Locking Mech and vac advance on HEI issues - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-26-2010, 06:12 AM
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Locking Mech and vac advance on HEI issues

I got a plate to lock my vac advance. I took off the springs and rotated the weights and plate to full advance and tack welded it. Now for some reason when I rev the engine the timing will retard about 4 degrees. Anyone know why it would do this?

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Old 04-26-2010, 10:24 AM
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Could it be the cam walking in or out?
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Old 04-26-2010, 11:20 AM
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Excessive end play on the distributor shaft will cause at least some of it. You want to set the play to ~ 0.017"-0.020". I've seen them over 0.100".

Mr Gasket and others have shims for the HEI. The shaft is ~0.490" OD.

What you're probably seeing is a combination of the timing set, distributor end play AND the cam being forced rearward.
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Old 05-20-2010, 10:23 AM
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Ok so the tack welds broke on the weights. How else can I lock my mechanical advance?
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Old 05-20-2010, 10:59 AM
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I just had to...seems like I'm more smart *** than helpful these days....weld it better?....just yankin yore chain man
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Old 05-20-2010, 01:11 PM
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why are you locking out the mech and vac adv curves?

The trouble with welding the advance locked is getting the thing hot enough to get a good weld, without frying the module etc.

Much better to just bind the stripped down advance mechanism with two zip ties so it is locked fully advanced. Then reset the timing.

This is only required for motors with big racey cams that require full timing at idle. If you street drive it why are you locking out the vac advance.
You might want to limit the amount to avoid excessive vac advacne but no need to eliminate it.

Search my old posts for pictures that show various methods to locking out and limiting the mechanical and vac advance that don;t include welding and are reversable.

What cam is in this motor? locked out advance is not a cure all for every driving, idling issue.
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Old 05-20-2010, 01:14 PM
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The timing is retarding cause of timing chain slack, distributor gear wear and the internal electronics delay in the distributor. sometimes 1 deg per 1000rpm.

Some timing lights don't read accurately at high rpm either.
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Old 05-20-2010, 05:46 PM
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If you really want to lock out your HEI by welding it then simply remove the weights and springs and weld it. One of the reasons that your timing retards as RPM increases is the fact that there is a certain amount of "Dwell" built into the module hence the reason for centrifugal advance. You really should toss the module along with the weights anyways and get yourself an aftermarket ignition box and fire it off of the magnetic pickup.
After you weld the distributor you should make sure it is phased correctly. Pay someone to put it on a distributor machine. It can also be done in the car but it takes patience.
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