Runs for only a short time

Hi All

I've just bought an 89 ST50 which starts first kick after cleaning plug. However engine will then only run for a couple of minutes, getting progressively reluctant to respond to throttle, until it conks out completely. Then only way to restart is to remove plug (which is now wet with fuel) clean it with wire brush and start again. Any advice please.
Thanks

Could be a number of things:
- try running the bike without the fuel cap on your tank
- try a new spark plug
- check and clean air filter
- check for air leaks (carb) and condition

- if it still doesn't work, check the cdi / ignition coil

Many thanks I'll try all of these then revert.

Best regards

Thanks Waynus

It is the Fuel filler cap. Take it off and the bike runs (it's not happy and is still reluctant to rev, but it runs). Just needs a tune I guess.

Many thanks.

Best regards

Graham

Glad I could be of service!

I must admit that my first thought was that the choke was on all the time, that would have the same symptoms wouldn't it?

I take it the cap is the one with the open/close vent, you haven't got it in the closed position have you?

No it's open, but bunged up after years of not being cleaned. I've popped it into an ultrasonic aquious washing tank and it has come up fine.

Thanks anyway.

Regards

Graham