IF YOU LOOK through the used motorcycle ads in the paper you'll notice that anyone who has kept his in a garage will make note of it. Well he might. Nothing is more harmful to a bike than to be left outdoors, particularly in the winter.

A bike parked on the street or sidewalk every night is an open invitation to thieves. It takes no time flat to hoist a motorcycle into a van and even the heaviest case-hardened steel chain can be cut.

Beyond that, rain and damp are murderous on a bike. The motor is exposed and if a machine sets for long stretches, as it inevitably will in winter, condensation is going to attack carburetors and cylinders and rust will eat away at the chrome.

Plenty of bikers have left their machines out all winter and found the engine locked solid in the spring.

It's not hard to find a garage with space for a bike. Most folks will rent you bike space for under $20 a month. It's worth it.

If that's impossible, buy a cover or make one out of canvas. Waterproof plastic won't do. It traps moisture and can be worse than fresh air.

A cover can fend off the bike robbers, too. They're never sure what's under the cover and the time it takes to remove a well tied tarp can foil a clean getaway.

Once you've found a safe place to keep your bike you'll want to think about maintaining it yourself. Bike shops are okay for big jobs - clutch replacements, valve jobs - but you'll save yourself piles of money if you can do the simple stuff yourself!

The first thing to learn is how to change your oil and grease what fittings your bike has. You should also be able to oil and adjust your control cables and chain, change a tire, adjust brakes, change burned-out bulbs and eventually tune your own machine.

None of these jobs is too hard for anyone with average smarts, given the right instruction. And once you've learned to do them regularly you'll spare yourself the agony of being stranded on some superhighway 90 miles from home. You cannot leave your bike by the side of any road and expect it to be there when you get back.

There are two schools of thought about acquiring mechanical knowledge. Some people can learn how to fix things out of books. I can't. I've only found one mechanic's book that made any sense, and that's Volkswagen Repairs for the Compleat Idiot. I know of no parallel for motorcycles.

The other way is to learn from those who know, first-hand. Harry Powell of Washington taught me all I know about bikes, and God bless him for it. There are others like him. If you hang around them long enough and try to keep from being obnoxious, eventually you will learn.

A few helpful hints: Changing Oil

It's the most important thing you can do for your bike. Motorcycles have small oil reservoirs and many have no oil filters. Change the oil every 1,000 miles if you can. To do it, look on the bottom of your engine for the oil pan. Somewhere on it there will be a big bolt. The tool kit that came with your bike should have a wrench to fit.

Loosen the bolt until you can turn it with your fingers. Then slide a big pan underneath and loosen it the rest of the way. Black, sludgy oil will issue forth. Leave it draining for five minutes so you get it all. Replace the bolt.

Now find your dipstick, which tells you how much oil you've got. It's somewhere on top of the engine, or under the seat. Ask someone to help if you can't find it.

Pour in new oil until the dipstick says it's reached the right level. If you have a manual it should tell you how much you need and what kind to use. Otherwise, call a dealer. He'll know. Greasing

Most bikes have few grease fittings. The most important one is in the swing arm - the frame member that runs to your back wheel. Find the rear axle and then work your way toward the engine along the frame tubes. Where the tubes meet the frame there should be a grease fitting or two. Borrow a grease gun or have any garage push grease into the fittings until it starts to ooze out any openings near the fittings. Do it twice a year. Changing Tires

You have to take the wheel off and that's hard, particularly on the back. It will mean removing the axle and the sprocket the chain sits on. This is one job that you'll need guidance from a fellow biker on.

Try it some time when you don't have a flat, so you'll know how.

Getting the tire off is hard, too. You'll need tire irons and the little tool that removes the valve that sits in the air filler tube.

Most motorcycles have tube-type tires. With a new or patched tube, the key is getting the tire back on without pinching a hole in the tube with your irons. That's something else you'll need first-hand help on. Oiling and Adjusting

Your brake, throttle and clutch cables are on the handlebars. The brake and clutch have little rings that you can screw back and forth by hand. They should be adjusted so there is a tiny bit of play. That keeps strain off the cable.

Cables should be lubricated whenever they become hard to work. Clutch and brake cables can take fancy graphite cable lubricants. Detach the cables from the controls and pour the slippery stuff into the rubber protective cable cover.

Throttle cables take only oil, because the lubricant eventually works its way into the carburetors and you can't run on graphite.The technique is the same.

You should adjust your drive chain once a week or so. You do it by changing the placement of the rear axle, thus adjusting the rear sprocket. The chain should have about as much slack as a properly adjusted fan belt on a car - a quarter-inch or so.

While you're adjusting, lubricate the chain by pouring heavy grade oil over it as you spin the rear tire. Otherwise your chain snaps while you're doing 50 and tears your engine and your legs to shreds. Unncessary. Tuneups

Some day you will be able to tune your own machine. All you need are wrenches, a pointgap gauge and a timing light. But it will take time.

Meanwhile, there are ways to trouble-shoot if your bike won't go.

Engines run on two things: spark and gas. Take away either and it won't run. The way to find out if you're missing either is to take a spark plug out.

If the base of the plug is all wet with gas you've gotten too much gas. Dry it off, blow on it. Then hook it pack up to its cable and set it against something metal.

Now turn on the ignition and kick the engine over. You should see a spark at the electrode of the plug.

If your plug is bone dry when you pull it out, you're probably out of gas or something's blocking the line.

If you get no spark you've got a dead battery, bad points, and bad coil or condenser or a bad electrical connection somewhere.

If a bike has been running well and suddenly won't start, it generally is a gas problem. Either it's flooded or dry.Those are easy problems to fix. If it's an electrical problem it usually is progressive, and you should have tracked it down long before the motor quit on you.

Next Week: How to enjoy your bike.