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Smoking 75 HP two stroke

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Snakeriversailor77

Contributing Member
Im working on a 2002 Mercury 75 HP two stroke 3 cyl carb motor. The engine began smoking at running speeds and according to the owner it felt like it was "missing a gear". It appears that it is over oiling somehow as the exhaust is spitting oil and fuel. Initially it was thought that new spark plugs, filters and a carb cleaning was the answer but that didn't change anything and if anything, it caused it to smoke more. The engine idles rough and misfires which belches blue smoke when it does. At idle, this misfire can kill the motor. It was suggested the problem was a leaky fuel pump diaphragm which was causing excess fuel and oil to get inside the motor through the pulse circuit, no pin holes or tears were found but new diaphragm and check valves were installed. No change. The recirculation/evacuation lines are clear. The check valve between the oil pump and the fuel pump was replaced. No change. The engine still smoked after disconnecting the oil line and running a shop tank mixed 50:1 and new good quality non ethanol fuel too. The timing is right on, and spark is good at all cylinders. A faulty meter showed the CDM's to be bad but new CDM's didn't solve the problem either. Compression is good at @125 per cylinder. Carbs were cleaned a second time with no change either. The only previous issue with this motor was a leaky oil cap. The discharge water is only a few degrees warmer than the river water after prolonged fast idle. Replaced the thermostat and poppet valve thinking that the cold condition was causing an over rich condition. Didn't solve anything and still smokes badly enough the Mosquito Board is thinking of paying me to run it! Help!
 
Take a look in the carburetor bores ( use a strong flashlight ) when the boat is up on plane.-------See if one carburetor is putting in more fuel than the other 2.
 
Not sure the engine will run strong enough to get up on plane. I have checked at a idle and found that each have roughly the same vacuum and choking off the carb will flood it with fuel and kill the engine.
 
After doing a number of diagnostic tests, and finding after disconnecting #1 cylinder that it still misfired and then repeating with #2 and #3, found that misfiring is being done on #2 and #3. Replaced spark plug again with a new one, no change. These cylinders have new CDM's. Has anyone experience a bad reed valve or valves causing excessive smoke especially at above idle RPM, misfiring and belches of smoke at idle. Could the reed valves be the culprit even when it has good compression on the gauge? I also am concerned that perhaps the crank seal between cylinders #2 and 3 could be leaking causing excessive fuel to enter those cylinders for firing. Oddly, when looking at the spark plugs, #1 and #2 are wet and oily and #3 is dry and appears to be firing fully. Dry and brown for #3. Any thoughts?
 
Sorry , but reed valves have nothing to do with the reading you will get from a compression test gauge in the cylinder.----The reed valves only have to do with compression in the crankcase.---------Remove carburetors to check reed valves.------Inspect the little check valves found in there as well.-----Check the flywheel key too.----What brand of oil is being used ? -------Have you considered removing the exhaust cover for a look at pistons and rings ?----------Possibly review your understanding of what a crankshaft seal between # 2 and # 3 does as well.
 
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The flywheel key is fine and the engine is firing in time with TDC on each cylinder. I am using Mercury Quicksilver 2 stroke oil. I am concerned that the seals between the cylinders are leaking or have failed causing the problem with excess fuel in the two cylinders. The crankcase seals would account for the sudden stop (as the crankcase was too full of fuel as it was compressed.) This does not happen at anything above idle speeds, which would allow the fuel to "vent" and not stop the engine. Haven't removed the exhaust cover yet and may do that after checking the reed valves. I inspected for damage from the carb manifold when the carbs were off being cleaned. No damage was seen but they were not removed for further inspection. Seems like all the usual suspects for the excessive smoking have been eliminated and don't want to just replace parts. The boat is in the water and having to remove the powerhead to split the case to replace crankshaft seals would be difficult to say the least.
 
Do you know how the seals between cylinders work on this motor ?-------Do you know how these seals are made / constructed ??------Do you really know the purpose of these seals.-------Please review them before tearing the engine apart.
 
rechecked both the trigger outputs and the stator outputs with a different multimeter. None of the trigger outputs were in spec. should have been .2 v to 5 v and the best was #3 at .189 v. Stator output was below spec as well at 163 v at idle. Replacing both now although running and firing it wasn't completely igniting the fuel apparently.
 
Replaced the trigger and stator, running smoother but still smoking, less smoke but still not right. Haven't done the reed valve check yet. #1 and #2 spark plugs are now wet but not oily as before. Thanks for the advice everyone I'm still not happy with it obviously but it's getting closer. Just doesn't make sense, nothing that usually should solve this problem has done so.
 
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