Lawn mowers and other lawn and garden tools with gasoline engines often will not start or are difficult to start after a long storage period. The engines can become balky for several reasons, but frequently a simple spring tuneup will put them back in good working order.

Here is how to proceed:

* Use fresh gasoline. If you are using gasoline that has been stored in a can or tank for a long period, pour it into your car's gas tank and get fresh gasoline. For best results, especially with tools that go for several weeks or months without being used, add gasoline stabilizer. This product, sold at most home centers and hardware stores, will help keep the engine's carburetor from gumming up during storage.

The safest way to remove old gasoline from a mower tank is to use a small hand pump. (Caution: Do not use an electric pump.) I use a plastic siphon pump of the type sold for filling kerosene heaters. Make sure to tag the pump "for gasoline only" so it won't be used for kerosene or other fluids.

* Clean or change the spark plug. Remove the spark plug with a wrench. If the plug has not been changed for two years or more, buy a new one of the same type. The plug's model number is usually stamped on the porcelain insulator. If you're in a hurry to use the tool, clean the plug's electrodes with an old toothbrush and reinstall it in the engine.

* Clean or change the air filter. This step is sometimes overlooked in a do-it-yourself tuneup, but a clean air filter is important for easy starting and smooth running of the engine. Remove the filter cover and inspect the cartridge. Dirty paper cartridges can sometimes be cleaned somewhat by bumping them lightly against a hard surface to dislodge loose dirt. If a paper cartridge is very dirty, it should be replaced.

Foam filters can normally be washed and replaced. Clean the foam in a solution of detergent and water, rinse and squeeze out most of the water, then let the foam dry. Pour or squirt a little auto-engine oil on various surfaces of the foam, then knead the foam to distribute the oil evenly. Reinstall the foam and replace the cover.

Here are a few more steps that can improve performance and make it last longer.

* Change the oil. Many mower manufacturers recommend changing the oil after each 25 hours of use. Oil should always be changed when the engine is warm, but not running. If necessary, run the engine for five minutes or so to warm it up.

Oil can often be drained from push-type mowers by removing the oil-filler cap and tilting the mower so the oil runs out the filler hole. Use a pan or tray to collect the oil and pour it into a suitable container. (I use old detergent jugs. Milk jugs are too thin and can spring leaks.)

Used oil should be taken to an oil-recycling station for disposal. Some auto-parts stores, quick-lube chains and filling stations run such services.

Refill the mower with fresh oil. Most mowers use either SAE 10W30 or SAE 30 auto-engine oil. If in doubt, consult the owner's manual. Do not overfill; many mower engines use less than a quart.

Some tools, such as string trimmers, leaf blowers and chain saws, do not require oil changes. Special oil to lubricate the engines is mixed with the gasoline in these tools.

* Check the blade. Do this when the engine is cold and the gas tank is empty or nearly empty. Disconnect the spark-plug cable and place it away from the plug or tape the metal end to avoid an accidental start. Tip the mower enough so that the blade can be examined. A blade should be replaced if it appears to be bent, cracked, very dull or is otherwise damaged. Replacing a blade is often cheaper and easier than having it sharpened. If a blade has only a few nicks in the cutting areas or is dull but in good condition, it can be touched up with a file.

Wear gloves to protect your hands when examining or working with a blade. To remove a blade, lock a C-clamp or locking pliers on the blade housing so the blade bears against it when turned counterclockwise. Loosen the nut on the mounting bolt with a wrench. Remove the nut and washers, put them aside for reuse and pull off the blade. Reverse the procedure to install the new or sharpened blade.

* Clean the housing. Carefully scrape any mold or dried grass from the inside of the blade housing. A plastic windshield scraper works well for this.

Questions and comments should be sent to Gene Austin, Box 861, Blue Bell, Pa. 19422. Send e-mail to doit861@aol.com. Questions cannot be answered personally.