you need to look at the end of the rocker arms. you'll need to pull the whole cam cover off with the rockers attached.
then look at the end that holds the roller bearings that roll around the camshaft.
if you have four 'chip marks' to hold the pin in = you have the old rockers.
if you have what looks like a 4mm pin inside a bush = the newer rockers.
easy to convert but will cost you a few quid.
BEST CHECK. symptoms are i'm afraid those of early rockers!!!! soorrrryyy!
bearings will be good although you're not far from a full strip and rebuild for that year. don't panic, i haven't done it yet either but we're close....
tuning? '53' MX cam, 501 downpipes and MX silencer along with the FCR carb. my MJ is down to 158 which means that the air stream speed through this carb is almost too fast and the next carb would be good.
i am porting the exhaust ports out to 29mm and that'll be tried next. as mentioned in a recent post. the inlet tract is huge and was designed for a 550/650 and NOT the 400.
regards
Taffy