I. CHECKS IN ORDER TO POINT OUT CAUSE OF IMPROPER OPERATION:
1. Remove the spark plug and insert compression gauge in spark plug port. Crank the engine by pulling starter rope a few times and read the compression. It should be not less than 100 psi (new approx. 120 psi). If compression gauge is not
available, crank engine without removing the spark plug, but disconnect spark plug lead. Engine should be cold. There should be a considerable resistance as piston approaches top dead center. Hold piston against compression a few seconds; the pressure is supposed to decrease very slowly. A rapid drop indicates poor compression.
Poor compression is usually a result of - worn piston rings, worn cylinder bore, - ring groove on the piston worn.
2. Crank engine slowly, checking for noise, scraping or other signs of
improper operation. These symptoms may be due to damaged bearings or to a bent
crankshaft or connecting rod. If the crankshaft will not' turn at all, the
piston may have seized in the cylinder. Remove the exhaust elbow and look into
the exhaust port. Force the piston in a position so that you can check if the
piston side surface is scored.
3. Move crankshaft back and forth to check for excessive play. Excessive play indicates worn rod bearings or a worn piston pin.
4. Check crankshaft seals and around the entire crankcase where crankcase halves join, for evidence of oil leaks. A leaking crankcase or seal will result in faulty fuel metering, hard starting, weak performance, and difficult idle adjustment.
II. CYLINDER AND PISTON:
The cylinder wall wears or scores. and the piston rings wear as a result of dirt,
improper lubrication, and running with lean fuel mixture.
To install a new cylinder, new piston or piston rings proceed as follows:
1. Remove exhaust.
2. Unscrew the 4 cylinder nuts and pull the cylinder straight out from the crankcase. The piston rings, even after being removed from the piston, may still appear to be in good condition and suitable for re-installing. However, there are a few ways of finding out if they are worn, which should cause weak performance of the engine:
