Faults:

Steering head bearings.

Top end needed rebuilt, but normal for a two-stroke.

One of the plugs in the crank web worked loose and needed to be welded in place. Had the crank rebuilt at the same time, even though it was still sound.

Rear wheel cracked, due to rider hitting a very big rock.

Parts aren't too hard to find, Yamaha interchangeability is great.

Overall it is very reliable for its age.

General Comments:

Power on the 1988 YZ250 is very strong in the low- end and mid-range. Top-end is OK. With Carbontech reeds, drilled out Boost ports and a flywheel weight, the low-end and mid-range is excellent for a bike this old. It is very tractable, and can be lugged like a four-stroke.

Front brake is good, back is kind of weak.

Clutch holds up to abuse well and doesn't drag. Transmission isn't easy to shift, but it doesn't miss shifts either.

Starts easy, runs clean and doesn't foul plugs.

Suspension is alright, nothing great, but it is 20 years old. Handling through tight trails is light and nimble.