MSD rev limit set at 3800?

[georgyporshy]

In my 83 911SC I recently installed an MSD 6200 CD box, MSD Blaster coil, and Magnacore plug wires as the final step in battling a Permatune system that would fail after 15 minutes, then work again after 30, then repeat. I had checked all grounds and other connections prior to deciding it must be the Permatune. The MSD unit worked great for 2 weeks, then my ignition started to periodically cut out at 3800 RPM +- when the engine had not reached full operating temperature. Now it has progressed to the point where I can’t go above 3800 rpm, as if there were a rev limiter set at this speed. The WUR is a possible suspect as it works poorly in cold weather (backfires) but seems fine right now. I do have a backdated exhaust with the Oxy sensor in place. Any ideas? Thanks.

[Por_sha911]

Does your unit have the removable chip that allows you to pick what RPM it cuts out at? If so, try swapping out the chips.

[sc_rufctr]

Have you done the MSD rotor mod?

I installed an MSD ignition system much like yours except I'm using MSD Super conductor wires with custom spark plug boots.
I was having similar issues to your but after doing the MSD rotor mod they all went away.

Now that I've posted this I'm sure someone will chime in and say it's not needed.
But think about it.. If your rotor has an in built resistor then you need to get rid of it when your using MSD ignition... (Or any after market system)

It takes about 30 minutes to remove the resistor and solder in a bridge.
You could easily spend hours trying to resolve the issue without success.

Please read this thread... MSD Rotor Modification

I'll post a picture of my old rotor as it looked when I removed it shortly. (Just need to find it and take a picture)

[sc_rufctr]

"georgyporshy"

This is a picture of my stock rotor after I had been running an MSD ignition system for about 1000kms...

As you can see the dark brown resin has lifted and deformed.
Note the shiny streak of resin that has leaked towards the rotor tip.
There is a resisitor under this resin that joins the tip to the centre of the rotor.

This confirms to me that the normal solid resin has become liquid and the centrifugal
force of the spinning rotor has caused it to streak towards the tip.

To All reading this post: This more than justifies the MSD Rotor mod.
Counter arguments welcome..............

[sc_rufctr]

Closer pic of heat damaged resin...

[georgyporshy]

Thanks for the advice so far. The MSD box has a set rpm limit, not adjustable. I checked the rotor and it looks new, but may try the modification soon. Strangely enough the car ran fine this morning - weather cooler than yesterday - so temperature may be the issue. The box is mounted in the engine compartment where the Permatune was.

[sc_rufctr]

I'm not surprised it ran well after sitting overnight because the resistor has not completely failed but I suspect if you took it for a long drive the problem would reoccur.

Your rotor may look fine now but the resistor under the resin may be failing.
If this is the cause of your problems eventually the resin would start to "move" and run like mine.

So.... Take it for a decent drive. Say 50 miles minimum... And if the problem returns remove the dizzy cap and check the rotor.
Touch the resin with a small screwdriver or similar and see if it has softened.

I have a brand new in the box unmodified rotor I bought from out host.
I'll measure the resistance and post the value here for comparison.

[georgyporshy]

Peter,
Thanks for your interest and help. The problem returned when I took the car out again for lunch. I'll go ahead and do the rotor as soon as I can - preping/launching my boat right now.
George

[sc_rufctr]

Cool. What sort of boat is it?

(I'll post the resistance value of the new rotor in about 1/2 an hour)

[georgyporshy]

Freedom 35 cruiser/racer sloop. I race out of Rochester Yacht Club on Lake Ontario. It's a pretty fast boat but still very comfortable for cruising - a quality boat.

George

[sc_rufctr]

Sound like a great way to spend the day

Anyways... The resistance value of the new rotor (930-602-902-00-M14) is 4.89 Kilo Ohms.

If the rotor you're using measures higher than 4.89 Kilo Ohms then it confirms that it's the problem.

When I measured mine I got a value of about 5.60 Kilo Ohms cold. (the one in the photos I posted)

That doesn't seem like much of a difference but it was measured when the rotor was cold.
I know from my electronics back ground that a resistor can recover some of its original value after being over loaded.

I never confirmed this but I'm pretty sure if I kept running the car without doing the mod this value would
have got higher and higher as it heated up and eventually it would fail.

Please post your comments after you do the mod.

[georgyporshy]

The problem is solved - a failure in the tach signal line was causing the MSD to limit revs to 3800. This is either due to a frequent but not always grounding or the fact I hadn't installed the tach adapter available from MSD. The tach worked fine without it, but I had this problem, intermittently. Now I'll install it and see what happens.

My local Porsche guru, Dan Deegan came up with this solution. He said to see if the rev limit happened both under load and when running free, if so it's an electrical fault, if not it's probably fuel delivery. If electrical, possibly the tach connection. I made sure it was failing when driving, then revved in neutral - it just kept bouncing off 3800 rpm. Pulled the tach wire, no more problem, lots more FUN!!

Thanks for your help.
George