Diagnosing a LM318......
Posted: Wed Jul 13, 2022 7:06 pm
Looking for a gut check on the diagnosis of an 88 LM 318 port engine. Performed my normal start of season routine on both engines. New cap and rotor. Lubed the weights on the Pertronix mechanical advance. Changed oil, oil filter, fuel filters, etc. Engine started harder than normal.
- Found a weak coil. It tested low voltage / high amps. Replaced it. Started better.
- Took it out for a trial run. Engine would not accelerate past 2500 RPM. Sounded like it was stumbling. Exhaust smelled heavy with fuel / really rich. Carb is a quadrajet. Pulled it off. Fuel bowl was full of crud. It looked like the primary rods were stuck open and not sealing at idle / slow speed. It was the original carb. Sent it out for a rebuild.
- Put the rebuild carb on, tuned it. Engine seemed to idle fine but did not sound as smooth as normal.
- Vacuum at idle is 14-16 PSI. This is normal for the engine.
- Barely out of the slip, increased throttle to 1500-2000 RPM to spin the boat and the engine started stumbling under load. Reduce throttle to idle. Shifted fine in and out of gear and did not stall. Went back into the slip fine.
- Fuel smells fresh. I always use fuel stabilizer and Marvels. Never had a problem.
My next step is pulling the plugs and checking compression. To me, the engine acts like it dropped a cylinder from a cracked head or blown head gasket. The engine oil is clean and not milky. I haven't checked the ignition module (Pertronix Igniter 2). Will likely do that before the compression test. However, my experience is those simply quit completely.
Thoughts?
- Found a weak coil. It tested low voltage / high amps. Replaced it. Started better.
- Took it out for a trial run. Engine would not accelerate past 2500 RPM. Sounded like it was stumbling. Exhaust smelled heavy with fuel / really rich. Carb is a quadrajet. Pulled it off. Fuel bowl was full of crud. It looked like the primary rods were stuck open and not sealing at idle / slow speed. It was the original carb. Sent it out for a rebuild.
- Put the rebuild carb on, tuned it. Engine seemed to idle fine but did not sound as smooth as normal.
- Vacuum at idle is 14-16 PSI. This is normal for the engine.
- Barely out of the slip, increased throttle to 1500-2000 RPM to spin the boat and the engine started stumbling under load. Reduce throttle to idle. Shifted fine in and out of gear and did not stall. Went back into the slip fine.
- Fuel smells fresh. I always use fuel stabilizer and Marvels. Never had a problem.
My next step is pulling the plugs and checking compression. To me, the engine acts like it dropped a cylinder from a cracked head or blown head gasket. The engine oil is clean and not milky. I haven't checked the ignition module (Pertronix Igniter 2). Will likely do that before the compression test. However, my experience is those simply quit completely.
Thoughts?