Which VAC to use??

[Larry T.]

Re: Which VAC to use??

At 18* initial timing you may have trouble starting a hot engine, I did and went back to factory specs. No further problems. Have fun!!

Thanks. Where are you at WOT?

[Tom L.]

Re: Which VAC to use??

It wasn't a "Not firing" issue, it simply wouldn't turn over. Too much advance combined with a hot engine and starter.

[Duke W.]

Re: Which VAC to use??

Thanks for the info. Lars Grimsrud recommends trying to be at 18 initial and 36 total, without vacuum hose connected. He says 12 initial is very low. He suggests trying to idle at 34* once everything is hooked up. These figures are ideal, but he suggests getting close. Do you have any thoughts on this?

You guys need to review the OE specs!. ...got a CSM or AMA specs?

The OE L-72 centrifugal advance is as follows:

0 @ 900
30 @ 5000.

The recommended initial setting is 8 nominal with a range of 8-14, but you might not be able to run more than 6-10 on modern gasoline.

Set the initial timing to as high as it will tolerate without detonation. You should be able to get the revs down to 700 without stalling, then bring the revs up to see when the mark starts moving. If it doesn't start moving until 900 then it's okay to set the initial timing at 700. If you have to, tie up the weights with a stiff rubber band to set the initial timing

If you want to set/check total WOT timing with a dial back light, you will have to free rev the engine to over 5000 revs. No big deal, but some guys don't like sticking their head a foot from the fan under those conditions.

Once you have found the maximum initial advance it will tolerate without detonation, you can try some lighter springs to bring the centrifugal in quicker, but detonation may preclude quickening the centrifugal.

With initial in the 6-14 range, the total idle timing in the normal configuration with full time vacuum advance should be initial plus full vacuum advance (16) at no more than 900, which should be 22-30 with the B20/B26 VAC, and the total WOT timing with the centrifugal fully deployed should be 36-42.

Duke