Change the spark plug first - these break down at higher revs but can operate normally at lowr revs and at start up when they are cold. Check to see if there are any sparks visble between the spark plug cap and the engine. If so replace cap.
But it is more likely carb float height (if it has a carb - is it injected)? If it has a carb the float height needs checked against the manufacturers setting - you need a manual or a local dealer to do it for you.
If it is injected check all gasket faces between the injector body and the engine for leakss - squirt some washing-up liquid round the areas and see if it bubbles or gets sucked in anywhere, that all petrol lines are unblocked and if it has a petrol filter that this is clean and not blocked. If the lines/filter are dirty you should also drain petrol tank to see if there is any contaminants in it, and if you have done all that and it still does it the ignition needs checked too - a manual is needed or a dealer check.
Russell Taylor, December 2007
