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I was doing some routine maintenance and thought to check some timing numbers. Removed the 2 vacuum hoses to the distributor and plugged them in order to check initial timing. Timing light indicated it was 17ish  BTDC when I remember setting it to around 8 BTDC. 17+ 24 mechanical is too much to have so I turned the distributor counterclockwise to retard the timing back to the 8 BTDC I wanted.

Did some other checks , in addition to checking mechanical advance and went back to the initial timing setting. This time it is ATDC somehow, so I advanced the distributor to get the 8 BTDC I wanted. Vacuum lines still disconnected and plugged. Idle set to around 600 RPM to avoid mechanical changing timing.

How does timing change with the vacuum lines removed and low idle? Engine was at a hotter temp than when I initially checked things but how does temp change timing, if it even does??




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How old is your harmonic balancer? I suspect that the outer ring on it is slipping.



“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein


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+1 with Don.  More than likely the problem.  About the only other thing that could cause it and is a long shot is the distributor shaft gear.  Most gears are pressed on and held in place with roll pin.  If the roll pin failed the gear could slip on the shaft.  Don't know what cam you have but some after markets cams require a bronze distributor shaft gear.  As these gears wear your timing will change also.      




"The only dumb question is the one not asked"

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Thanks, both good things to check out!

I'm also wondering if it could be related to a sticking dizzy breaker plate or some issue with the dual diaphragm vacuum canister. Odd that the timing changes with the vacuum hoses removed and plugged. 

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I'd check the breaker plate for free movement and that the white slides are in place. The factory "White Lube" turns to glue at some point.  Might be worth lifting the plate to inspect the springs and weights. Let us know what you find. Chuck

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