Hi , my 4rt has developed a misfire, if you quickly open the throttle it will splutter and die. If you gradually open the throttle it will rev to the maximum with out fault .It is not the plug as I have tried more than 1 new ones.Any help would be greatly received.

It sounds like your bike has too lean of a mixture when you open the throttle quickly. Four Stroke motorcycles are more susceptible to this problem than 2 stroke bikes - and the carbs for 4 stroke motorcycles have used carbs with vacuum controlled slides or more recently accelerator pumps to solve this problem.

I don't know what carb you have and the method it may use to control the mixture during quick throttle opening - but I would start by giving it a good cleaning and see if this solves the problem.

EDIT:(Sorry for the "carb" statement....I am not a Montesa owner and did not realize it was fuel injected. It still is a problem that is related to the mixture being lean upon sudden throttle opening for some reason. Hopefully the other peoples suggestion about the exhaust will solve the problem.)

My Sherco 4T just developed a problem off idle and I took the carb apart - and I found some black sludge and coffee grounds looking bits in the float bowl. I have absolutely no idea where it would have come from as I use fresh 96 Octane Shell racing fuel, my tank has a strainer, and the rubber fuel line was in good shape. I put it back together with new Tygon fuel line and an inline filter, and it runs great again after cleaning.

Edited June 27, 201115 yr by 1oldbanjo

....I don't know what carb you have...

???????

TURBO,

I'm no 4rt owner but I have seen a few faults with them.

If you're lucky, it's a leaking/loose exhaust manifold...if it's not then you're looking at a pump? or such

an exhaust leak (at the cylinder) can cause a mis-fire.

an exhaust leak (at the cylinder) can cause a mis-fire.

Mine did this and I tightened it up and all fine now

How old is the sparkplug?