Mercruiser 4.3L Thunderbolt IV Rotor Rotational Play

Mercruiser 4.3L V6
Thunderbolt IV Ignition
Stock cam

Before I quit trying to correct some low WOT issues last night I pulled the distributor cap off and found that I can turn the distributor rotor clockwise and counterclockwise by hand - basically it has some slop in it. I'ts not 90 degrees or anything, but maybe 20 degrees or so? I don't know if it's just the rotor turning or if it's the rotor and shaft turning - but I plan on checking that when I get back home this afternoon. I did not notice any side to side movement of the rotor (no bad bushings) or vertical movement.

Should the rotor be pretty solid in the housing without this "slop"?

Maybe it's not possible to have a loose rotor on the shaft - which would mean bad distributor drive gear... And if that's the case, would I need a special replacement for this gear or would one from an auto parts store work for a same vintage GM 4.3L V6?

Thanks!

Re: Mercruiser 4.3L Thunderbolt IV Rotor Rotational Play

Yes, the rotor should be solid on the shaft.
If the gear is moving on the shaft, the roll pin is probably sheared, and not a bad gear. The Thunderbolt distributor is not an automotive distributor, so hard to say what automotive distributor the gear came from.

Re: Mercruiser 4.3L Thunderbolt IV Rotor Rotational Play

My guess would be end play in distributer shaft to housing. When you turn the rotor, it rides up the gear and turns in the process. There are shims that go between the housing and distributer drive gear to take up the play.

If the play is between distributer shaft and rotor, then rotor plastic is sheared or slot in shaft is worn.

You might notice some fluxuation in the initial timing from this. Under load, the gear would generally ride up and stay in place, until the rpms are dropped. Something to be fixed, but probably not the cause of low wot rpms. It would be more important to make sure you are getting the full advance at wot.

Re: Mercruiser 4.3L Thunderbolt IV Rotor Rotational Play

Just for an update...

After I had time to inspect it - the distributor drive gear was a bit chewed up and the rotor was solid to the shaft. In addition, the sensor wheel was loose on the rotor.