Vacuum

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J

JFenton

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Just bought a restored 71 Demon have not had 1since high school. Live in WI now so it's a little early to put her on the road. But on my test drive to buy she has a mild cam and although she came w/disc and power brakes she was hard on the brakes to stop. This car was spared no expense to build and the owner said because of the cam not much vacuum. Before I get her on the road is there anything I could do to help the situation short of changing cams?
 
Yeah you can add a vacuum storage canister on it. That will help. But before all that, make sure it's all in good tune. If the timing is retarded, that will cause manifold vacuum to be low and result in poorly performing power brakes, among other things.
 
I will check the timing first and if she not retarded I will add a canister--thank you very much
 
JFenton said:
. is there anything I could do to help the situation short of changing cams?
Click to expand...


Why don't you try Rhoades lifters? They are variable duratiion. That means that they "bleed down" at idle taking out about .025" lift and 15° duration. Then "pump up" (don't bleed down) above 3500 rpm. So it's like having a smaller cam for idle, then a larger cam for the top end. You get your cake and eat it too. It will increase your idle and vacuum. I used these on a .484 284 MP cam and got it to idle at 800 rpm.

They are good lifters and work! I've been using them for over 25 years, and it took me 3 years to convince me to try them. They sell at Summit racing for $100 - $115. Give them a try, I think you will like them and is a great alternative to changing cams. Don't try the competetitors variable duration lifters, they don't work like Rhoades.
 
One small commonly overlooked thing also, check your vacuum check valve. Simple to check . Sometimes the little diaphragm goes south. When that occurs you will be using manifold vac for assist as opposed to accumulated vacuum in the chamber.
 
Invest in an inexpensive vacuum gauge. Check the vacuum directly off the manifold at idle, the closer it is to 17" the better. As mentioned before, check the fitting on the booster, the timing, not to mention the idle screws on the carb. Try adjusting them to see if you can get more vacuum out of the motor. If all else fails, put a canister on it. JMO
 
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