hotroddennis

Hell-o every body. I have a 1968 Corvette with the 350 hp 327. This an all stock motor,including the points distributor. When I bought the car it a Holly carb,not a holly on the street fan,but my plan was to replace it with a Quadrajet anyway. I gave her a good tune up and replaced the coil with an Accel coil.When the car is up to temp.and I put my foot in it,it would back fire through the carb,I blamed the Holly and replaced it with original quad. The back fire was still there until one day the fire went out and she would not start,I put the old coil back on and she fired right up,o.k,I got a bad accel.so I put a new one on,ran fine,I could put my foot in her and she ran like a 327 should run...until this weekend...the back fire is back,now this is two coils in probably 1000 miles...any ideas?,thanks

Easy Mike

A back fire is usually timing related. Check the timing. Is the valve lash corrrect?

MelWff

have you checked the voltage at the positive terminal of the coil, both when running and when cranking? have you tried installing another new points condenser?

hotroddennis

points and condenser, yes...have not checked voltage at coil, where should it be? thanks for the reply

Peterbuilt

With the key ON and the wire disconnected from the + side of the coil you should have 12 volts. (battery voltage)

MelWff

you should only get the 12 volt reading when cranking, in the ON position the resistance wire should reduce it to about 9 volts.

vette_jim

Also check distributor centrifugal advance & vacuum advance mechanisms to make sure they are moving freely and not sticking at times. Had a bad vacuum advance once that allowed the breaker plate to float around at will, not good.

Big2Bird

So, you have burned up 2 coils?

hotroddennis

it appears that way,don't know why,I have never had a standard 12 volt ignition coil fail,I'm wondering if the ignition wire is bad

Big2Bird

The stock wiring harness has a resistor wire that limits the primary current. A separate wire to the starter relay enables full current for starting.
If that resistor wire has been replaced or bypassed, the coil will overheat and fail.

Peterbuilt

Your correct about reading less voltage when ON and running, here's a past conversation about readings: