ETZ251 carburation - engine 'running-on'

[rossma49]

Any suggestions to cure a nasty habit of my 251 to 'run on' at high revs when I come to a halt, say at a junction or traffic lights?
When I've been travelling at a good speed and then stop, the bike the engine continues to rev at about 2500-3000rpm instead of dropping back to the normal tickover, about 1200 rpm.

The slide does not stick and the cable is free, in every other way carburation seems excellent. The bike is fitted with a Bing carb.

Any ideas please?
Thanks
Richard

[djsbriscoe]

Have you had a look at the spark plug? Is it oiled up or maybe the heat range is wrong. The 2 strokes often benefit from a high speed motorway run occasionally to burn out any carbon deposits in the exhaust downtube and cylinder head. I'd also check the timing is set at 2.5mm BTDC with an MZ timing guage (do a search of the forum-I posted a picture a while ago). Different oil and oil mix ratio could be tried as well (should be 50/1 petrol/oil).

If you read the links I posted earlier in reply to the ETZ125 owner that may be of help.

David.

[Old Dog]

Sounds like a cable/ routing issue to me. Is the cable new or used?

Steve

[Old Dog]

Personally I cannot see how petroil mixture would cause this but hey, if you've nothing better to do and an oiled plug is more likely to kill the motor.. I would also check for airleaks in the inlet rubber and around the carb. Have you given the carb a good clean, with carb cleaner?

[rossma49]

Thanks for the ideas and suggestions.

The plug colour is about as perfect as I could wish for.
The engine is injector for oil - not pre-mix - and I'm using very high quality oil.
The cables are new and the cable run smooth and without any sharp angles.
Carb has been cleaned and seems well set up.
I've fitted new air hoses and have a good seal to the cylinder intake.

So I don't think it's any of these things that is causing the problem.
If I stop and just let the clutch take up the drive, putting a slight load on the engine, the revs drop down to their normal tick-over at once.

Maybe there's nothing wrong - just a characteristic of the motor. But it's annoying and can't be completely right.

[Old Dog]

I have to say that all mine can be a bit slow off the throttle - difficult to tell without hearing it. If you open the throttle wide open (engine off) an then let it go does the throttle snap back and do you get the clack as the slide returns home?

[Old Dog]

It occurred to me that you might want to check the gearbox oil. See if it smells of petrol. If so your mainseals are blown not a huge job.

[davegsm82]

I had a similar problem with my 301 with bing carb, never pinpointed it to one particular area, however I did find a cracked spacer between intake manifold and engine, as well as fairly poor joints between the carb and manifold itself.

Fixing those points as well as cleaning up the rubber choke bung seemed to cure the problem.

Dave.