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This motorcycle runs perfect at full throttle, pulling strong and fast all the way up through the RPMs. At slow throttle increases, particularly in first and second gears, it almost dies on me around 3500-4000 rpm. When in neutral and I ease the throttle up through the rpms, I can feel it breaking down all the way up through, not a steady engine firing.

I have proper adjustment of mixture and idle screws; pump adjustment to 6ccs after 20 throttle turns; floats adjusted right; tried smaller and larger needle jets and main jets; new coils and plug wires; properly set Dyna III electronic ignition; new plugs properly gapped. Where do I go next with this?

Appreciate any and all help. I have another R90S and an R100Rs that run great and this one is driving me crazy!!!

DLC3 wrote:This motorcycle runs perfect at full throttle, pulling strong and fast all the way up through the RPMs. At slow throttle increases, particularly in first and second gears, it almost dies on me around 3500-4000 rpm. When in neutral and I ease the throttle up through the rpms, I can feel it breaking down all the way up through, not a steady engine firing.

I have proper adjustment of mixture and idle screws; pump adjustment to 6ccs after 20 throttle turns; floats adjusted right; tried smaller and larger needle jets and main jets; new coils and plug wires; properly set Dyna III electronic ignition; new plugs properly gapped. Where do I go next with this?

Appreciate any and all help. I have another R90S and an R100Rs that run great and this one is driving me crazy!!!

Make sure the enrichers are shutting off completely, no cracks in cable boots on these either. Shrink tube over a piece of fuel line works well and lasts longer than the stock rubber ones.
Check for loose screws on covers.
Which method did you use for float adjustment? They are very fussy about float level.
What notch are needles set at?

You are very fortunate to have a store of known good parts to swap over. I'd start by swapping the carbs from your other 90S. If they work you can start thinking about the ignition system. Welcome aboard, by the way.

Appreciate the thoughts. Regarding the enricheners, I actually removed the enrichener cables on these carbs and installed the old levers just because of what you think could be the problem. If you are unfamiliar with the levers, they screw into the carbs where the cables screw in and you lift the lever to enrich. For float adjustment, I tested the float level that I set by measuring the gasoline in the float bowl which is working out to be 55 ml for each carb... just the right amount. Another note... I had these carbs rebuilt by Charlie Laughridge at Bing Agency International in Kansas. He does a good job rebuilding Dellortos and has all the parts. While he was rebuilding them, I put on a brand new set of Dellorto PHM38s and had the same problem I am having now.

When was the last time you cleaned and re-greased the advance unit? Could it be sticking at lower rpms?

Advance unit should be ok. This performance issue was happening when I had original points and was still happening when I put in the electronic ignition in 2011. However, along with you, I am believing this is not a carb problem. More thoughts please!!!

Did you put the timing light on the advance unit to check for smooth operation? That is the best test. You can see jerky operation with the light on the actual unit while it may not show up with the light on the flywhee.

I doubt that this is it, but check to be sure.

Duane...
If the timing is off, would it still run very smooth at full throttle? I will check it the way you suggest... have never tried it that way, and also do a check with a static light just to make sure. My first Dyna III electronic ignition was defective so perhaps I need to get more focused on that. Thanks!

Timing can be way off and run well to the top end. Having full power tends to indicate that you have good compression, but not much else. Static timing is useful as a start, but that is all. Since you have a strobe timing light, always put it on the advance mechanism. This way you know what it is suppoesd to do.

A sticky advance mechanism can really cause trouble. Again, I doubt that is the trouble, but you MUST check everything that is basic to the running.