To much mechanical advance?? - Team Camaro Tech
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post #1 of 7 (permalink) Old Jul 24th, 03, 06:38 PM Thread Starter
 
Join Date: Jan 1999
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Question

ok i rebuilt a gm hei with msd stuff, cleaned the weights and put in the silver and blue springs. when i fired it up it had like 55 degrees timing(i used a dialback light), could not get it to lower by rotating the distributor. so i went back in and put in the heavy springs and the timing lowerd to about 45 degreees at idle. what gives? where should i look to fix this?
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post #2 of 7 (permalink) Old Jul 24th, 03, 06:43 PM
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My uncle-in-law is a race engine builder and curses GM HEI distributors for advancing too much. He modifies them somehow. I believe he drills a hole in the advance mechanism and installs a pin to limit the advance. I don't know the details, but you're experiencing a common problem with GM HEI distributors.

Are the bushings that the weights pivot on in tact. Usually they desintigrate and have to be replaced. Seems like the originals are nylon, but brass bushings can be installed. If the holes in the weights look to be about twice as big as the pin, the bushings are gone.

-dnult

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #3 of 7 (permalink) Old Jul 25th, 03, 03:40 AM
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The first thing I would check is the TDC mark to see if it is correct.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968
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post #4 of 7 (permalink) Old Jul 25th, 03, 06:03 AM Thread Starter
 
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but should i not be able to adjust the advance by rotating the distributor?
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post #5 of 7 (permalink) Old Jul 25th, 03, 01:16 PM
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I think that answer to your question is yes, depending on what you mean by advance. Turning the distributor doesn't change the advance really - it changes the timing. In other words, if the distributor is advancing 30 degrees over base timing, rotating the distributor wont change that.

-dnult

Dave
========================
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI
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post #6 of 7 (permalink) Old Jul 25th, 03, 06:50 PM Thread Starter
 
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sorry, that one went over my head???
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post #7 of 7 (permalink) Old Jul 26th, 03, 04:42 AM
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What he is saying is that the spark advance is controlled by the advance curve in the distributor, advance means 'to move forward'. The advance mechanism rotates the trigger mechanism of the distributor, advancing it in relation to the distributor shaft. The amount of advance is controlled by a pin in a slot that limits the travel of the mechanism. The rate of advance is controlled by weights and springs. If you have a 25° advance curve, it will advance the timing 25° within its rpm range of operation. If you set it to start at 15°, it will advance to 40°. If you set it to start at 5° it will advance to 30°.
Spark timing is the point at which the spark is triggered, and this is controlled by rotating the distributor body in relation to the shaft. Rotating the distributor will always change the timing, and it will set the boundaries of the advance curve.

"For those that will fight for it...FREEDOM ...has a flavor the protected shall never know."
Semper Fi! L/Cpl Edwin L. "Tim" Craft, B Co 3rd AT's, Khe Sanh Combat Base, February, 1968
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