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low vacuum after upgrades


JAS

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I recently rebuilt my A15 and added an overlap cam, high lift, 264 duration. Duel webbers, dcoe 40, now I can't even get 710"hg vacuum to run my vacuum advance... I tied the runners together for a common vacuum port. Vacuum Gage shows shows max 10" hg and shows a late exhaust valve close? Won't even activate the vacuum advance, runs pretty good otherwise... any advice? Look up instagram for pics or build, or my profile on facebook...

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A high overlap cam will do this at low speeds. This is the time when the intake hasn't closed yet and the intake is already opening. Both valves open at the same time is going to destroy intake vacuum. This is not a problem at very high RPMs because the time both are open is too short.

 

Vacuum advance only happens at part or light throttle and deceleration when the cylinder is weakly filled with air and fuel. A cylinder that is less than full needs more time to burn so the spark is advanced an appropriate amount. Vacuum advance has no effect on full throttle driving but makes part throttle more efficient. Mileage and drive ability improves with a functioning vacuum advance.

 

You may be able to put a weaker spring in the vacuum unit.

 

Depends on the distributor but vacuum advance can begin with as little as 2 inches of mercury, but 12 -15 inches are needed on most. You would certainly get this amount on deceleration. If your maximum vacuum can be pushed above 10 inches I would check the hot valve lash. A tight valve can prevent it closing tightly and vacuum will leak.

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