See also Rebuilding An Engine.
Nice link on the odd-fire engine: http://home.off-road.com/~merls_garage/oddfire.html
But it is confusing when you get to the distributor cap, so ignore that part. The SM can be considered to be two 3 cylinder engines with one crank, spaced 90 deg apart.
- Distributor Rotor
- Check the resistance of the each tip to brush contact on your rotor. It is nominally 5K ohms.
- If they are not close to the same, one of the resistors has bought it and you are likely running on three cylinders intermittently. The high resistance will eventually destroy the rotor.
- The resistor is there by law to reduce radio interference. In an emergancy you could dig the resistor out wire across direct.