unexplained engine revving.

[phil99]

Series 2 with Italian GP 200 engine,all standard except varitronic ign.

Returning home from I.O.W.rally I live 6 miles from the ferry medium traffic 40 mph.

I pulled into drive and stopped put scoot on stand with engine still running because I wanted to adjust the tick over as it had seemed to have increased in speed on the ride.

I took the side panel off but before I could put it on the grass the engine started to rev on its own and with in seconds it was at screaming pitch,

I pressed the cut out button but it didn't make any difference after a few seconds I switched the fuel off it carried on until it ran out of fuel.

Following day I checked it over took the cylinder head off to see if there was any piston damage, there was nothing to indicate what had caused it to rev on its own.

Has any one else experienced this ?

Thanks Phil.

[MickYork]

Air leak ?

With hindsight it's always worth doing a leak test before taking things apart. If it was the cylinder head leaking you may have now fixed the problem ......and will never know for certain if that was the cause.

Give the engine a full leak test when put back together.

[Storkfoot]

The first thing I’d look for is an air leak around cylinder head, cylinder base, exhaust gasket or inlet manifold.

[bike grim]

Or something as simple as a fraying throttle cable getting snagged in the outer. Happened to me in Belgium this year

[Fast n Furious]

I think you have more than one problem there. The kill button should have cut the engine regardless!

[Adam_Winstone]

We've had a number of instances of this in our riding group. One was down to a well-known dealer's inlet manifold gasket degrading over time and a leak ensuing. The second example, which happened repeatedly was caused by fuel starvation; initially improved by a change of fuel tap but dodgy float and float needle may also have been an issue.

Both of these examples would not turn off by the key, suggesting that there was red hot carbon allowing the motor to 'diesel' of its own accord.

Air leak needs to be your first consideration.

Good luck resolving it.

[L111amo]

If it’s running that hot & weak it would never stop from the kill switch, these are scavenger engines

[Warkton Tornado No.1]

I've observed this Post without comment but have had it occur with tuned engines that have not had air leaks in which case the set-up has been too close to optimum & need richening up as the cure or due to the wrong plug grade. Stalling in gear is the quickest way to kill the engine.

In the instance of the OP's engine the opportunity to prove an air leak has been lost so a methodical rebuild seems the only solution. The reassembly should ensure head volume & squish are not contributing to the previous issue.

My advice is to do what is necessary to eliminate the need for a head gasket. I never use them & have not done so since the 70's. As the 'meat in a sandwich' they introduce an additional flat face prone to failure regardless of how good a sealant is incorporated. IMO the best solution is to spigot the barrel & recess the head which not only seals the joint but centralises the squish.