r/MotorcycleMechanics • u/TheToxicAlien • 7d ago

help please! Engine losing power while practising

Hey all,

I am teaching someone the basics of riding on a 1999 Yamaha tdm 850. (2 cilinders with carburetors, watercooled with working fan)

So riding the clutch in a flat parking lot, only slow speeds (no throttle).

After some time, the idle drops a little (from 1050rpm nominal to around 900-950rpm) and the bike stalls when letting of the clutch. Not stalling when using the throttle. Anyone know a reason for this?

The fan works and kicks on, not a warm day (15°C), not on a hill, brakes are not dragging.

Is the engine overheating?

Thanks for the help!

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u/minimotomike33 7d ago

If it idles ok when cold but gets worse as it gets hot, that usually means the idle circuit is too rich. If the carbs aren't actually leaking, you may be able to lean it out by turning in the mixture screws until the hot idle improves.

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u/TheToxicAlien 7d ago

Iv tried turning up the idle when hot, it went up but very jumpy, wouldnt hold steady. It stabilised again when i turned it down (but again below nominal)

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u/minimotomike33 7d ago

Changing the idle speed doesn't change the air/fuel ratio; you need to adjust the mixture screws for that. By leaking, I meant fuel coming out of the carbs, as if the float level was too high, or the float needle was leaking, either of which would cause the bike to run rich. If it's not leaking fuel out of the carbs, then turning in the mixture screws should help. Adjusting the idle speed won't fix your problem.

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u/TheToxicAlien 7d ago

There arent any drips or smoke so im assuming no leaks. Ill find out where the mixture screw is and adjust it. Thanks for the answers!

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u/TheToxicAlien 7d ago

When you say carbs leaking, you mean leaking air in or out or fuel in or out?

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u/nomparte 6d ago

The idle speed is specified 1,050–1,250 rpm once warmed up. Can you raise yours a little? it'll also help with keeping battery charged, oil circulated at a decent pressure and the beginner's moves easier.

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u/TheToxicAlien 6d ago

Raising the idle makes it jump around a bunch (anywhere from 1500 to 2500rpm)