I have a 2002 315R that shows an intermittent problem:

Every so often the engine note becomes a lot more 'poppy' i.e. more pronounced pop, pop, pop... than usual (it does sound like a lawn mower) and the power drops to about half of normal. Usually just cutting it out and then restarting it makes it go away fro a while - but that's not much use when you're in the middle of a climb on a section as happened yesterday.

I've had the carb apart (a Keihin PWK 28 copy (Koso) put on by the previous owner) and have cleaned the jets out but it made no difference.

Has anyone experienced this or have a clue as to where I might look for a solution?

Many thanks

What you describe happens when you are not running a "resistor" type spark plug. What plug are you running? It must have an "R" in it's designation.

NGK BR6ES is what you want.

I will check the plug, but why would the problem be intermittent if that was the case?

As Martin says, had the exact same problem .

Martin is spot on ! the coil heats up from the lack of resistance , you may need a new coil soon . And make sure your grounds are clean and tight .

Glenn