Engine missing and backfires. HELP!

[cwalton]

Hey everyone, I have a problem on my hands. I built a 383 stroker out of a 1986 sbc 350 and swapped it into my 1971 k5 blazer a while back, and when I took it out for its maiden voyage things didnt go so well.

It went fine for about a half hour, then all of a sudden it would completely stop firing. I would push in the clutch, and it would then come back, and then occasionally backfire big time. It did this at all different RPMs, it was very odd. Does anyone have any suggestions as to what might be causing this?

It has a carburetor and has the original HEI distributor from the 86 engine on it.

Help me get this old girl back on the road!

 

[kennyw]

If my engine was missing I would go looking for it...

But, check to make sure the vacuum advance is still working properly. Does the HEI have a vacuum advance? My old '86 K20 had extra electronic controls on the HEI for knock sensors.

Is the carb new or rebuilt?

 

[cwalton]

If my engine was missing I would go looking for it...

But, check to make sure the vacuum advance is still working properly. Does the HEI have a vacuum advance? My old '86 K20 had extra electronic controls on the HEI for knock sensors.

Is the carb new or rebuilt?

Yes it has vacuum advance, I'll double check through that. The carb is a relatively new edelbrock. I had it on the '71 350 prior to the swap.

 

[Carcinogen]

Check to make sure the distributor is clamped down tight, my brother had his spin on the 383 in his Camaro and had similar issues.

 

[diesel4me]

It could just be the rotor or cap has a carbon track or crack,and lets the spark ground out instead of firing the plugs when its under a load--I've had HEI rotors get punched thru under the spring things and ground to the distributor shaft--the carbon button in the cap can fail too..or the coil might be dying,weak spark will crap out under a load but usually run it fine in neutral,a coil will oftenm work OK till its been run awhile and heats up

..it could be the module or pick up coil dying too...you can do some checking with an ohm meter to do some resistance checks and see if the coil and pick up are OK,but sometimes replacing them is the only way to be sure..

 

[Keitha]

Did you break in your cam correctly?

 

[cwalton]

Thanks for the replies everyone. I had been leaning towards the coil going. I only drove it the one time on its maiden voyage, but it seemed to start bogging down more often the longer I drove it, with the backfires getting progressively larger.

I know the distributor is clamped down tight. I rechecked the timing after driving it and it was right were I had set it originally.

I am confident it is not the cam or lifters worn out as I broke in the cam properly.

I'm leaning towards getting a new distributor since it is a relatively old one anyways, a replacement couldn't hurt. I just hope that solves the problem.