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

rads <[email protected]> writes:

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

> Thoughts anyone?

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


--
____________________________________________________________________
Pete Young [email protected] Remove .dot to reply
"Just another crouton, floating on the bouillabaisse of life"

On Mon, 21 Jun 2004 11:22:28 +0100, Pete Young <[email protected]>
wrote:
>
>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

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

Still plausable (sp) to be a holed piston?

David

rads typed:
> On Mon, 21 Jun 2004 11:22:28 +0100, Pete Young <[email protected]>
> wrote:
>>
>> 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
>
> Except there are hot exhaust gasses (and some, though not excessive
> smoke) at low revs.

Crank seals gone too .. perhaps ..

> Still plausable (sp) to be a holed piston?

Yup .. ;(

--
Paul ...

(8(|) ... Homer Rocks

On Mon, 21 Jun 2004 10:20:10 +0100, rads
<[email protected]> wrote:

>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


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

On Mon, 21 Jun 2004 10:20:10 +0100, rads
<[email protected]> wrote:

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

--
ColonelTupperware
spouting bollocks on Usenet since 1997
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"Colonel Tupperware" <[email protected]> wrote in message news:[email protected]...
> On Mon, 21 Jun 2004 10:20:10 +0100, rads
> <[email protected]> wrote:
>
> >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?
> >
> UK.Rec.Motorcycles
> is that -----> way.
>
> --
> ColonelTupperware

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

Andy
--
SWB Series 2a ( dressed as a 3) "Bruce"
It's big it's mean it's really really green

On Mon, 21 Jun 2004 08:54:22 -0700, "Paul - xxx"
<[email protected]> wrote:


>> Except there are hot exhaust gasses (and some, though not excessive
>> smoke) at low revs.
>
>Crank seals gone too .. perhaps ..

Seal between gearbox and crank case replaced.
>
>> Still plausable (sp) to be a holed piston?
>
>Yup .. ;(
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

rads typed:
> On Mon, 21 Jun 2004 08:54:22 -0700, "Paul - xxx"
> <[email protected]> wrote:
>
>
>>> Except there are hot exhaust gasses (and some, though not excessive
>>> smoke) at low revs.
>>
>> Crank seals gone too .. perhaps ..
>
> Seal between gearbox and crank case replaced.
>>
>>> Still plausable (sp) to be a holed piston?
>>
>> Yup .. ;(
> 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!

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