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Tossed a belt on my 81 500sl. Belt tensioner bearings were seized.
New bearings, valve job, belts, rings, cleaned, washed.

Starts better than it ever did, but when you rev to 5k, it breaks up.
Ocasionally it will rev, but only for a minute, then we're back to stumbling and riding along at 50mph (if lucky). It may be dropping a cylinder.

Carbs cleaned, accelerator pumps checked, still have the problem.

I'm guessing that the ignition system won't advance, or a bad ignition module? I guess I could reach down and pull a spark plug lead while I ride it along. On the bench I can't tell.

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sounds like you have not sync'd the carbs correctly yet. look in the CARB AND FUEL SYSTEM aread for the procedure explained

This response may be a little late for you, but perhaps it will help others with a similar problem. My F1 had exactly the same symptom and it turned out that the ignition pick-up gap was way too small. The result was poor spark and no ignition advance. My bike backfired badly which led me to understand that there was no ignition advance. Once the gap was set correctly, it felt like the power output was doubled.

Dean

Hi.
I bought in March 2007 a Pantah 500SL in good condition. At least I believe. I cleaned and balanced the carbs, changed some cables and the engine starts easily. There is no misfirings and no afterburning in the silencers. When I ride the engine can climb easily to 10000 rev/min on the first, second and third gears, but on the 4th and 5th it doesn't run above 5000 rev/min.
I checked the timing. It is nearly correct, maybe a little bit too early.
By the way, I read somewhere the mainshaft turns as the rear wheel.
Can somebody confirm? And give some clues to correct the problem. It's a pity to be limited at 150 km/h on a bike able to reach 190 ...

Check the float levels are correct.If the levels are too low the engine will run out of fuel. This is most common in the higher gears as the throttle is open wider and longer, drawing more fuel than in the lower gears. Also check that the petcocks flow freely as this will cause the same symptoms. Oops! almost forgot, make sure the fuel tank cap vent is clear too.

My 600 TL has a similar problem,it revs to well above 6000 but there is no point as there is no topend power,lots of noise but short shifting at 4500
makes it accelerate much stronger,I suppose its going to make the engine last and BTW I get fantastic fuel economy ,even going hard it easily tops 60mpg,thats Imperial gallons

- I have recently changed the carbs gaskets and checked the gas line (tank, filters, taps and tubings) altogether. Maybe I did an unnoticed mistake.
I will verify the float levels at the first opportunity.
- I have not yet checked the valves. What happens on a desmo when the clearance becomes wrong from wear?
- Vince's problem looks as if only one cylinder was operating properly (at least at high revs).
- I had too an unanswered question on the relative rotation direction of crankshaft vs rear wheel .....