3000 rpm miss

[Sprottrod]

I have a slight problem. In my malibu I had a 350 with a small cam, performer RPM intake, 600 edelbrock, Holley Annihalator billet distributor and ignition box, long tube headers ect. It was running fine then started missing anything higher than 3400 rpm. It got down to 3000. I had another motor so I put it in. That is the motor that I was running in my 76 vega and it ran good in there. I put it in the malibu and it has the same miss. The only thing I changed from the vega to the malibu was the distributor and ignition box. I have tried changing carbs and there was no differance. I put a stock HEI back in and it got alot worse. I dont know what else to try. Any ideas or suggestions would be much appertiated. Thanks.

 

[megaladon6]

if i read this right, you changed the engine but have the same dist. and ign. box. have you checked the distributor cap for cracks, or corrosion/charring on the contacts?

 

[85 Cutlass Brougham]

Sounds to me like an electrical supply problem, not an engine problem. Most misfires are at their worst lower in the RPM band if it is an internal issue with then engine due to problems with cylinder pressure that are made worse at slower engine speeds. When I had this problem, it turned out to be the main battery cable at the starter was loose. This is the main terminal bus for the entire car, so it is important that it has a good connection. In your case, it may also be a bad connection under the dash or a break in the pink ignition hot wire that runs to the coil. At lower RPMs, the coil has more time to saturate and would be less likely to miss due to lower amperage. At higher RPM's, this reverses and is probably the cause of the miss. The capacitive discharge box may have somewhat compensated better than the HEI for some reason (how it is wired?) and was able to saturate the coil longer. Without seeing the car all I can do is guess and this is the most logical conclusion I can come up with.

Oh, one more possibility would be a fuel delivery problem due to a clogged line, bad in tank pre filter ( "sock") or a failing fuel pump. If running an electric pump it could be overheating and running too slow (had this happen on a Holley Blue) and the better ignition would then have been able to keep combustion going longer with the lean mixture than the HEI.