MK1 Piston seizure

[drillam]

Heading up to Eastleigh on Sat morning to go on a trail ride with the local TRF. I took the motorway as I was running a wee bit late and it was only about 10 miles. Doing a steady 70mph and the engine cut, no warning, just died like the kill switch had been hit. Briefly twisted the throttle a wee bit but no effect so then disengaged the clutch and coasted to the side. Doing about maybe 20mph on the emergency lane and dump the clutch again just to see if it'll catch and the back wheel locks this time. Come to a stop, get off, remove me helmet and Camelback and try kicking it. the engine is free now but I can turn it with my hand, no resistance.
Missus comes to collect me in the Tranny and get it back home and remove the head expecting to see a big hole in the piston, but instead this is what I see.

http://i14.photobucket.com/albums/a340/ ... C02906.jpg

http://i14.photobucket.com/albums/a340/ ... C02909.jpg

I'm not great with s strokes but thought the plug looked OK ish

http://i14.photobucket.com/albums/a340/ ... C02914.jpg

I rebuilt the bottom end a few months back and it's been out on the trails half a dozen times since with no dramas. I usually van it to the trails and then back home again as it's often around an hours ride to the start of Salisbury Plain or up Basingstoke way which is where I normally ride. I've had it up to 70 a coupla times on brief bursts and it's felt fine.
Since I rebuilt it, it's felt very hesitant about midrange when giving it a handful. Felt like a four firing on three, so had to drop a gear or two then rev it through that patch and then it's extremely willing.
I had a feeling this was an over rich mixture and as when I got it a coupla months ago the needle was set on the second from top groove (one weaker than the centre of the five grooves) I decided to put it there again after dropping the carb in the ultrasonic tank to give it a good clean.
Set of with the new needle setting and it felt much better, barely a hint of the previous hestitancy.
It felt willing on the motorway just prior to the seizure, not stressed and plenty left in hand, but I was happy at 70 and didn't want to overdo it.

1 So what do you think happened?
2 Why did it cut out?
3 Why did it not seize immediately when it cut out?
4 Why did it seize shortly afterwards?
5 Why no compression?
6 It's on +1.50, do I have enough for another rebore?
7 How much is a new piston and rebore?
8 Anybody want a MK1 with a small fortune in receipts once I've rebuilt it? (again)

cheers
Millard

[drillam]

piston and barrel removed now, more pics

Exhaust/nearside
http://i14.photobucket.com/albums/a340/ ... C02916.jpg


Inlet/offside
http://i14.photobucket.com/albums/a340/ ... C02917.jpg


http://i14.photobucket.com/albums/a340/ ... C02918.jpg

millard

[fallenmikethebike]

That's bad luck, IMO, you have had two events happen, for reasons as yet unclear, the bike has got very hot,and lost compression, the sudden loss of cooling has in turn lead to the heat seizure, that once the engine cooled has released.
Why?
The needle position, shouldn't be quite that critical, by itself, but if other factors are contributing, say, an unknown air leak, that indeed could lad to the failure shown.
Does it still have sparks?
What main jet size is in the carb?
The temp, light didn't come on?
What plug is in there?
Mike

[AndyM]

Hi sorry to hear about your bad luck,
The reason why the engine cut out may of been down to the debris from the piston & bore effecting the way she fires or fouling the plug, and because of the ware on the piston & bore gave the conrod the free play to seize up.
No compression might be down to the air no escaping through the worn piston + bore.
Thats about all I can think of.
Thanks
Andy

[knackeredMk1]

Assuming that the coolant is still in the radiator -

Could be lack of 2 stroke oil. - Check oil pump pointers line up.

Thermostat has broken - unlikely.

Most likely running lean due to jetting or air leak. Nipped up at first, then caused more dammage as going though layby.

Needs rebore, new piston etc.... Double check all possible air leaks when rebuilding. What jets have you got

[drillam]

That's bad luck, IMO, you have had two events happen, for reasons as yet unclear, the bike has got very hot,and lost compression, the sudden loss of cooling has in turn lead to the heat seizure, that once the engine cooled has released.
Why?
The needle position, shouldn't be quite that critical, by itself, but if other factors are contributing, say, an unknown air leak, that indeed could lad to the failure shown.
Does it still have sparks?
What main jet size is in the carb?
The temp, light didn't come on?
What plug is in there?
Mike

Sorry for the delayed reponses, been away for a few days

yes it still has sparks.
jetting, plug etc all as standard with the excpetion of the needle lowered one notch, as stated.
I didn't notice the temp light come on, but was doing 70 on the motorway so wasn't looking down.

[drillam]

Assuming that the coolant is still in the radiator -

Could be lack of 2 stroke oil. - Check oil pump pointers line up.

Thermostat has broken - unlikely.

Most likely running lean due to jetting or air leak. Nipped up at first, then caused more dammage as going though layby.

Needs rebore, new piston etc.... Double check all possible air leaks when rebuilding. What jets have you got

I set the pump up a few months ago when I rebuilt the bottom end but will be checking it out again when the barrel goes back on this week. It's at LT for a rebore, should be back soon.

It didn't nip up first though, it just cut out, it then seized when I dumped the clutch a wee bit later on the emergency lane, that's the bit I don't understand.

[knackeredMk1]

A seizure can just nip the piston rings so that you get very little compression and the engine won't run but will turn over quite easily. I've had this happen twice.

Forcing the engine to turn over will then cause further damage. eg. an engine turning over at 6,000rpm - the piston goes up and down 100 times per second. So even a fraction of a second can cause big damage.

[drillam]

A seizure can just nip the piston rings so that you get very little compression and the engine won't run but will turn over quite easily. I've had this happen twice.

Knackered, that sounds like exactly what happened in the first few seconds, so that probably explains a lot, at least to me who is ignorant of 2 stroke lore.

Forcing the engine to turn over will then cause further damage. eg. an engine turning over at 6,000rpm - the piston goes up and down 100 times per second. So even a fraction of a second can cause big damage.

But what about the arse end locking up when I dumped the clutch say ten or twenty seconds later?
If it didn't happen imediately then why did it happen just a few hundered yards up the road?

[knackeredMk1]

Initial seizure happened when everything was hot and you didn't run it long enough after it stopped making power. When you pulled in the clutch and coasted the barrel cooled down a little making it grip the (still hot) piston tighter. Also once the aluminium of the piston starts rubbing against the iron of the bore without lubication it generates very high temperatures and smears itself all over the bore in rapidly expanding patches. You can see the aluminium dust on the piston crown in your photos.

If you had not pulled in the clutch it would still have happened - just at a higher speed.


Any other options


You need to find the leak or change your jetting when you rebuild. The leak might not still be there