Chevrolet Repair: Chevy Truck misfire, vacuum leaks, distributor shaft

Question
Van,

I have a 1985 Chevy K-10 4X4 pickup.  It has a V8-350 and is carborated.  The coil is mounted in the top of the ditributor and has HEI ignition (module is inside the distributor). It is misfiring badly.

As a little background, I was picking up a ton of rock about a week ago.  The truck drove fine all morning.  Once the rock was loaded on the truck it started fine but as soon as I pulled out on the highway it stalled.  It restarted briefly before stalling again.  It would turn over but not run.  I checked to see that fuel was getting to the carburator and it was.  I changed out the plugs (although the set that was in there had less than 1000 miles on them).  Once the plugs were changed the truck started up and ran fine.  However, it is now running very poorly again (misfiring).

 It has new plugs, and the wires, rotor, and distributor cap visually look ok.  Is it possible that I have bad components in the distributor?  Could the coil be going bad?

I have thought about buying a reman distributor from a local parts store and replacing the entire thing.  I feel pretty confident that I can do this with the help of the Haynes manual but I don't want to throw money at the truck and not fix the problem.  Do you have any suggestions?

Answer
Hello David,
The HEI has a very powerful coil, and there is a rubber washer under the coil that would sometimes crack and allow the spark to arc to the distributor shaft, and cause a miss.
A vacuum leak is also a possibility. The EGR valve, and the brake booster are possible sources of vacuum leaks, as well as the vacuum hoses, transmission modulator valve if it has an auto, and heater a/c controls.
A vacuum guage connected to the manifold is still a good diagnostic tool.

The plug wires can be tested with an ohm meter, but if you decide on the new distributor, I would change the plug wires with it.

Van