Rough running at constant throttle

[Marly]

Went out for the first ride of the year over the weekend.

Bike seemed to run fine for most of the journey until a few miles from home it started to sound restricted at constant throttle.

It happens at any revs, but not during acceleration.

It sounds like it's switching to running on 3.

Only happens when revs/throttle held in position.

Any thoughts on where to start looking?

It's a 1992 NC30.

Thanks.

[missis sumner]

Icky fuel? (This is my stock answer to any rough running, of any bike, that's been left standing for more than a couple of weeks )

Try putting fresh fuel in and taking it for a blast.

[magg]

Might also be worth draining the tank and checking for any contamination of the fuel tap filters.

[Marly]

The fuel wasn't great so the tank was almost completely emptied and refilled with fresh about 60 miles before it started running odd.

Maybe some gunk has blocked up a filter.

Thanks for your help.