Erratic spark

JOELCOOK

MEMBER
1990 PONTIAC FIREBIRD
5.0L
V8
2WD
AUTOMATIC
200,000,000 MILES

I took out v6 and put in 350 Vortec with old style headed an Eldelbrock 1406 carburetor and intake an Accel distributor with super coil with vacuum advance timing set at 35 total 42 with vacuum. Ran great, but now it misses. I used my timing light on all the wires an light was erratic, not steady at idle. But not while light throttle or without power. What would cause this?

3 Replies

HMAC300

MECHANIC
48,601 POSTS

Run the timing back to something like 12 initial at idle or 10 even. Check distributor cap for cracks and resistance of coil. You may have pre-ignition running that much timing and made the valves tulip.

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Tuesday, March 8th, 2016 AT 6:59 AM

JOELCOOK

MEMBER
2 POSTS

Set at 10 initial have 23 mech. Total at 33 still rough idle and unsteady spark an no power loss.

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Tuesday, March 8th, 2016 AT 8:37 AM

HMAC300

MECHANIC
48,601 POSTS

Check your compression and for vacuum leaks. If you installed a big-lift cam in the car that could be your problem as well as it will need to idle faster due to vacuum loss. Also check the coil for leakage as it may have cracked, spray water around it and make sure distributor cap wiring is in the correct order.

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Tuesday, March 8th, 2016 AT 9:32 AM