Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Opinions, dumb idea: Using vacuum canister + vac advance to avoid hardstart 38'
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Old 06-01-2010, 02:32 PM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by bubbahotep
This motors ideal timing 38' total, anything less is a loss of power on this motor. I idle at 30'

I know about vac advance and mechanical but had to remove the centrifugal weights to use the MSD timing box. Its their suggestion to lock the dizzy only because the msd computer does the timing.

All that aside I dont yet see why using the vac advance to give me my full timing is a bad idea. I know its not being used properly. Im using it more as a 'full advance' after startup with no regards for fluctuating engine vacuum
Mostly the problem is a lack of vacuum. In a"normal" motor manifold vacuum is quite high at idle and part throttle. Under that circumstance the vacuum can pulls in it's advance. As the throttle is opened the the vacuum falls and the advance it creates goes away while the centrifugal takes over with advancing RPM. At 10 inches of manifold vacuum at idle you're in a situation where a normal vacuum advance has retreated. The reduction in vacuum just falls further as the throttle is opened, so the resource you need for the vacuum advance keeps becoming less. The vacuum brake reservoir saves up a bunch of low pressure air, but it can't make the vacuum deeper than what's being produced in the manifold. It will, however, be able to supply some advance based on how deep the vacuum can be retained, this is a good idea and will work, but you'll have to trim an adjustable vacuum advance to work with the available low vacuums.

Way back you said this engine was cammed but not with all that much-do you know how much? Your description of this engine speaks to a lot of cam, or a miss timed cam if it isn't a big whomper stomper stick, or the compression is lower than you think it is. An engine that needs 38 degrees to run good also speaks to a slow burning chamber. So some or all of these could be players if this isn't a huge cam, what ever that really means. Please comment to these details if you know them. such as

-Cam duration and lift, timing events if you have a card or the cam maker and part number so they can be looked up if the cam is fairly recent and not an obscure maker.

- Cylinder head, whose and what chamber size.

- Pistons need crown configuration (flat, dome, dish & whether that dish is round or D, compression height.

- Rod length

- Cam coordination to the crank, straight up, advanced or retarded and by how much.

- Status of the distributor gear and if a roller cam status of the distributor drive gear on the cam,,i.e. wear.

- Carb mixture ratio, lean will give also you some of the headaches you have.

- Vehicle weight, gearing and rear tire size.

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