Timing bouncing around -whats the cause?
 

I have a 2.7 RS spec motor with S MFI. The timing mark sporadically jumps about 8 degrees advance. The timing mark sits is steady and then jumps every so often. It seems pretty random and occurs at idle on up at least to 2000 RPM. Seems to get more sporadic at high RPMs. I'm running new points and the dwell is spot on. The engine is on my dyno so I can't tell you how it's driving but I can tell you that it's getting half the power that it should be getting. The plugs are new but the wires are not.

Will a Petronic Ignitor cure this situation or is my distributor hosed? How can check if the distributor is bad? Could it be the wires?

The engine was just rebuilt with new pistons and a complete valve job.

TIA...

Avery,

The jumping is probably due to sticking advance weights ... because of rust, gunked-up lubricant, or other collection of contaminants on the pivot posts, or disintegrated bakelite rubbing wafer under the advance weights.

No, a Pertronix conversion won't help ... because the camshaft [that the trigger magnet wheel would be mounted to] is controlled by the advance weight mechanism, and any play or variation would also be present in the trigger signal generated by the Pertronix unit!

The following thread may help with cleaning your distributor after partial disassembly:

http://forums.pelicanparts.com/showthread.php?s=&threadid=5432&highlight=distribu tor+lube

I agree with S man, but if it turns out not to be the advance mechanism, it could be your plug wires. Depending on what kind of inductive pickup you have on the timing light, you might be picking up a signal from an adjoining wire. A lot of guys put the pickup very close to the distibutor because it is easy to get to. That places it into a pretty closely bundled mess of wires, making this problem worse. Try it right down as close to the plug as you can get; on a dyno it should be easy. If the advance no longer appears to jump, you may have found the problem. Bad wires and the resultant weak spark can rob a surprising amount of power.