RPM Limiter (rotor): How Do They Work?

[Sunroof]

I ran a thread here a few weeks ago on my stumbling idle after the engine started great at warm up and ran well til idle. I thought it was a fuel filter issue, but several recommended a check of the points, cap and rotor for starters.

I removed my distributor cap to check cap contacts, cracks, etc and noticed my rpm limited rotor (6500 rpm) for my 1973.5T (CIS) was damaged. I could not identify exactly what was damaged but on the top of the rotor where it bulges out their appears to be this black coating or cover material. OI do not know what is under it, but about 50% was burned thru. I pried some of this off and alot of burned debris came out. Crazy, that the car still ran with this damage, and only the idle was affected.

I replaced the rotor and the stumbling stopped!!!!

That raises the question of just how that rpm limiter rotors work. I never quite understood how it shuts down the revs or holds the revs to 6500 rpm. Most of all what could have burned and what caused the burn? Past 6500 rpm does something self destruct? Are these rev limiter rotors only good for a one time shot over a 6500 rpm load? Just what was under that black plastic looking cover material????????

Thanks

Bob
73.5T

[mthomas58]

PM sent on another topic

[BK911]

Centrifugal force. Look at your rotor. There is a 'doohickey' which is spring loaded. As the rotor turns faster this doohickey gets closer and closer to grounding out.

[moneymanager]

As the revs climb, a small spring loaded piece slides outward and eventually shorts the circuit and slows the motor. Can't answer your other questions, but these are a regular maintenance item with suggested replacement every 15k miles I think along with the cap. Sounds like yours just burned up somehow. Note that you can break off the springs and pull the sliding piece out; car will run fine without the limiter. Just keep an eye on the tach.

[bkreigsr]

There is a bullet-shaped capsule in the rotor that is forced outward toward the grounding spike by centrifugal force at the outer edge of the rotor. At full-tilt it actually bounces off the spike causing a temporary loss of spark to the plug that happens to be firing at the position where the contact is made. They are meant to be serviceable for the life of the rotor.

The different rev limits are achieved by using different spring tensions in the groove that contains the capsule, or by varying the weight of the capsule itself.

If you have a concern that it's causing your trouble, you can disable it by removing the sliding capsule.

Bill K

[R_Builder]

I ran across a similar issue once on a friend's 73.5T... it had an intermittent hesitation in the midrange during acceleration. It was traced back to a failed rotor... the black potting material on the contact to the cap had a crack in it. So, this is a good (easy) place to look for a solution to a sudden "running" problem, as you just found out!

[GH85Carrera]

It really is a pretty simple design that works.

Many years ago my brother stuck a rotor from a VW in my 914 as a joke. As I took off, the rev-limiter stopped the engine at 4,200 RPM. It only took a couple of shifts to figure something was wrong. When I drove back to where he was he had a big grin on his face. I did not think it was so funny.

[Sunroof]

Interesting about the spring activation. Appreciate the education. I thought that spring headed north at night RPM.

But, whats with that black coated spot?? Why would it crack or burn? Whats underneath it that obviously burned up? Counter weight?

Bob

[moneymanager]

Amazing what 20-30,000 volts will do.

[Zeke]

Originally Posted by moneymanager

Amazing what 20-30,000 volts will do.

Or more. What kind of ignition box is on this '73.5? The rotors don't like things like Crane or MSD.

[Sunroof]

Yep, nothin like a slug of voltage to create a stumblig idle! I think that black potted material was used for counter weight on the rotor. Peelin it back I did not see any remants of a wire or anything metallic. Just black and brown dust came out through the burn through portion. Perhaps its sacrificial in the event of a voltage surge.

Either way, a replaced rotor did cure the stumbling idle.

Always learnin....................

Bob

[Sunroof]

Milt...

I believe I still have the original CDI - three plug unit onboard. I have a pretty good set of records and receipts since day one when it was purchased in Birminmgham, Alabama. No record of a CDI replacement. Since I have owned it now (eight years) only a coil change, but not the OEM Bosch coil.

Bob

[bkreigsr]

see if you can track down an oem black coil. I ran a the blue Bosch on mine for a while because it was a lot cheaper, but went back to the original for better overall performance.
I think they are nla, and the last time I saw one listed nib it was over $500
Bill K