This is TOTALLY ShadowShack's! But I'm reposting and stickying it here for the benefit of our members.
Thanks again ShadowShack!
(a rehashing of the Customarily Minded editorial from the Beginner Bikes days)
Here are some pointers on what to look for on a used bike purchase. Take a clipboard with you when viewing the bike, and look for the following items:
Check the oil prior to starting, is it clean? Or at least filled to the correct level? Check the coolant (if applicable) is it filled properly? Check the air filter as well if possible. After all the checks, start the motor...does it start and run smooth? Any visible smoke coming out of the pipe(s)? Does the throttle rev smoothly? Any odd noises coming from the engine, like valve ticking or coughs, spits, sputters, or rattles? Does the clutch engage easily and does the shifter click naturally as you go into gear? (many bikes have a side stand safety feature that prevents the motor from going into gear while parked, raise the side stand before doing this...and then put it down while it's in gear/clutch in to see if the feature works). Check for oil leaks around the motor, any trails of dust/dirt build-up at any of the engine seams?
Are they new? Decent? Worn? Look for any hairline cracks in the sidewalls, this is an indication of replacement time regardless of tread depth.
Check the pads and look for the wear indicators, most pads have a groove carved into them and if this is gone the pads are thin, needing replacement. Also check the rotors, are they clean and smooth or filled with grooves? Drum brakes often have an indicator mark on them, when the pivot arm is aligned with the mark it's time for new pads.
Does the front end bind at any point during full motion, from left lock to right lock? Do any of the lines and cables pinch at any point? If possible, raise the front wheel off the ground (center stand/hold the rear down or a car jack) and grab the lower forks, yank forward and aft to check the steering head bearings. If there is any play they'll need some work.
Are the seals leaking? Telltale sign here is a grime ring above the lower/slider, the seal pushes the dirt and leaking oil up and it collects at the end of the travel distance. If the bike is already clean apply the front brake and compress the forks down a few times and run your finger along the fork tube, it should be dry all the way around. Any slippery feeling or noticeable oil indicates new seals are needed. Also are the forks straight, no dings or dents or any other damage?
Is it on solid? Jack the rear up (center stand or car jack) and grab the swing arm by both sides and try to move it from side to side, if it moves new bearings are in order.
Test all the lights, bells, and whistles...high/low beam, directionals front and rear, tail light and brake light, horn, indicator lights (such as neutral, turn signal indicator, high beam on, etc).
Is it clean inside, no rust or other deposits? Does it smell like fresh fuel or is it dingy enough to curl your toenails? Does the gas cap pop off and on easily?
Is the chain tight or loose? most bikes should have 1/2" to an inch of slack at the center point of the chain. Is it lubed or dry? Are the sprockets good? The teeth should be tapered to a squared point and even, if they are pointy or broken they need replacement ASAP. If the bike is a shaft, check the oil level in the transfer case. If it's a belt drive feel the snugness of the belt and examine the lugs in the pulleys.
Does the bike fit you when you're sitting on it? Can you stand the bike upright and plant both feet firmly on the ground, heels and all? Can you do the same with the bike on its side stand? Can you reach the pegs comfortably and work the controls? Do your elbows dig into your ribcage during full left to right turning motion? Can you pull yourself off the seat via the handlebars without having your feet slip off? All this can be modified later via the aftermarket, but it's always nice to be able to ride the bike right away...
Don't expect this, many owners won't allow it. You may need to strike up a mutual bargain here, let the owner hold the cash and you hold the title while you ride it, if you dump it it's yours and if you like it you don't have to come back! Or you can ask the owner to take you for a ride as a passenger, at least you can feel how smoothly it works. You'll want to see how smoothly it accelerates, does it bog at certain revs? Do the gears change smoothly? Do the brakes work smoothly? Do the wheels/tires spin smoothly and straight or is one end shaking a bit?
Check on this before buying any bike (new or used), can you afford the rates? If you're 18 years old and looking to get a ZX-7 Ninja or a CBR 600 don't expect a fair rate, $3000+ per year (and that's for used bikes, new ones are even more) is a common rate for sport bikes in this age bracket, assuming the company will insure them at all. You can get better rates on bikes that are 7 model years old or older, the next break comes at 25 year old riders and again at 29. And tell the insurance company what the bike is, don't pass off something like "EX500" instead of Ninja or "VFR750" instead of Interceptor. The low rates they quote you may not endure once they run the VIN and sock you for the actual rate. If anything, ask for the actual rate before commiting and then insure it at "the lower rate"...at least you'll know what to expect if it happens and will know whether or not you can afford it when it does. You don't want to bring a bike home and then find out you can't afford the insurance later.
Do that and someone else could be visiting you with this list before you ever get to enjoy the bike.