Check air cooling (fins, airflow, mixture)

An air-cooled engine loses heat through the metal fins on the cylinder and head into the moving air. It cools better at road speed than in slow traffic. Scooters and some step-throughs add a fan and plastic ducting (forced air cooling). An air-cooled engine overheats when the fins are blocked, the airflow is cut off, the mixture is too lean, the oil is low or old, the timing is wrong or the engine is worked too hard. This check covers the causes a rider can find.

Safety

• Let the engine cool fully before touching the fins. They stay hot long after the engine stops.
• Do not spray cold water on a hot engine. It can crack the cylinder or head.

You need

• A soft brush, a plastic scraper, water with a little detergent, a torch and a plug spanner.

Steps

1. Check the oil level with Check the engine oil level. Oil also carries heat away; low or old oil makes the engine run hotter.
2. Look between the fins with a torch. Mud, sand, oil-soaked dust, plastic bags or nests block the air. The K-Light chart lists fins clogged with sand and oil as a cause of overheating.
3. Brush and scrape the dirt out with plastic tools. Wash with detergent water on a cold engine and rinse gently. Do not use a high-pressure jet near the plug cap or wiring.
4. Check for broken fins. One or two chipped fins are acceptable; large missing areas mean a hot spot.
5. Look at anything that blocks air in front of the engine: aftermarket covers, luggage, a mud flap, a crash bar wrapped in cloth.
6. Scooter or fan-cooled engine: check the fan cover is complete and fitted, the fan is not broken, and the air inlet grille is clear.
7. Check the air filter. A blocked filter usually makes the mixture rich (black plug), but a filter soaked in oil or water can also cause problems; see Clean or replace the air filter.
8. Remove and read the spark plug. White or blistered means lean or too hot; black means rich. See Read a spark plug.
9. Check for an intake air leak if the plug is white: Find an intake air leak.
10. Check the exhaust is not crushed or choked. A blocked exhaust traps heat.
11. Check that the brakes do not drag and the clutch does not slip. Both make the engine work harder.

Check your work

• Fins clear and clean. Plug colour light grey to light brown (K-Light: light grey is good).
• Engine no longer smells hot, pinks or loses power after a long climb.

If it did not fix the problem

• Still overheating with clean fins and a normal plug: Air-cooled engine overheats to check timing, oil supply and compression.
• Pinking under load: Pinging or rattling sound when accelerating (pinking).