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b5aar b5aar is offline
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Join Date: May 2010
Location: up and down the east coast
Posts: 131
Thanks guys....should have searched some more before posting...here's Jim from an older thread

""Dual port mid eightees USA 930 distributor

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What Mark said about timing during boost is correct.

The factory instructions for setting timing on a mid eightees counter clockwise rotating USA 930 dual pot distributor is remove both vacuum lines (don't bother plugging them because that has no effect on anything) and hold the rpms at 4000 rpm and line up the 26* before top dead center line on the engine pulley with the vertical line on the fan housing using a timing light on #1 spark plug wire.
The instructions are on a sticker on the bottom right side of the engine lid and in the owners manual.

26* BTDC is full centrifical advance with no vacuum advance, vacuum retard, or boost retard from the vacuum pots moving the vacuum advance/retard plate and adding or subtracting timing on top of centrifical advance.
Centrifical advance turns the seperate upper half of the counterclockwise rotating distributor shaft with it's attached star shaped reluctor wheel and the rotor a little bit farther counterclockwise as rpms rise up to 4000 rpms and thats your centrifical advance.
By 4000 rpm the centrifical advance is maxed out so if you can't see the tachometer you can just rev the engine higher within reason while watching the timing mark with a timing light until it stops going any higher - that accomplishes the same thing.

When cruising at steady speed and accelerating before boost pressure starts to happen the red vacuum advance hose coming from a tiny hole through the throttle body just above the butterfly and connected to the advance pot has vacuum in it and pulls the advance/retard plate with the stator windings mouted on it inside the distributor clockwise which is the opposite direction the distributor shaft, star shaped reluctor wheel, and rotor are turning and timing is advanced more than where centrifical advance has it.
Let off the gas and decelerate and vacuum inside the throttle body drops to near nothing and intake manifold vacuum below the throttle butterfly and vacuum inside the blue vacuum retard hose increases and the vacuum retard pot diaphram pushes the advance retard plate counterclockwise which is the same direction the distributor shaft, star shaped reluctor wheel, and rotor are turning and timing is retarded.

Then when boost comes on everything including both the vacuum advance hose and the vacuum retard hoses become pressurized but the way the vacuum pots are designed internally the vacuum retard pot diaphram does nothing and has no effect under boost pressure but the vacuum advance pot diaphram goes into reverse moving the opposite direction it did under throttle body vacuum and becomes a boost retard pot and it pushes the advance/retard plate with the stator windings counterclockwise inside the distributor and thats the same direction the distributor shaft, star shaped reluctor wheel, and rotor are turning so that retards timing under boost.

I did not know that until about a year ago when I pressurized both the vacuum and retard pot at the same time with a mighty vac tool while it was removed from the distributor and watched what happens.
Do that while the motor is running at 4000rpms and you will see the timing drop down to around 18* BTDC with a timing light. That is where timing will be with boost retard working at .8bar boost or above.

I hope that explanatioon helps and I hope I reread it enough times to fix my typo mistakes before posting this.""


Thanks Jim...and you didnt have to type it all over again
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Old 04-03-2014, 10:14 AM
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