Rotor toasted (MSD)

[project 911]

Okay, so I know this problem has been around, and I've seen most of the archives, but does anyone have a solid answer for this problem? I just spent a huge amount of time installing this system, and hate to put the old system back in.

What I'm running:

Basically stock 2.2T (1970) with Zenith carbs.
MSD 6TN (same as AL but with external rev limiter)
SS coil (not canister type, but ok for 6 series box)
Accel spiral core wires with Beru ends.
Bosch dizzy
I cleaned the plugs and re-gapped them to .040

Car seems to run fine, but toasted the rotor during about a 20 minute drive. I bought a VW solid rotor but it doesn't fit the shaft in my dizzy.

Any help would be appreciated. I have a long drive tomorrow, and don't dare do it with this stuff happening.


Look at that... It has a hole with melted plastic coming out of it.

[Early_S_Man]

Steve,

Fairly common problem with MSD ... and plug gaps that are too wide! Reduce the gap back to 0.032" and problem will probably go away!

The exact VW rotor that is needed is for the old Bosch '010' distributor from VW transporter/bus around '56 - '58.

You can fix that burnt rotor by cleaning off the burnt, mass of plastic goo and resistor element with a Dremel tool and rotary bur ... then bridge the gap normally filled by the resistor element with 4 or 5 parallel 22 ga bare wires, soldered, the epoxied in place. I will post a pic of the proposed fix a bit later ...

[john walker's workshop]

that's the best burnt rotor i've ever seen. larvae!

[project 911]

Thanks Warren,

I'll change my plug gap. Would this rotor be useable in this condition?

[Early_S_Man]

Steve,

In my opinion, no ... because the 2nd stage after this level of burnt resistor is to burn through the rotor body/insulation to the distributor shaft. Then you wouldn't have spark to the plugs at all!

The repair I described ... bridging the burnt resistor on the rotor would make it usable.

[project 911]

Thanks Warren,

I just finished grinding out all the filler in the rotor, now I just have to figure out how to solder a bridge in there... If you have any pics, I'd love to see them.

[project 911]

Well I ground out all the filler and soldered in a section of quarter inch wide brass strap to bridge the gap. I get a good solid reading on the multimeter. If I fill it with epoxy, should this work?

[randywebb]

My guess is you don't need epoxy - but see what Warren says.

Now, what lens & camera are you using for those sharp macro pics?

[project 911]

Randy-

I went ahead with the epoxy just in case...
That last pic was pretty fuzzy, but I'm using my old Sony digital 2.1 megapixel on the macro setting.

[project 911]

Well, I backed my plug gap down and soldered in the brass, sealed it with epoxy and it did the trick. Went for a long drive today and nothing melted or exploded. Thanks all-

[Nitrometano]

I need to do the same to my car.

[B D]

If you have a rev limiter go ahead and remove the one from the rotor as well.

[Vintage911Racer]

I agree with Early S man.

However Instead of closing the gap in my Plugs I simply replaces the Coil with a Bosh Blue one instead of the Crane one I was using. Problem solved. However, I always dremel out the resistor and solder in some wire just in case. I have about 4 or 5 spare rotors new that have this modification already done to it to be on the safe side. they work great that way unless you need the 7100 or 7300 rev limiter in them.

[randywebb]

Use a msd coil with the msd box - Steve Wiener posted on this not long ago.

[Nitrometano]

I will do the rotor modification. Tomorow I will buy a new one in a VW store. I will change the MSD blaster 3 coil for a new Accel 140001 coil. The Accel coil have 50,000 volts contrary to the MSD Blaster coils that have 45,000. Sure my rotor will burn in fire. So the modification will be necesary.

[Lorenfb]

"The Accel coil have 50,000 volts contrary to the MSD Blaster coils that have 45,000. Sure my rotor will burn in fire."

Not only will the excess voltage, i.e. way beyond what's necessary, damage rotors,
it also stresses all the ignition system elements, e.g. the wires, the plug connectors, & the cap.
Porsche & other OEMs could have easily used very high voltage coils, but didn't because
of the problems many have noted on this thread & because no benefit is gained.

Everyone seems to ignore the fact that besides voltage, a good spark requires a minimum
energy level to fully ignite the charge in the combustion chamber. As with voltage, too much
spark energy can also be a problem, i.e. without excessive energy, voltage alone can't
burn rotors. For a CDI system, energy equals 1/2 C V squared, where V is voltage stored
before application to the ignition coil. Remember, the ignition coil in a CDI system only functions
as a pulse transformer & as such only steps up the voltage applied from the CDI storage capacitor.

The peak voltage is only necessary to CAUSE the spark given; the cylinder pressure & spark
plug gap & the rotor gap. Once the spark occurs, it's the energy of the spark which is key.
The energy is dissipated by; the resistive coil & plug wires (if used), the resistve coil
connector, the resistive plug connectors, & the resistive plugs (if used). The remaining energy
is available for the spark.

[RoninLB]

Originally posted by Lorenfb

Remember, the ignition coil in a CDI system only functions
as a pulse transformer & as such only steps up the voltage applied from the CDI storage capacitor.

afaik in the old "standard" ignitions, points/condensor/coil, delivered only enough voltage that was demanded. As demand changed thru operation so did secondary voltage output.

if I'm correct and I transfer this "afaik" then a CD should only deliver the voltage necessary to create the spark? If there is more spark than necessary then it would look to go elsewhere? Elsewhere would be a voltage leak to ground somewhere? The amount of amps in the spark is the controller for the amount of work the spark can do, I think?

so just because a coil can deliver more voltage it doesn't mean that the voltage will necessarily be used, I think?

I think when Loren is around, for sure.

thx