Diagnosing a LM318......

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ericinga
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Location: Cumming, GA

Diagnosing a LM318......

Post by ericinga »

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?
Eric Spies
1989 32 SDFB
Twin 318s
Lake Lanier, GA
Marinette Boat
BlueSkye
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Re: Diagnosing a LM318......

Post by BlueSkye »

Hope it's your plugs. I always start the season with a fresh set. I do a lot of canal travel and lots of time at/near idle is a killer.
1971 32ft Express 2x318
Seneca Lake, Erie Canal, Lake Ontario, Rideau
Fastjeff
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Location: Rock Halll, MD

Re: Diagnosing a LM318......

Post by Fastjeff »

Careful check to be sure the plug wires are all going to the right cylinder. (Its DAMN easy to mess them up on these engines since the wires dive under the manifolds.) My old boat planed off just fine with two wires crossed on the starboard engine, but the motor refused to make more than about 2,500 rpm--sound familiar? The marina did a compression test on the motor, and THEY had switched to two wires.

The way I stumbled onto it was by pulling the wires from the cap, one at a time, and seeing how the motor reacted. When two wires did nothing, I switched them and all was well again. I'd do this in any case for it will tell you something about how each cylinder's running.

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington
jralbert
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Re: Diagnosing a LM318......

Post by jralbert »

Suspecting OP is very careful about these things but when I hear stumbling at low rpm's I always look back at my experience with the little filter at the mouth of the fuel feed into the carb. For the quadrajet it was a cg-11 filter about the size of the tip of a finger. Could that have been overlooked?
-joel-
former owner 1988 '32 FB Sedan
Chesapeake Bay
twin 318 / 240 hp
Potomac MD
Fastjeff
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Re: Diagnosing a LM318......

Post by Fastjeff »

Good idea to check that as well.

Jeff
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington
ericinga
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Joined: Mon Jul 14, 2014 11:21 am
Location: Cumming, GA

Re: Diagnosing a LM318......

Post by ericinga »

Well, drained fuel tank and filled with fresh fuel. All fuel filters new and clean. Pull spark plugs and replaced. All cylinders running very rich but no white / bright plugs. Wanted to eliminate those variables. Pulled anti siphon valve. Clear and working fine. Ran the engine again. Under load, engine will initially run up to 3500 RPM but stutters and drops down to 2000 - 2500. New diagnostic fact: Oil is now milky and there's a ticking lifter on cylinder 5. Thinking it is a leaking intake manifold. Preparing to tear the top end off this week and stop messing around. I replaced the lifters in 2016 due to an intake leak causing milky oil and lifter collapse. At that time, decided not to remove and rebuild heads. Might tear the entire top end off this time.
Eric Spies
1989 32 SDFB
Twin 318s
Lake Lanier, GA
Marinette Boat
BlueSkye
Tin star
Posts: 70
Joined: Sun Jul 20, 2014 9:38 am

Re: Diagnosing a LM318......

Post by BlueSkye »

If you have water in the iol you have to fix that before going on with the troubleshooting. Good luck!
1971 32ft Express 2x318
Seneca Lake, Erie Canal, Lake Ontario, Rideau
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