high revs misfire

[berfinroy]

I just "rebuilt" my distributor [e.g., disassembled, cleaned incl. trigger points, cleaned and reset points and relubricated everything] . New correct non-resistor plugs, too. Engine starts, idles and generally runs great.
HOWEVER. It starts to breakdown @ 3,500 rpm and really won't go any higher.
SECOND HOWEVER. Its been so long since I tried to run the engine at high rpms I really can't say that it wasn't doing that before!
I'm thinking bad coil?
Other thoughts?

[Idle]

Have your ignition timing check with a scope. A misfire at high speeds is usually a result of incorrect timing advancing. This can be cause by your points not being up to the job of higher revs (which is known as 'floating' the points) or your vacuum advance having a small leak in the internal seal.

But before you start replacing parts check it with an ignition scope. You might be able to rent one of these or have a garage run a check for not too much cash. Then you will know just what needs to be replaced.

[Frank Reiner]

Originally Posted by berfinroy

I just "rebuilt" my distributor [e.g., disassembled, cleaned incl. trigger points, cleaned and reset points and relubricated everything] . New correct non-resistor plugs, too. Engine starts, idles and generally runs great.
HOWEVER. It starts to breakdown @ 3,500 rpm and really won't go any higher.
SECOND HOWEVER. Its been so long since I tried to run the engine at high rpms I really can't say that it wasn't doing that before!
I'm thinking bad coil?
Other thoughts?

For an engine that will run nicely to 6000 RPM (gallop), 3500 RPM is a canter. Not high at all. Possible causes: Points, Coil, Switchgear.

Originally Posted by Idle

Have your ignition timing check with a scope. A misfire at high speeds is usually a result of incorrect timing advancing. This can be cause by your points not being up to the job of higher revs (which is known as 'floating' the points) or your vacuum advance having a small leak in the internal seal.

Too much advance - pinging.
Too little advance - lazy engine.
But no misfire as a function of advance.

Very weak point spring may be a cause of misfire.

[Tomguy]

I agree with Frank. Also as a "Been there done that" warning, if you snap the carbon trace in the distributor cap, it'll run fine to about 3000 RPMs or so and struggle to get higher. It'll also feel like you lost all your ponies if you try to get it to move under WOT even at lower RPMs.

So points (or their gap), coil, switchgear, cap. Non-resistor plugs, and non-resistor wires (with resistor wire boots).

[Sugar Bear]

What are you working upon?

[Tony H]

Are you running the original ignition amplifier? I switched to Pertronix ignitor/coil and cured a high speed miss on my 3.5. The PO had thrown a lot of parts at the problem.

[berfinroy]

My car is a 1972 300SEL 4.5. Solid core spark plug wires and new non-resistor plugs. Original ignition amplifier and coil of unknown age.
The ONLY thing I changed from original was to substitute a different centrifugal advance mechanism with weights marked "200" vs the originals that were marked "238". The "200" set seemed (purely from handling; no measurement) to provide a more aggressive advance curve, which a surmised would be good for performance, but I really don't know that. It was an experiment.
Does anyone know what these weight markings mean? Grams?
Incidentally, I have a third set marked "285" from a distributor that is definitely
from a later 450 series car, and the movement of the weights is unquestionably more restrictive.

[Tony H]

I would ditch the ignition amplifier and install Pertronix. It is miles ahead of the original ignition. You can leave all the original in place for a stock appearance if you wish.

[Benz Dr.]

Last time I had a problem like this on a real 350SL it turned out to be a bad throttle switch. The part was new in the car but it was still junk. Replaced it and it was like a different engine. Pulled hard right up to redline in every gear. It wouldn't rev much higher than 3,000 RPM and would only climb very slowly or not at all with the defective part.

Definitely threw us a curve. New parts = never ever worked

[berfinroy]

Originally Posted by Tony H

I would ditch the ignition amplifier and install Pertronix. It is miles ahead of the original ignition. You can leave all the original in place for a stock appearance if you wish.

I have an 1885 kit. There are 2 suggested ways to install it. One way is to by pass the original switchgear and use a flamethrower coil, and the other way is to retain the switchgear and original coil.
Which way is "better" and why?

[Tony H]

The original switchgear will do nothing but cause potential problems and is not needed. I would use the Flamethrower coil with the Pertronix unit.

[berfinroy]

[QUOTE=Benz Dr.;4057456]Last time I had a problem like this on a real 350SL it turned out to be a bad throttle switch. ]

Nothing more frustrating that a faulty NEW part. I removed, inspected and cleaned and lightly touched up the sliding contacts on my TPS the other day. Looked good to me.

Thanks, Benz Dr and Tony.

[berfinroy]

[QUOTE=Benz Dr.;4057456]Last time I had a problem like this on a real 350SL it turned out to be a bad throttle switch.

What were the symptoms of a bad TPS? I just removed, inspected and cleaned mine, and now my car runs worse! Can't imagine how I could have damaged it, but those are the facts.

[Tomguy]

Did you just reinstall, or calibrate too? You need to calibrate the switch (set the adjustment right). I lost the manual for D-Jet but I'm sure someone still has it that can send it.

[berfinroy]

I THOUGHT I put it back in the same place as I found it, give or take a millimeter.
I have a service manual that should tell me how to set it. Hard to believe its that touchy.