I would try something else first before an attempted bump start which will cause more harm if you have a stuck valve and could trash your engine.
Remove the left side engine cover with the plugs out and try to turn the engine with a ratchet spanner on the crankshaft nut. The engine cranks anti clockwise as i recall, so try turning it the other way and see if it will free up whatever is dragging.If it will move at all in either direction and you feel a solid "clunk" when trying to turn it further I'd say you had a stuck valve. (I had this on a GSX engine - found two bent valves and one broken) if I had tries to start it with the starter or a bump start I may well have damaged the engine even more, so I know a little about this........

I suspect however that it's not in the transmission, because if you pull the clutch in the engine should be able to turn over without the gearbox being an inhibitor.
Just my 2c worth.

 

+1 use a ratchet to crank it on counter rotation (clockwise) see if it releases

I'm thinking the starter clutch is the culprit here some how like the starter is
staying engaged you could pull the airbox out and apply some gentle taps of
the old persuader to the starter see if it helps.

 

Thanks guys

I'm not going to bump it.

 

I believe I had the same thing happen to me.
I had done a valve adj, carb sync and when I first started it up it ran fine.
I shut it down and geard up for a ride but when I hit the starter it cranked about 1/2 revolution and locked up!!!
Rear wheel spun in neutral but the engine still wouldn't turn over.

I turned the engine backwards ( as was suggested) it turned fine. But when I hit the starter the engine spun 1/2 rev and jammed again.
I tried turning it backward several more times and eventually whatever it was freed up and the engine cranked over and started right up!!!!
No noises and everything was fine until I serviced it again later.
SAME THING HAPPENED!!!
This time I put it in gear and pushed the bike backward and bumped the engine in reverse.
It freed up and started fine.

It hasent happened for over 5000 miles and several services.

No solution but you're not alone !!

Btw first time was at 7000 miles, so it's not a milage thing.

 

all the more reason to suspect the starter clutch mech ?

 

Hmmm very interesting.

That is absolutely EXACTLY what happened to me. Thanks for the tip. That's a total of 3 individuals that have now said to try the reverse bump method. I will tentatively give it a go once I get time.

Anyone know if I am to gently try this, would it be easier without the plugs in to remove the compression equation, or does anyone suggest keeping the compression there to aid release (pulling apart the obstruction)?


Thanks very much
Pedro

 

Before trying a reverse bump, i would use a ratchet with the plugs out to ensure things free up. Cause as suggested earlier in the thread getting her to run with a stuck valve could cause a massive engine failure. JMO

 

+1
And if it is your starter clutch I have a spare..............

 

Well, have you freed her up yet???

 

Not yet. Sorting the garage first, but I will keep you posted.