melted already three bosch rotors

[R&TSWB]

Hi everyone, recently got a moto rebuilt 2.2s bosch distributor , installed a petronix ignitor, new caps and rotors and replaced the marrelli distributor on my 2.2T engine
Timing was checked and carb tuned, the engine was running smooth at idle and reved up smooth and strong. After a minute or two , it starts to idle rough and as i checked the ditributor, i noticed the rotor was hot and melted.
Replacing the rotor with another new one, the engine started, idled and reved great, but then again it started faltering and the rotor was melted in the same fuse area!?!?
Melted three rotors and am at a loss? Also using MSD6al with blaster 3 coil.
I'm thinking the coil could be the culprit? Any help or ideas will be greatly appreciated! thanks and regards romy

[john walker's workshop]

run a ballast resistor inline with the 12V to the coil or get rid of it. if you replace it with a regular coil, be sure it's a resistor coil.

[Steve@Rennsport]

Did you replace the plugs, wires & connectors?

That too, will generate large resistances and the MSD will simply increase its current output to fire the plugs; voila',...burnt rotors and caps.

[RoninLB]

bosch pn 04038

[barney911rs]

I have an MSD 6AL and a blaster coil, which I've heard is a better choice then the Bosch coils when running the MSD. I have not any issues in many years, so you should double check all the wiring, and the plugs and plug wires like Steve said.

[bit]

I had the similar problem, check my old thread at What the heck happened to my Rotor and Cap?!? . I removed the MSD 6AL and put back the Bosch CDI unit and my problem disappeared. I'm still running the Blaster coil though. Seems this was simply a failing 6AL in my case.
Cheers,
.Steven
1980 911SC

[jpnovak]

There are rotors available that do not have the resistor between the center cap pin and the outer contact. This is what you need. The resistor is for noise suppression.

you can also take your rotors that are blown, use a dremel to mill the epoxy out and solder a brass contact in place. Then reseal with epoxy. I have done this to several rotors and they last for several years.

[euro911sc]

Originally posted by Steve@Rennsport
Did you replace the plugs, wires & connectors?

That too, will generate large resistances and the MSD will simply increase its current output to fire the plugs; voila',...burnt rotors and caps.

Steve,

Are you saying that if you replace with all new stuff you run the risk of generating huge current? This does not make any sense to me... I thought the older stuff got the higher the resistance???

Best regards,

Michael

[Lorenfb]

"the MSD will simply increase its current output to fire the plugs"

DO WHAT???????

The output of the MSD is the SAME as any other CDI, i.e it's a voltage
source (a charged capacitor) driving a coil. It's the ridiculous high
voltage of the JUNK MSD unit & coil the causes the problem.

Save the MSD system for the Chevy engine install in your 911
and stay with the Bosch system (40+ years of use for many).

[Bobboloo]

Originally posted by euro911sc
Steve,

Are you saying that if you replace with all new stuff you run the risk of generating huge current? This does not make any sense to me... I thought the older stuff got the higher the resistance???

Best regards,

Michael

The older wires may be causing resistance, which means heat buildup. The higher current from the MSD/coil will burn the rotor if the higher current is not allowed to flow to the plugs due to resistance from the plug wires.

Also check your plug gaps. Too large of gap will cause more resistance as well.

[RoninLB]

output delevered is only what is needed to fire plug

[Lorenfb]

"The older wires may be causing resistance, which means heat buildup."

Right! But ONLY within the wires themselves.

"The higher current from the MSD will burn the rotor if the higher current is not allowed to flow to the plugs due to resistance from the wires."

No! Heat is mostly developed at the rotor to cap arcing AND the rotor's internal
resistor (if it has one).

Remember, the greater the overall resistance, e.g. wires, the LESS current and
less heat (power):

Power = current squared X resistance.

Thus, using resistive wire reduces the current (and RF noise - AM radio).
A bad coil wire (high resistance) or bad spark plug wires reduce/eliminate
the spark current causing an inadequate spark.

[pwd72s]

I'd like to add that it could be the rotors themselves...they are no longer bearing the "made in germany" stamp...now farmed out to who knows....

A 914-6 friend suffered a similar burn-out. I gave him the "made in Germany" rotor from my 1972 S distributor (I'd switched to an RS distributor) and as far as I know, the used german rotor is still
working well for him. He's running Bosch ignition.

Doesn't sound likely in this case, though...

[Early_S_Man]

The resistance element in Bosch rotors is 5000 Ohms, and the power rating is unknown, though probably on the order of 2 - 5 Watts, max. I have been told that the MSD puts out on the order of 100 mA - 115 mA. Ohms law provides that power = I ^ 2 x R = 0.1 x 0.1 x 5000, or 50 Watts for the minimum case. This particular phenomenon has had several threads here on Pelican with similar results!

[dvkk]

Bosch rotors that are orange/brown like in the pic Ronin posted have the resistor. Bosch rotors that are black have no resistance.

[RoninLB]

Originally posted by dvkk
Bosch rotors that are orange/brown like in the pic Ronin posted have the resistor. Bosch rotors that are black have no resistance.

1k ohm = 1000 ohm


i just measured the posted rotor pic at 3.5 k ohm
... the rpm limiting rotor tested at 350 k ohm






also MSD series starting with a 6 has spark energy [millijoules] of 105-115 mJ
- Then the catalog says "Spark energy is a product of voltage, current, and time with the result being measured in millijoules. The spec shown with MSD CD ignitions is the amount of energy stored in the capacitor which is all delivered to the coil for every firing."

One joule is the equivalent of one watt of power radiated or dissipated for one second.

[Lorenfb]

"also MSD series starting with a 6 has spark energy [millijoules] of 105-115 mJ"

The energy of the Bosch & MSD is about the same:

Energy = 1/2 X capacitor X voltage squared.

Since the peak voltage for both systems is the about same (~400-450 volts),
the current developed is the same, i.e. the output is a VOLTAGE SOURCE
and NOT a current source for both. The external spark elements determine
the current, i.e. the ignition coil, the rotor/cap, the spark plug wires/connectors.

It's the power per spark that's problematic for the MSD.
Power is the rate of energy applied over time. Since the MSD supplies multiple sparks
this results in more power being supplied to the system, e.g. the rotor resistance, which
causes the damage.

If the MSD supplies 5 sparks at idle (versus 1 at 3000 RPM) that's 5 times
the power that the rotor resistance has to dissipate versus as single spark
system.