2004 merc 90 saltwater(2 stroke) motor issue, please help

[scankdog]

i have had this motor since being brand new. it has given many trips and has never left me in the water except once and it was a gas line not motor. I have since had carbs cleaned recently, power pack changed(cause it went out) and few other minor changes. here is my problem....

the motor after boating out to mainly the same fishing spot(running great) and then when im finished fishing, im boating back about 3/4 of the way back to the landing, the motor starts to run up and down like it loses power and then kicks back to normal and repeats this at quick intervals. when I slow down the rpms or speed it tends top balance out and run a little better. sometimes I have stopped the motor for a couple of minutes and throttle back up to max speed and the problem is gone and then other times it continues. this happens like clock work and always on the way in almost closer to the dock it starts to do this seems like it is over heating(water running thru motor fine) or ???? I have new plugs and wires on motor also(could one of them fouled again?). what does this issue sound like? total miles when im experiencing this is when it gets to around a total of 20 miles(12 miles one way to fishing spot and about 8 miles heading back it starts to do this of losing power and and goes up and down forcing me to slow down the rpms to balance out the continuous power)

could really use some leads on what to check for...this issue has to be simple to fix, I just dunno what the problem is(battery , connections? etc ).....I will be on here looking to answer questions u have...thanks in advance

 

[scankdog]

Could really use some advice

 

[scankdog]

not one person including moderators have an answer to this topic? lol...wow what a forum

 

[Texasmark]

Just suck it up a minute. Not the usual paper and glue problem.

First thing that makes me curious is that it occurs when returning, not on the outbound trip. What's the difference? Into the waves, time of day, with the waves, load on the boat, fuel and cargo changes. A fouled plug usually won't surge. When it fouls is fouls till you correct the cause. How long are you out at your fishing grounds before returning....engine had time to cool off? If you had water in the fuel it would bother you both ways. If your tell tale is showing a good solid stream of water then you should be ok in that department. Besides your engine has an overtemp alarm that will sound the horn when your block water temp reaches 195F and by then your tell tale would show some signs of steam.

Your fuel tank could be gasping for air. Do you put something on it or near the vent that could choke off replacement air on the way back in? If ignition overtemp I think that once it started failing it would continue until you shut the engine off and things cooled off. However the 90 has 3 cyl and 3 CDI packs. I have had them fail due to temperature problems on a '02 90 triple I had recently.

Again, the up and down sounds like possibly water in the fuel, or clogging fuel tank vent. Very confusing that it would be either since it only occurs on the return trip and after so many miles. So that's back to temp and CDI's. The signals that feed triggers to the CDI's from under the flywheel are magnetic devices which do not manifest problems in that manner. If they work, if they don't they don't no wishey washey.

So where did we get with this?

Check the tank things I mentioned especially on the return trip. The next time it happens remove the cowling and feel around for something getting hot. the only thing that can hurt you is the spinning flywheel (or falling overboard) and it has a plastic protective cover over it. Feel the back of the block, it should only be warm to the touch on the way back in at reasonable rpms. Check the squeeze bulb when you have the failure. Is it soft or sucked flat? Does squeezing it change anything?

Edit: On the way out which direction is the wind blowing and the waves traveling vs your heading? How large are the wave?. On the return same question? How do you take on these waves? What kind of boat do you have?

Get back with some answers and I'll attempt to follow.

Mark