I plan on sending the MSD unit in for repair. I am going to keep the Edlebrock 500cfm carb in place. 

Interesting thanks for telling what it was. If you sent it back did MSD say what the failure was? Did you ever use a module with it? Are you going back to the Summit carb?

56 Vic, B'Ville 200 MPH Club Member, So Cal.

my misfiring issue turned out to be related to my MSD 8383 distributor . Apparently these fail rather often.  Swapped out cap,rotor ,mag pickup still had issues. Finally installed a new distributor. No more misfiring. How frustrating this has been for me. 
The MSD unit was  2 yrs old.   

How is the fire at the plugs? The MSD 8383 needs 12 full volts or the module can be damaged. Maybe using the ballast resistor caused some weird glitch?

Durham Missouri

292 .060 over  ECGheads
motor was running well  and had about 500  miles on rebuild
1. had a backfire thru exhaust and this started the misfire
plugs were carbon fouled . I was running a Summit 600 cfm

here are details of what I have tried
set valve lash to cam specs
 re set idle mixture with a vac gauge . Multiple times finally decided  to run a smaller carb. went to new 500cfm edlebrock
new mechanical fuel pump
I have the E4 isky cam , I don't have a tach but the vacuum reading is 15 at low idle. The vac increases with rpm and holds between 17 to 20. needle is steady   
changed the carb gaskets
new plugs and wires. I had autolight 46 in with gap at .035  ( when this started)
Currently have Champion 22 .  
cant detect any vac leaks . have used propane and starting fluid
I have msd 8383 distributor and new blaster 2 coil. I initially had a ballast in the circuit. I have removed this and get good voltage at coil 
have tried plug gaps .030   .035  .045(current)
compression showed 145 on all cylinders ( cold)
 played with timing a lot. seems to do better with retard. I was been running 10-12 deg advance timing 

car starts easily and has a slight miss at idle/ gets worse at higher rpm

I am sure there lots of folks here that know more than I do so feel free to give advice   thank you     

 
 

I would think you should be getting much higher vacuum.. unless you have a 'cam' in it , is it an auto?



"Came too close to dying to stop living now!"

A bent pushrod may or may not show up on the compression check depending upon how bad it’s bent but should definitely show up during a valve lash check.
  
Your misfire question likely needs more information to solve but try this.  Misfire at idle or under throttle?  Intermittent or all the time?  If at idle, then unhook the vacuum line going to the distributor and see if the misfire goes away.  If that’s the case, then move the distributor vacuum line at the carb end to a ported signal at the carburetor rather than a direct vacuum signal.  If under throttle, then it could be anything from carburetion, timing, spark plugs, spark plug wires, spark plug gaps, intake manifold seal, etc. without more information to further isolate the cause.

Lorena, Texas (South of Waco)

Been chasing s misfire. Changed plugs, wires, coil, compression shows 145 on allcylinders. Vacuum is steady at 17. Engine has 500 miles since rebuild. Checked valve lash . Would bent pushrod show on compression check?