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Rule of Tim - OT- Twin Cylinder Motor Cycle engine

Friend of mine has some sort of horrid, noisy little motorbike, with a

250cc twin cylinder two stroke.

It runs OK at low revs, but at full throttle, one of the pots seems to die, and the thing loses power.

Slightly oddly, when the problem occurrs, the engine will spin at 8000 rpm on one pot, but there is NOTHING (not hot gas, no cold air, no smoke, nothing) coming out of the exhaust pipe on the other side. I would have thought that even if there was no spark, that cylinder should still be working as an air pump and I should be able to feel something.

He has tried new coils, plugs and plug leads.

The reed valves, to a casual inspection, look fine.

There is some sort of "power valve" in the exhaust, which apparently tunes the exhaust ports for different rev ranges.

Thoughts anyone?

David

Reply to
rads
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Been tuning it, has he? Sounds like a bad case of hole in the piston syndrome.

Good thing about 250 cc 2-strokes is that it only takes a few minutes to whip off the head and barrels.

Pete

Reply to
Pete Young

Except there are hot exhaust gasses (and some, though not excessive smoke) at low revs.

Still plausable (sp) to be a holed piston?

David

Reply to
rads

Crank seals gone too .. perhaps ..

Yup .. ;(

-- Paul ...

(8(|) ... Homer Rocks

Reply to
Paul - xxx

Has he made sure the exhausts arn't blocked?

How about primary compression, test with a plug out.

Yamaha's had a tendancy to blow out the crankcase seal on the side behind the genny, usually if wrong plugs fitted.

HTH

John

Reply to
Lofty

UK.Rec.Motorcycles is that -----> way.

Reply to
Colonel Tupperware

Ah but he invoked the Rule of Tim which is a bit like a get out of jail free card

Andy

Reply to
Andy.Smalley

Seal between gearbox and crank case replaced.

Oops, forgot to mention, we did a compression test yesterday, and with the reed vales off (reed valves look OK), kicking it over on the starter for five kicks, achieved approx. 110 psi in each pot.

Will suggest "crankcase seal on the side behind the genny" (thanks Lofty).

re uk.rec.motorcycles

There is a newsgroup for this particular bike, but it seems to be full of polishers, rather than grease monkeys,

It seemed that for advice on illogical, oily, awkward and recalcitrant pieces of mechanical kit, there was only one place to come!

David

Reply to
rads

Sounds just like an LC ... ;)

Maybe the power valves are stuck, they were prone to problems in early versions, or are not working quite as they should. They might be opening enough to 'breathe' at low revs while killing it higher up the rev range. It's a loooong while since I worked on them .. ;)

-- Paul ...

(8(|) ... Homer Rocks

Reply to
Paul - xxx

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