Early S Misfiring - burned rotor - ???

Kevin G. · 10-27-2002, 02:02 PM

This is a follow up on my early S with severe misfiring between 3K and 4K rpm. Warren suspected the distributer, and another with lots of MSD 6 experience said the symptoms matched a failure of the MSD unit.

I attempted to determine if the MSD unit was at fault, and got nowhere. So I decided to go for it and remove the distributer. Popped the top off and......

I have never seen a melted and burned rotor before. Now I have. My main questions are - could this be the reason (alone) for the 3-4 K rpm problem? And the big one - what the heck could have done something like this? The guy at the local import parts place thought the coil may have a problem at first, then he noticed the rotor was not a Bosch, and suspected a cheap copy.

I guess what I'm really asking is the MSD unit or Bosch Blue coil pumping out too much current for the system? Did I fry the MSD and/or coil? Did Muffy really run away with the garbage collector and leave Ted at the altar just so she could race his 935?

Inquiring minds want to know.

BTW - the distributer may still be a problem - when set a 2degrees at 900rpm, I cant get more than 27deg at 6K.

Thanks for any help or advice you can provide!

Early_S_Man · 10-27-2002, 02:21 PM

Kevin,

That isn't an aftermarket rotor ... unless it is the best counterfeit work on the planet! Looks genuine Bosch to me, and if it has 7300 and '206' stamped on the underside that would confirm a stock 'S' rev-limiting rotor.

The 'burned' area is the resistance element, sealed in Glyptol High-Voltage corona dope ... and it has a crack present probably caused by being dropped from four feet or more onto a concrete floor or driveway! The crack eventually exteded to the resistance element, but the CDI-unit or MSD was able to jump across the gap ... and as the gap 'grew' from the arcing, the miss you described was the ultimate result. No big surprise, or mystery, if you understand where the crack started and how the problems progressed!

jeff m · 10-27-2002, 02:24 PM

I'm sure Warren will put you right, but until he gets here!

Output of the MSD is a 300V pulse.

Your car should have the black Transformer with the lighning bolt on it. (looks like a coil ) this transformer is expecting the three hundred volts.

The blue coil will produce too high an output to your distributor.
Having said that, I have read in posts that many people use the blue coil esp. with the replacement ignitions (with the same output)

Jeff
911T

J P Stein · 10-27-2002, 05:15 PM

Check out this thread.Rotor Fried

Kevin G. · 10-27-2002, 07:42 PM

Thanks, Warren.

I'm surprised that a Bosch part doesn't have the name on it but I'm certainly not going to dispute you. It does have the 7300 screenprinted on the side. The rest is obliterated due to the burning.

The new rotor & cap come in tomorrow, so I can get her up and running again.

Thanks to your explanation, I can breathe a little easier.

Jeff M - I confess to being confused. I was told specifically not to use the black coil, and that the Bosch Blue coil was the one of choice. Then again, a lot of opinions become fact once they have been repeated a few times. I still have the original black one, so I can put it back.

Thanks for everything, people!