air out of it.
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Re: carb problem
I agree with squirrel vacuum leak = higher idle rpm
test your brake booser with a mightyvac - see if it holds vacuum for several minutes after you pump it up.... if not - you have a continuous vacuum leak through the bladder of the booster... and your power brakes aint as good as they should be.
- it also seems like your power valve is too low, most popular is 6.5, 5.5 might be a safe bet based on your 7" at idle.... 3.5 seems way low.
Are both your idle mixture screws out abou 1.5 turns from bottomed?
Are you using ported vacuum for your vacuum advance?
(port on the side of the metering block is ported - any port on the base plate is manifold vac)There's always something new to learn.
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Re: carb problem
if you lean out the idle mixture more, it will probably drop rpm when it gets a vacuum leak.
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"If it don't go, chrome it!" --Stroker McGurk
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