The 18 HP Onans are B48M-GA018 engines. They have points unless someone spent the money to upgade to electronic ignition. That's what killed the B48M-GA019 19 HP at the shop. Upgrade is $1000 which is ~ half the price of a Kohler Command Pro 18. Plugs are easy to get to. PSI reading should be 90 or better with oil in the cylinder. Intake manifold failure is something to watch for as the joint between halves can breakdown. Sounds like the linkage isn't adjusted right.