immortal1 (linn)
+1y
Might be able to help - assuming the problem is in the distributor. The electronic pickup coil sits on a plate in the distributor that is held in place by 2 spring loaded cams. As the RPM increases, the cams rotate out and advance the timing. To help improve fuel milage, the plate is also connected to engine vacuum (typically "ported" vaccum - explained later).
The spring loaded cams should be at full advance by 3,000 rpm. The vacuum advance typically advances the timing to full advance around 1,300 RPM. If you push the pedal down harder, vacuum drops, timing retards and you accelerate without "pinging".
Now your problem. I don't thing the vacuum advance is causing the problem. But why would the spring loaded cams effect running at 3,000 RPM and not say 2,500 RPM? Doesn't seem likely either. You have plenty of power under 3,000 RPM so I would rule out fuel supply to the carb. By 2,000 RPM or so you would be on the main jets so I don't thing the fuel delivery is the problem either.
Another interesting problem I ran across on GM HEI distributors, as the advance moved the pickup plate, the tiny wires on the pickup coil would bend and break connection. But in your case this should also cause missfire when the vacuum moves the plate. So that doesn't sound right either.
The most likely cause might be the ignition coil. It simply looses the ability to supply the necessary voltage to the plugs at around 3,000 RPM and above. If you have a timing light, hook it up to the coil wire, and run the RPM's up past 3,000. If it's a decent timing light, it should keep up even at 4,000 RPM. If the light stumbles around 3,000 I would definately say the coil is the problem.
Sorry for the long reply - just the way my mind works I guess. Assuming everything above is correct, try a new coil.