Tuning Gasoline Engines

Your engine will start promptly and run smoothly each time if you follow these maintenance and tune-up procedures.

Spark Plug

Remove the spark plug after every 50 hours of use or once a year, whichever comes first, and check it for wear. Just looking at the tip of a spark plug can tell you something about your engine’s performance and help you decide whether to replace the plug. For example, if the plug is coated with a light brown substance, your engine is probably functioning properly See Analyzing Spark Plugs to find out what else to look for and when to install a new plug.

If you have to replace a spark plug, use the size and type recommended by the manufacturer—you must match the original plug or the replacement won’t work properly (e.g., a plug that is too long will be mashed by the top of the rising piston). Spark plugs come in many different sizes and lengths, so choose carefully

Also check the owner’s manual to find out what gap is required on the plug. The gap is the space between the side electrode and the center electrode. Its width determines the timing of the spark. New plugs come with preset gaps, but check the gap anyway with an inexpensive spark plug (wire-type) gauge that you can buy at any auto parts store. For example, if the required gap is .025, select that wire on the gauge and slip it between the two electrodes. You know the gap is correct when you feel a slight drag on the gauge. If it is too tight, use the tip of a screwdriver to increase the opening slightly. If it is too loose, tap the side electrode lightly with the wire gauge to tighten the gap.

Air Filters

One main reason why engines run poorly or don’t start is because the air filter gets clogged, thus reducing the ratio of air to fuel in the carburetor. You should change or clean the filter each time you change the engine oil, or about every 25 operating hours (more if you work where it’s dusty). To change or clean any type of air filter, follow these steps:

  • Oil-Saturated Filter: Loosen the screw that holds the filter housing above the carburetor. Remove the unit and lift off the cover. Remove the foam filter and wash it in kerosene or hot, sudsy water. Squeeze it dry and then saturate it with 30-weight oil. Squeeze lightly to remove excess oil, then replace in filter housing. Make sure lip on top of foam extends over edge of housing so that no dust can get past it. Replace top and remount housing.
  • Pleated Paper Filter: This is easy to replace each time it gets dirty, but cleaning will give you an extra season’s use. Unscrew the nut on top of the housing and remove the cap and the paper filter. If there is a foam sleeve around the paper filter, slip it off. Tap the filter several times on the sidewalk or on a piece of wood to dislodge dust and debris. If the filter is obviously dirty and tapping doesn’t remove much dust, replace it.
  • Paper Filter: This is less common; it is mounted beside the gas tank and connected to the carburetor by a hose. Remove the hose clamps and hose; then remove the filter. Tap it to clean it. If it’s still dirty, replace it.