Cb400n valve clearances, cold, 4 inlet and 5 exhaust, that's thou.
From memory the 250 was 5 and 6 thou cold, but will check later.
Pictures of cam area, perhaps we can spot something.
Any evidence of sealer at the front of the engine in the middle, or oil leaks there ?
Balance chain.
You should see a relatively large slotted plug close to the rev counter cable ?
Using a good fit washer, undo this plug. You will see a 12mm af lock nut in a slotted arm. Any slot left at the top ?
Engine cold and stationery, slacken the locking nut. DO NOT drop anything inside the engine !
The arm is spring loaded and should automatically rotate anticlockwise. It may be suffering from "sticktion" and need some excercise.
When it auto adjusts, nip up the lock nut.
Any slot left at the top ?
Looking thru the hole, at about 2 o clock, you will see a 14mm self locking nut. The threaded end protruding thru the nut has a "dot" which is a bit difficult to see, but not impossible.
Using a clock face, what position is it in ?
Engine stationery, using a long thin probe, with practise you can "feel" slop or slackness in the balance chain.
Looking thru the hole, looking thru the slit between casing and main bearing ladder casting, you will just about see the balance chain, but this takes practise.
The spring loaded arm is repositionable on its shaft, but clutch cover etc. off for that pita of a chore.
So, cam box area for pictures and what position is the dot ?
Any "lean back" on the cam sprocket timing marks ?
Noisey cam chain, balance chain and clattery vales is not good ?
Do not put these chores off, or oil and filter changes.
Engine done about 35k miles ?
How long you had this bike ?
The 400's are better than the 250's, more get up and go.
The 250's are great on back roads here in Snowdonia, Wales, UK.
You know of CMSNL for the very informative exploded views ?
Get a manual with wiring diagram.
Take your time, don't break anything, those cam cover bolts, and enjoy...
