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Old 05-14-2019, 03:03 PM
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larry medina
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1975 C3 corvette with a 350. I just replaced a defective ignition module The timing fluctuates at idle. When I reattach the vacuum hose, the engine dies. Do I have bad springs or weights in the ignition distritutor? When I disconnect the vacuum hose from the distributor it seems the engine runs ok.
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Old 05-14-2019, 06:01 PM
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HeadsU.P.
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If you mean the timing mark fluctuates at idle, that's par for the course with some timing lights. They have difficulty receiving a good signal from the plug wire then relaying that impulse through the light. The light beam will jump between two known marks.. You have to figure out which is correct. Usually the lesser mark..

I doubt the actual timing is moving but maybe. If the engine dies with Vac, you must be too advanced. Plug the Vac off, loosen the dizzy and set your total around 34-36 at a high RPM, above 3K. Once that is set and locked down, then reattach the Vac. By the way, the Vac line moves the plate under the weights & springs. Two different systems: Mechanical Adv and Vac Adv.

On the springs & weights? Hard to say unless the timing TRUELY is all over the place. But for now get your Total Timing (w/o vac) established.
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Old 05-14-2019, 06:36 PM
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your timing is way off. advanced too far. get a timing light.
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Old 05-14-2019, 08:15 PM
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Originally Posted by larry medina View Post
1975 C3 corvette with a 350. I just replaced a defective ignition module The timing fluctuates at idle. When I reattach the vacuum hose, the engine dies. Do I have bad springs or weights in the ignition distritutor? When I disconnect the vacuum hose from the distributor it seems the engine runs ok.
Just an FYI item to check. On old vehicles with an HEI distributor, it's not uncommon for one of the two wires connecting the pickup coil to the ignition module to fatigue/break due to the wires flexing every time the vacuum advance canister rotates the "breaker" plate. Perhaps a resistance/continuity check of the pickup coil while moving the "breaker" plate might be an item to consider checking.
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Old 05-15-2019, 03:35 PM
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larry medina
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Originally Posted by HeadsU.P. View Post
If you mean the timing mark fluctuates at idle, that's par for the course with some timing lights. They have difficulty receiving a good signal from the plug wire then relaying that impulse through the light. The light beam will jump between two known marks.. You have to figure out which is correct. Usually the lesser mark..

I doubt the actual timing is moving but maybe. If the engine dies with Vac, you must be too advanced. Plug the Vac off, loosen the dizzy and set your total around 34-36 at a high RPM, above 3K. Once that is set and locked down, then reattach the Vac. By the way, the Vac line moves the plate under the weights & springs. Two different systems: Mechanical Adv and Vac Adv.

On the springs & weights? Hard to say unless the timing TRUELY is all over the place. But for now get your Total Timing (w/o vac) established.
Thanks for your help. I'm going to get a good timing light and check it again. Prior to that, I've ordered and new ignition coil module. I noticed the three wires to the coil are frayed and may be arcing or miss firing.
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Old 05-15-2019, 03:37 PM
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I'm going to purchase a new timing light. Mine is over 40 years old.
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Old 05-15-2019, 03:39 PM
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I've ordered a new ignition coil module. Mine looks frayed. Thanks for your help.
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Old 05-15-2019, 05:18 PM
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Originally Posted by larry medina View Post
Thanks for your help. I'm going to get a good timing light and check it again. Prior to that, I've ordered and new ignition coil module. I noticed the three wires to the coil are frayed and may be arcing or miss firing.
That reminds me of something else. Remove the coil dust cover, three phillip screws. On one corner of the "coil in cap" is a GRD wire under one of the four mounting screws. Sometimes the coils are painted so well that a good Grd is impossible. Remove that Grd wire and screw. Take a small wire brush and remove the coating on the coil where the GRD wire goes. Put it all back together.
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Old 05-15-2019, 05:45 PM
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Originally Posted by 69427 View Post
Just an FYI item to check. On old vehicles with an HEI distributor, it's not uncommon for one of the two wires connecting the pickup coil to the ignition module to fatigue/break due to the wires flexing every time the vacuum advance canister rotates the "breaker" plate. Perhaps a resistance/continuity check of the pickup coil while moving the "breaker" plate might be an item to consider checking.
replace distributor pickup assembly ... Standard Ignition pn LX-302 ... or ... LX-302T


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