2002 Mercury 40-HP Two-stroke-cycle

[whalerbob]

Hi, I need your help on this cause I'm out of option now. I bought a 2002 Boston Whaler with a 2002 Mercury 40-HP two-cylinder two-stroke-cycle one-carburetor engine with no automatic oil mixing. The engine has less than 200 hours and looks new. From what I can see with original paint on the engine [there is] no heat problem.

When I got it there was a problem with the stator. I changed the stator, the trigger, and two CDM [please explain what this acronym means--jimh]. I checked all grounds and connectors. I installed new spark plugs. I rebuilt the carburetor with a new gasket. I rebuilt the fuel pump with a new kit. I adjusted timing to 8-degrees BTDC. I adjusted the idle mixture screw.

The peak cylinder pressure is 150-PSI on both cylinders. [The distance the] spark [will jump] is 7/16-inch on each coil. There is nice output from the water pump. I checked the reed valves and they are like new.

The problem: the idle is really rough. If I remove the spark plug wire from the upper cylinder [there is] almost no change, but the spark is good, still has 7/16-inch on the spark tester. If I remove the spark plug wire from the lower cylinder, the motor will stop quickly. I checked the trigger of each coil with opposite mark on my flywheel: no miss, no crossfire, very steady flash on each cylinder. I switched the coil [and found] no change [that is] the problem [remained on the] on same cylinder.

I changed spark plugs [and found] no change [that is, the] problem [stayed] on same cylinder. Reversed the stator CDM charging coil wire (white and green) [and found] no change [that is, the] problem [stayed] on same cylinder. (This is described on a CDI website for troubleshooting the stator.)

If [the cause of the rough idle] is not an ignition problem, [the cause of the rough idle] has to be fuel. Do you agree?

If I check the spark plugs both are wet with mixture of fuel and oil. I checked the crankshaft upper seal and no leak there.

Give me any suggestions.

[jimh]

Based on your spark test by removing the spark lead from the spark plug of each cylinder, the problem is the upper cylinder is not firing. It may not be getting the proper mixture of fuel and air. The plug is wet because it never fires off the fuel.

Also, check the spark with an in-line spark tester to be sure the plug is firing.

[whalerbob]

Thanks jimh--I already checked that my upper cylinder spark plug is firing and at a good 7/16-inch gap with a spark tester. When I said "wet", I mean it is oily. but even the lower cylinder is oily also. Spark is there on each cylinder and right on time (8 BTDC) and that is for each cylinder 'cause I made a mark on my flywheel for TDC for cylinder number 2 (lower)

[jimh]

Based on your description, the upper cylinder is not firing. Something is wrong with the fuel-air mixture. It is not igniting.

[flymo]

That's a bit of a stumper, Bob. If the spark is OK, and it seems like it is, then it's got to be a fuel problem - but how a single-carb twin has problems only on one cylinder is beyond me. I would have said "reeds" but you've already checked them.

You characterize this as a rough idle problem - does that mean that the motor runs OK once you get above idle?

[whalerbob]

thanks for the help Jimh and Flymo
This morning, I check the spark again. good 7/16 on my upper cylinder. switch spark plug. Same thing. It is not running fine above idle either.
I stop the motor, put a cup under the carb and empty the bowl to see how much fuel I got in there. Almost 40ml so it is not a fuel starvation problem. The bowl is full. Plus got all clear fuel lines. So I can see a smooth flow of fuel between pump and carb.
But with all these trial, I found something else. I took a small light that you can stick thru the spark plug hole to inspect the piston and cylinder. Found a lot of carbon deposit on the piston. Went out and bought some Seafoam. Put some thru the spark plug hole when the motor was hot and let it sit for an hour. start the motor. spray some seafom with throttle 1/4 open and motor running. lots of smoke. Let it run for 5 minutes at 1500 RPM. Now I can see a little difference when I remove the spark plug cable from the upper cylinder (bad one) and the motor will still run if I unplug the spark plug on the good cylinder. But the idle is still rough.
Could it be that the carbon deposit is igniting the mixture but at the wrong moment ????.
I will read more on the right way to do a de-carb and might try some other stuff instead of seafoam

[flymo]

Bob, I think it could be pre-igniting off the carbon if there is a ton of it. The carbon could also be fouling the intake ports on the one cylinder.

If there are big chunks of carbon in there, it might be easier to just pull the head and have at it with a popsicle stick.

Tim

[whalerbob]

Thanks Tim
The problem with this Mercury 40hp is that it is a blind bore. The head is welded to the block. you can be sure that it would be the first thing I would have done during my troubleshooting. There is a guy on bassbarn website. His name is Dunk. He seems to know his stuff. He gave a good method for decarbonizing.

[flymo]

Yeah, I guess Sea Foam is better than cracking the crankcase open:-). I think you're on the right track!

[whalerbob]

Good news. Seafoam is doing the job. the motor is getting responsive and better idle with every treatment.

Since I know that it is probably carbon buildup and sticky rings, I will remove the powerhead and

--try to find a way to pressurize the crankcase to see if I have a vacuum leak somewhere
--remove pistons and have a look at the rings
--take some pictures and post them here. everybody have been so helpful, it is the least I can do.

Merry Christmas and happy new year to you all
thanks

[Jefecinco]

A fiber optic viewer or camera is a wonderful tool to have when examining a combustion chamber is indicated. Due to cost not many non-professionals have one in the tool box but I believe they can be rented for a day or half day. I've used a Panasonic to inspect large diesel engines and the lens/light will pass through a typical spark plug of fuel injector aperture in the chamber. Depending upon the depth of the cylinder at the bottom of the stroke the top of the chamber or "head" may or may not be visible. However, if the top of the piston is heavily covered in carbon deposits it's highly likely the head will be the same.

[whalerbob]

Jefecinco
When I said that I look thru the spark plug hole with a light, it is really with an endoscope that has a light at the end similar to this one
http://www.renovasurgical.co.uk/flexibl ... pe-repair/

I use that tool to check inside SF6 high tension (300KV) bus bar and breaker in sub-station. It has 2 ring under the optic viewer that let you direct the camera. UP-DOWN or Left and right. Another ring let you adjust the focus. You can see inside the cylinder like if you were there. The only thing I can't see is the rings.