1999 GM 350 Misfire Issue (Sparking 90 degrees out)

[Farmin]

HI Everyone

 

I have a 1999 GM 1-ton with a rebuilt 350 in it.  We put on new distributor, cap, coil, plugs and wires.  The truck starts and runs good.  but when under certain driving conditions (full throttle, or certain revs) the truck will backfire, hesitate etc.

 

When a timing light is used the truck is timed right, but every so often it will throw a spark when the crank is 90 degrees past the timing marks.  It is random when it happens, but it always happens in exactly the same spot.  So it will fire 3 or 4 times on the mark and than once 90 degrees past and than back to the marks.

 

My question is what could be causing this?  I think 90 degrees on the crank is 45 degrees in the distributor, but how could the rotor be lined up to the next spark plug but jump back to #1?

 

Any guidance or insight would be greatly appreciated, Thanks in advance.

[bronyaur]

Did you set the CMP offset on the distributor?  To be within spec it should be between +/- 2.  A trouble code of P1345 might get stored if it is out of spec, although I've seen it as much as -14 and the code hadn't been set (but the engine hesitated and backfired as you described).