Top end issues

oehlin

oehlin

Junior member
Location
Sweden
First start today after changing the regulator and the charging lamp went out.
Hurray.
After 1 minute of warming up, alarming sound were suddenly heard from the top and the engine was stopped.
Upon inspection, it turned out that on the right spark plug, the side electrode was compressed and minor damage to the piston top could be seen.
Manual rotation of the crankshaft seemed ok until the piston reached the top where it was stopped. The same result in the other direction.
What has happened 😭
 
When that happened to me it was a section of 2 pistons from the ring lands near the inlet valve that had broken off and caused havoc banging into the plug tips. All I noticed was a loss of spark that time. The next time the same thing happened there was no loss of engine performance, just massive smoke with full throttle coming out of a bend, part throttle was ok and I managed to ride home like that. All put down to detination from my highly tuned engine. Yours sounds different and terminal, sorry to say. My guess and it's hard to believe a sudden loss of crank control, but enough to hit a plug, really?
 
Agree with Lothar - you have to determine what was done to the engine. eg, if all you did was remove and replace spark plugs, then the plugs are the problem.

If you had the head and barrels off then maybe there's a gasket issue. Work backwards.
 
What's the history of the engine? New to you? Recent engine work?
I would suggest starting with removal of the valve cover and see if all the valves are moving freely. Sounds a little like many years back when I had a valve seize due to being too tight in the guide. But that was on a freshly built engine, it wouldn't normally happen if the engine has been in regular service for some time.
 
It sounds like you should take off the head. You can see that the spark plug has hit the piston, and it happened suddenly while the engine was running. First you could do as Maurice suggests and see how much slack there is between your piston and the crankshaft, by rocking either side of TDC. It is strange because usually a piston will be stopped by the squish band or overlapping valves before the piston can hit a spark plug, and I am sure I have heard that people have used protruding core plugs in SF motors without problems. If you have a protruding core plug, change it for a non protruding as recommended all the time - B8ES, N3, etc. and run it again.
 
chrisk said:
Too long, if they're gunna hit, why did it happen after a minute of running?

The alarming sound was heard and then the engine was stopped. Not: Then the engine stopped.

Batto taught me to read the first post properly ;)
Click to expand...
Hi Chris, thought about that, too (and yes, I read it... ;-) ), but a running engine produces heat and heat causes parts to expand... sometimes it's just that bit to much.

Still - we need more information. The worst Scenario I could tink of is a streched rod or a valve that lost a part which then caused the dent in the piston top, not really the spark plug. Time to take the head of I would say... or at leasrt inspect it with an endoscopic camera...
 
Previous owners have had the engine disassembled and replaced seals and ground the valves.
Bought the bike with the engine mounted but not started.
In connection with the preparation of the bike, the engine has only run hot on a few occasions without any problems.
The spark plugs are B8ES.
Manual turning of the crankshaft seems smouth until the piston reaches TDC where it stops transversely.
Visually, the valves look good from the outside.
I will have to lift the top and see what happened.
Is it enough to tilt the engine down to do this? or do I have to remove it from the bike
 
Before I would take the engine out I would follow the proposal of Tippie and Maurice. I assume it is a crankshaft issue, big end or main bearing, but at first I would control whether the valves are working well.
 
I think its a twin we are talking about here, and if that is a "yes" then tilting the motor forward does mean that you can remove the head, but its such a lot of faffing around, because at that stage their are only two bolts left to remoive the unit, so do that and then on the bench you will have all round vision and accessability, but is this required? it does seem so, you had the correct plugs and the piston hit one, sounds like a big end.
CLEM
 
I doubt it's a big end issue. If it's lost or re-distributed its rollers because of a failed cage or something, then that shouldn't make the piston hit the head when the crank is turned by hand. It's more likely that the piston would fall short of its full stroke.

It sounds to me like something has gone awry in the combustion chamber. Could be any number of things like a stray nut that was lurking in the inlet tract got sucked in (if it hasn't been run since someone else built the engine, that's a possibility). Or a piece of broken ring land like Vince suggested. Or maybe a valve seat has dropped out, not allowing the valve to close. A loose valve seat jangling around behind the head of the valve could bash the plug electrode even though it's still captured by the valve.

Chances are your piston will be fucked if you can see damage down the spark plug hole.

An endoscope camera down the plug hole might be informative.

An interesting problem. Please keep us informed.
 
you are forgetting the centrifugal sling of a big end with collapsed or missing rollers Cam,when the crank pin get to TDC with extra clearance, and starts to decend the rod and piston will carry on by inertia and then get yanked down as the pin contacts the eye again.
CLEM
 
Has now lowered the engine and removed the valve cover and loosened the top bolts and then the engine goes around.
Tightened the top again resulted again in a stop at TDC.
May now disassemble the top to see what the inside looks like.
Does it matter what position I loosen on the cam chain?
Has now also disassembled the cam chain tensioner.
 

Attachments

  • thumbnail_IMG_2112.jpg
    thumbnail_IMG_2112.jpg
    144.3 KB · Views: 43
  • thumbnail_IMG_2113.jpg
    thumbnail_IMG_2113.jpg
    109.9 KB · Views: 43
It's more important how you actually mount it back together... ;-)

As the engine has a "sandwich" construction, where the top valve cover and it's nuts presses the whole engine together, it is anyway necessary to exchange at least the head gasket, potentially even the one between engine and cylinders. Pull the sucker appart - you have no other chance anyway to find the root cause...
 
You must log in or register to reply here.
Back
Top