My 1988 NSR250MC18 R2J would appear to be doing the same thing(front cylinder). The bottom end has never been apart, so could this be caused by the seals degrading through age.
The engine still sounds as it should and there is no erratic idling or poor starting. The machine has been stood/ garaged for approx. 8 weeks and was fine the last time I rode it.
I was also wondering where the best place would be to buy all the neccessary seals and gaskets.