Check Frame & Sub-Frame
Check the sub-frame by sighting down the rear fender. A bent sub-frame will most likely make the rear fender look out of alignment. Check the underneath side of the bike. In particular, examine the frame tubes on the bottom where the footpegs bolt on. Look for crushed frame tubes from slamming down on rocks. A heavy rider can do some frame damage riding it hard in big rocks. While you are at it, check the frame steering headset bearings for excessive play. Lots of folks neglect steering head bearings so they may need to be replaced or greased.
Check Engine Oil
After the engine has ran for a few minutes lean the bike way over and put your finger as far in the crankcase as you can. Examine the oil under intense sunlight or bright light. If there are any metal flakes present in the old oil, then you might need to think about it. When you roll the oil around in the container, look for a fine metallic sheen of gold or silver, like on a candy-apple paint job. This means the engine is shedding metal internally. Could be expensive. Of course, check the oil screen, or filter if you can, and if there are any chunks of metal large enough to be trapped in it then don't touch that bike. The engine and frame are big ticket items. If they check out OK then the rest is affordable to fix.
When you kick the bike over, cold, there should be some fair resistance. Not enough to stop the kick, but you should feel some compression building up in the cylinder (depending on the size of the engine). Some bikes come with an auto decompression mechanism so, unless the auto-decompression cable has been removed, you won't feel massive amounts compression. If there is no resistance at all to the kick-starter then be suspicious. Remove the spark plug, see if it looks new or old and worn. What color is the insulator? Black? White? Tan? Was it hard to remove the plug? Do the spark plug threads look good in the cylinder head?
Suspension & Wheel Bearings
Another test you could do is grab the frame tube under the back of the seat, and with the bike off the stand, jerk the back tire straight up off the ground. If you can feel or hear any clunks, then something might be worn in the shock, suspension linkage, or shock mounts. Maybe not expensive, but, good to know what you are getting into. Put the bike on a stand and spin the wheels, front and back. Check to see that they are warped or bent. Do they spin freely and straight?
Modificatins & Other Items
Ask about any modifications done to the bike. If there are mods, do they look sloppy or improper?
Other items to look at or ask about:
- Is the muffler or exhaust system stock?
- Has the airbox snorkel been removed?
- Has the carburetor had any jetting changes?
- Is the air filter element stock or aftermarket? Is it clean?
- When was the spark plug last replaced?
- Are the tires needing replacement? Are they original?
- Are the brake shoes needing replacement? Are they original?
- Does the front wheel point straight ahead when you sit on the bike with the handlebars straight?
- Are the handlebars bent?
- Are the plastic on the bike all scratched to ratshit, or still shiny?
- Is the seat cover ripped, torn or patched up with tape?