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Old 12-29-2005, 01:19 PM
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Low Vacuum

I built a 350 a while back and it has always had a low vacuum which leaves my brakes practically useless at idle (I back out of my driveway in neutral and rev it to 1000 in order to stop). The engine has a 280 isky, performer intake and a holly 750dp. I initially thought it was just due to the cam i installed and planned to get either a vacuum canister or a vacuum pump, but after reading some of the posts it looks like I may have other problems. Is a 280 isky enough to cause a low vacuum or am i running subpar compression? Does my low vacuum mean i just need to buy a vacuum canister or does it mean i need to start tearin my engine apart?

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Old 12-29-2005, 03:44 PM
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The engine creates the vacuum signal. If the engine is tired, it will reflect. I would try a compression or leakdown test.
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Old 12-29-2005, 03:47 PM
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While a 280 is certainly enough for street use and low vacuum signal is primary result I think with careful tuning you should be able to get the needed vacuum to at least run power brakes. You'll need 10" or better for power brake booster assist. Make sure it's in tip top shape and put as much intial timing as you can and limit the total to prevent burning it down. 36 degree's total. Probably better to error with 34. A stall convertor will help keep the vacuum up because of the decreased load at idle but I still think maximizing the idle control circuits and high timing will get you power brakes. Raising the idle speed to 900-950 in gear will help with several things. But you will suffer with curb idle speeds at or above 1000.
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Old 12-29-2005, 03:54 PM
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Here`s another trick you can try. You`ll need some rocker arm clips so oil doesn`t splash everywhere. With the engine at operating temp at idle, raise the idle speed slightly up to about 1300 rpm. Back off a rocker arm at a time until it starts to clatter, then tighten it slowly until the clatter stops, then give it another 1/4 turn. This is called "zero lashing" hydraulic lifters, it`ll help it to idle smoother and see less duration at low rpms so the vacuum signal will increase.
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Old 12-30-2005, 01:26 AM
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thanks for all the tips guys, i'll keep ya posted
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