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So a few days ago my 87 decided it had enough and blew, prior to that for about a couple of hours i knew something was up, under hard acceleration it would bog a little but not much, yet was still smokey so no lean bog. and a few pinging or clicking sounds in the motor, i use 98 octane the highest grade here in aus. when it happened i was chasing a yamaha raptor 660 happy though as he couldn't outrun the 87 old beast. she sounded like she was being shot with a machine gun of pinging for a few seconds then completely let go. i was expecting to find a piston up in the head somewhere due to absurdly low compression but when i took the head off i was in shock. The head, cylinder and piston were in immaculate condition. brand spanking new not a single scratch or sign of wear and the plug was perfect colour tan. this is after 2500km as mine is regoed. yet has no compression, havent done a test but it would be 30 psi maybe. any thoughts?. i dont have a flywheel puller so i cant get the magnets off but under the cover looks clean?

how was your coolant level ?

l bought one of these, works beaut and the holes like up with 175 200 and 250 flywheels.

http://www.ebay.com.au/itm/MOTORCYCLE-F ... 43d1444e88

If the magnets had come off the flywheel or were loose you would definitely hear them !


Sounds like you have overheated the bike and the rings have lost tension.

diymirage wrote:how was your coolant level ?

Coolant ? That's an '87 air cooled kdx isn't it ?

Cheers

Geoff.

KDX Geoff wrote:

diymirage wrote:how was your coolant level ?

Coolant ? That's an '87 air cooled kdx isn't it ?

Cheers

Geoff.

my bad, i meant wind speed, how was the windspeed ?

(does sound like an overheating issue to me)

dont bad crankseals present themselves as an airleak ?

Wouldnt overheating show a white plug colour? I need to get the flywheel off to check the magneto side seal. Can overheating cause rings to loose there tension?

Flywheel key shear and allow too advanced timing?

kdx87 wrote: yet has no compression, haven't done a test but it would be 30 psi maybe. any thoughts?.

If there's little or no compression then its either going past the head gasket, spark plug or rings.

So if the heads tight and no leaking gasket, that the spark plugs tight and no leaking gasket, that only leaves the rings.....

kdx87 wrote: i was expecting to find a piston up in the head somewhere due to absurdly low compression but when i took the head off i was in shock. The head, cylinder and piston were in immaculate condition.

Have you had the barrel off to check the rings and piston ?

Will it role start ?

How about some pictures ?

Had the head off but not the barrel, no leaking head gasket or plug for sure. Compression so low i can kick the motor over with my finger. I would have thought rings go over time not at one sudden.

Broken ring/s

Rings jammed in ring land and/or broken.

KDX Geoff wrote:Broken ring/s

Rings jammed in ring land and/or broken.

ive seen that before, sounds about righr here.


some pbblaster will free the rings, if they arent rolled over

left side crankseal would be air leak bike would run lean , right side would lose tran oil and foul plugs more

Yep, was seized rings. Should have been the first thing i checked. Anyone propose how this would have happened? Plenty of oil in the crank and under the piston 30:1 ratio. Symptoms before motor died was it would bog once it got to 5-6000 rpm, yes i got a tacho on the aircooled.

kdx87 wrote: when it happened i was chasing a yamaha raptor 660 happy though as he couldn't outrun the 87 old beast. she sounded like she was being shot with a machine gun of pinging for a few seconds then completely let go.

Piston's nipped up in the bore (overheat caused by above or running lean....) and crushed the ring lands jamming the rings and no compression is the result. The bike stopped before the cylinder got damaged. Are there marks on the piston to confirm this ?


.

yep, rings partly seized and got the 4 side seizure on the piston. the bore is scratched up, these cant he honed can they because of the plating? On another note i had a spare motor and was putting in the piston and to my shock its a 243cc engine for an 88. its also been port and polished. i always wondered why the old bike went alot harder thought it was just me. i wonder what caused the overheating all of a sudden it had done a 3000 previous kilometers fine, maybe dirty fuel filter or bad floats?

For your future reference, your compression comes from the combustion chamber sealing up well. On the downstroke, it is pushing air/fuel mix up into the combustion chamber, on the upstroke, those ports get closed off and the mixture gets compressed. If crank seals are leaking, and it's pulling air and/or trans oil in, the compression is still going to occur in the combustion chamber. So little to no effect there. You need a leakdown test to confirm leaking crank seals. Make sense? Anyone have that awesome 2 stroke gif handy that shows the path of air/fuel mix through the engine?

Yes i understand thoroughly now thankyou, anyone know how the big bore kit does on the aircooled motor?

kdx87 wrote: its a 243cc engine for an 88. its also been port and polished. i always wondered why the old bike went alot harder thought it was just me.

My '83 has a 243 kit in it too ! There's an ADB test from '86 in this link.

http://forum.ozvmx.com/index.php?topic=35684.0

Cheers

Geoff

Thats really interesting jeff, anyone able to help me out with reading jets. i pulled jets out of 2 carbs and they read 300 & 410 on the main jets. however i see people talking about 150 and numbers much smaller. does mikuni have a different numbering system? im going to follow what people seem to be doing and im going larger on the main jet and smaller on the pilot.

Sure. The jets you are looking at are Mikuni. When you see folks talking about 150, 140, etc. those are keihin jets.

Alright thanks.