Here is my guide to buying a second hand bike. I'm no mechanic, this is an educated layperson's view. All corrections, comments, additions welcome.
Buying a second hand bike is always a bit of a risk, especially if it's an old one. Here are my thoughts on the subject. I've given some rough idea of costs, but it's only rough. Parts prices vary wildly and labour charges/times do too. Now... what you do when you find a problem is up to you. Some things are just points to argue a bit off the price, some are walk away jobs. I've tried to note the ones you should walk away from.
Take note of signs of hamfisted mechanics - a bike which has been in such hands is often too much trouble. First off, preparation. See if you can locate one of your target model before you go looking at ones for sale. This gives you a chance to have a baseline of feel and engine noise. Even better if the owner can tell you "No, that noise is peculiar to my bike, but they all make that other noise." If you are after a Euro bike, then hunt the local owner's club, or ask on the net for someone vaguely local. Otherwise try to look at least two of the model you want before buying one.
Read the various classified papers and Just Bikes and Motorcycle Trader to get a feel for the going price. Ring up a few to check for the mileage you get for that price. Now when faced with an actual machine you are thinking of buying...there are 4 things you are looking for: - evidence of good or bad ownership - evidence of crash damage - evidence of likely engine nasties - consumables that need replacing.
So, first thing is to take a general look round. Is it clean and in good nick? No obvious oil leaks, no worn wires, no duct tape on the seat. Are there any obvious crash marks? Check handlebar ends, lever ends, exhausts, footpegs. Are there any rounded bolts or stuffed screws? Check cam covers, switchgear, engine cases. Have the cables been oiled recently? If it looks neglected and unloved then probably best to walk away unless you are prepared to spend money. Sometimes a neglected bike is an ugly duckling waiting to turn into a swan, other times it's an ugly duckling not even fit for the pot.
Any owner with any sense will clean the bike. So look for oily places that might have been missed, and look again after the test ride for new oil patches. Old bikes do tend to be a bit oily, but more than a bit is a danger sign.
OK, now start at the front and work back. (Try all of these on a good bike of some kind first to give yourself a baseline.)
1) Wheel bearings: put it on the centrestand and grasping the front wheel, move it about. You are trying to see if you can woggle it on its axle. Move it side to side and twist it. If you feel looseness or graunching, there's a dead wheel bearing. Probably about $30 for the bearing and a couple of hours labour.
2) Wheel: spin the wheel and look for dings. A flat spot indicates the owner has hit a rock, or wheelied too hard. Gouges indicate a ham fisted tyre changer. Run a screwdriver over the spokes of spoked wheels. If you hear a different note, the wheel may not be completely true. Also look for broken spokes, especially in rear wheels of trail bikes. While you are there, check the brakepads. Most calipers have a little plastic cover you can pop to see the pad thickness.
3) Forks: run your hand over the fork tubes, feeling for pitting. Any pitting in the bit the slider travels over means fork seals will always die. Push down on the front end to compress the forks fully. When they come up again, look for oil marks - oil means a new fork seal needed. A little bit of oil mark is OK, more than a smidgeon, worry. Replacement will be about $30 per seal and a couple of hours labour.
4) Steering head bearings: have it on the centrestand again, and grabbing the forks push forward and pull back. You are looking for clunks and graunch feelings in the steering head. move the bars from full lock to full lock. They should move easily and there should be no graunch or notchy feeling. Replacement will be maybe $150-$200 incl labour.
5) General front end: run an eye over all the bits - fork sliders, headlight, tubes, axle nuts. Look for scrapes, gouges, bent bits. Look for grungy wiring.
6) Electrics: test all the electrics. Everything should work. All lights, all indicators, all switches. Dead electrics can be simple - bad connection, bad bulb. They can be horrid to track down though.
7) Switch gear and instruments: look at the switch gear, looking for buggered screws, bent bits, bad wires etc. Look at the instruments - all idiot lights should work, all dials should work. Look for crash damage. Look for swapped speedos - does the fade level of the tacho numbers/background look the same as the speedo? (That's a *dead* giveaway on my Duke single, and no one has ever asked about it..) Are the handlebars straight? Is the front wheel straight when you hold the bars straight? If no, then walk away. If he hasn't fixed the bent bits there may be a reason. It might only be a bent handlebar, but be sure that's all it is.
8) Battery: lift up the seat and check the battery. Sulphated terminals and low fluid level are a sign of poor maintenance or a bad charging system.
9) Chain/sprockets: look at the chain and sprockets. Can you pull the chain back from the sprocket teeth at the very rear of the sprocket more than half a tooth? If so, the chain is dead. Are the sprocket teeth worn? A new chain and sprockets can cost a couple of hundred bucks. Are there any signs a chain has jumped? Damaged chain guard, welded cases, scratched swingarm. If the cases have been damaged and welded, walk away. Gawd knows what is under the weld. Some bikes cope OK, some have horrid consequences if the chain jumps. Are the chain adjusters about the same either side? If they are markedly different and the bike tracks straight with hands off the bars then the bike is bent and the rear wheel skewed to hide it from that test, so walk away.
10) Tyres: check tyre tread. Remember how expensive the damn things are. Are the tyres matched? Correct pressure? (If the owner doesn't know the right pressure, then that's a giveaway in itself). If they are tubeless make sure they haven't been plugged for a puncture.
11) Rear wheel/swingarm bearings: put it on the stand, get someone to lean on the front so the rear is steady off the ground. Grab the rear wheel and push the swingarm from side to side. It should not move and be firm. Any give or woggling or graunching is a dead swingarm bearing. Can be expensive. Now twist the rear wheel same as the front looking for dead bearings.
12) Rear brakes: check the pads for thickness. If it has a drum rear, it will usually have a pad thickness indicator on the brake arm. If not, then the angle of the arm is a clue: the angle should be 90 deg or close. There should be plenty of adjustment left on the adjuster arm too.
13) Shocks: these are hard. About the only thing that will show up is if the rear shock is absolutely dead. Take it off the stand and bounce the seat up and down hard. If the thing bounces several times instead of going down and coming up once the damping is dead and you need new shocks. This will only catch a really really dead shock. A ride can catch bad shocks, but it can be hard to tell. Expect bad shocks on any bike with more than about 70,000km unless they've already been replaced.
14) Start it up from cold. If it's warm when you get there, walk away till it's cold again - too many things are hidden by a warm start. Does it start easily? How much choke? Does it blow smoke? Does that clear when the choke is off? Most bikes will blow smoke on the choke, but should settle down when off choke. Kwaks are notorious for revving their heads off on choke but most bikes will idle more or less.
15) Motor noises: its really hard to know what a motor *should* sound like without having heard good ones. For example Guzzis are all tappety, but a dead camchain is distinctive.
16) Stationary tests: does it rev easily? is the throttle stiff? Does it have trouble returning? Does it change into first OK? How heavy is the clutch? Does it feel notchy? (If it does, likely to be a new cable needed. Same if the throttle feels gritty.) Anything obviously oily now the engine is running? (Feel under the crankcase.) Turn the headlight on. Rev the motor. Does the light brighten? If not, walk away as the charging system is stuffed. (You can test this with a multimeter if you need to. You should see the DC volt needle jump when across the battery. Now, the ride... If it seems bent or the engine or gearbox has ickinesses you can't live with, walk away. It's all expensive to fix.
Does it track straight at 10-20 kmh with your hands off the bars or does it veer to one side? (Note - road must be flat, no camber)If it veers, it is bent. Make sure you are well balanced for this test, you can make it veer with body movement. Also try some tightish corners, one each way. Is it easier to go round one way than the other? How are the brakes? Some bikes (GT550 Kwak and similar) have absolutely awful feeling brakes but they pull up OK. If it doesn't stop OK, it may be as little as new pads, ($30 a caliper), it may need new lines ($100+), or it may need a master cylinder kit ($100+).
Will it rev freely, and pull OK? Does it jump out of any gear when revved *hard*? Are all gears easy to get to? Any false neutrals? Talking of neutral, can you find it at a standstill? Does the neutral light lie? (All Italian bikes will fail both these tests...) If the gearbox is temperamental and misses gears and such, can you live with that? It will be bloody expensive to fix. Does reserve work? If not, may just need a cleanout. Does it shake, get rough, or otherwise object at any revs bar very low ones? (most 4cyl bikes have a "rough" rev range where the thing feels like it is grinding something. my 550 Kwak did it at 4000 but not at 3900 or 4100).
Do a lot of stop/start and clutch slipping. Does the clutch cope? New clutches can be expensive depending on bike model. My old Italian bikes seem to "grow" the clutch in stop/start traffic and I have to continually adjust the cable. I live with it. If you can take it somewhere where you can go at speed, how is it once you get to 100kmh? If you can find a corner, does it weave? If it wobbles at 100 or going at medium pace into a corner, the shocks are gone or possibly steering head (make sure tyres are OK). Get someone to look at the exhaust when you accelerate. If it blows smoke from one or both pipes, it may have ring/piston problems. (2 strokes excepted of course, they don't call them "blue smokes" for nothing.)
Do not forget to have a good look at all the paperwork. Make sure the numbers match the bike, make sure the compliance plate is there and correct. If the seller is not the owner why not? If the address is not the same as on the paperwork, why not? Most states have a Registration of Encumbrance, ring your Rego Dept. They can tell you if it is stolen or has a debt on it. Get a receipt that shows the rego, frame, and engine numbers as well as your name and address and the seller's name and address. Be sure to have all the transfers signed properly.
Remember - there are lots of nice bikes out there. Don't let your new bike fever call the shots - your bike will be waiting for you, don't buy a lemon because you can't wait for it.