Mercury two stroke cutting out

[Gregg927]

I have a Mercury Force 120 horse power outboard. At higher RPM the engine momentarily kicks up its speed as though I have an intermittent spark plug firing problem. I have been through the carburetors and wirings, all seem to be good. Part of the problem is that the system will not permanently fail, thus, when checking I cannot find the devil. I'm wondering now about spark plug wires, capacitor discharge modules and the stator; I'm not well versed on testing methods for these. Any thoughts would be appreciated.

[flyingfin]

When was last time you put in new plugs? Did you buy OEM as specified by Merc or NGK? Be careful buying anything recommended by Auto parts stores, keep with the OEM.
If that isnt the problem, you probably have a coil going bad. Buy a new coil and keep it in the boat with tool to remove / replace. The old coil will see it and hang on to life for another year until it , but then you are prepared!

[jeepbluetj]

Speed Increase? How would that be spark? (i.e. If all the fuel in the cyl was burned by the 'correct' spark, what would an incorrect spark ignite?)

Unless the hypothesis is that the 'typical' spark is poor, and every now and then it's 'correct.' That would lead me to think coil or something in ignition.

If it's running great then revs up, that sounds much more like fuel or throttle butterfly (return spring) issues to me.

[jeepbluetj]

Speed Increase? How would that be spark? (i.e. If all the fuel in the cyl was burned by the 'correct' spark, what would an incorrect spark ignite?)

Unless the hypothesis is that the 'typical' spark is poor, and every now and then it's 'correct.' That would lead me to think coil or something like an intermittently bad wire.

If it's running great then revs up, that sounds much more like fuel or throttle butterfly (return spring) issues to me. - Or oil injection injecting more oil than normal. (I'm not familiar with the exact specs of that engine)

[Gregg927]

Originally Posted by flyingfin

When was last time you put in new plugs? Did you buy OEM as specified by Merc or NGK? Be careful buying anything recommended by Auto parts stores, keep with the OEM.
If that isnt the problem, you probably have a coil going bad. Buy a new coil and keep it in the boat with tool to remove / replace. The old coil will see it and hang on to life for another year until it , but then you are prepared!

Thank you for your thoughts. I did replace the plugs with NGK's, that was the recommended type per some Mercury website I investigated. I like your coil recommendation, and the lurking girlfriend threat use thereof. I had thought of buying one and then marching the replacement down the cylinders until I, hopefully, got lucky. I appreciate your help.

[Gregg927]

Originally Posted by jeepbluetj

Speed Increase? How would that be spark? (i.e. If all the fuel in the cyl was burned by the 'correct' spark, what would an incorrect spark ignite?)

Unless the hypothesis is that the 'typical' spark is poor, and every now and then it's 'correct.' That would lead me to think coil or something like an intermittently bad wire.

If it's running great then revs up, that sounds much more like fuel or throttle butterfly (return spring) issues to me. - Or oil injection injecting more oil than normal. (I'm not familiar with the exact specs of that engine)

Thank you for your thoughts. I leaning toward spark being the problem. The fuel is premix not injected. I'm in synch with your typical spark description; it is as though the bad guy has a button and occasional presses it to briefly spark the mystery cylinder.

[bourque]

Check your fuel system from tank to carb. Could be air bubbles.

[Navicula]

had the same problem......I found out that it was the wires...the insulation broke down and I was loosing power to the engine housing......I found this out while the cowling was off and I was playing with the wires moving things around when I recieved this huge jolt of power through my body...

I wrapped it with electrical tape and all worked fine....but I was ready to purchase a new motor....it was driving me nuts

[HopCar]

When I read your post my first thought was propeller cavitation. It can cause that exact symptom. Air or vacuum getting to the prop will unload it causing the rpm to increase.

Is there anything new on the hull that could be blocking water flow to the prop? Did you raise the engine? Is the engine trimmed all the way in? Is the prop damaged? How about the engine skeg? Growth on the hull or lower unit? Do you have a spare prop you could try?

I think I'd be looking at the lower end of the engine before the power head.

[hobopacket]

Spark causing rom increase? Don't think so unless a one isn't firing regularly to begin with.

Is it running rough regularly?

I would start with as stated above. The prop. It could be cavatating or better yet the hub is spun.

[David M]

What is the color of your spark plugs telling you?

[SV Bacchus]

It was mentioned previously and dismissed so I'm sure you will dismiss this as well.

I have a 65MP Merc that I converted to premix, as is yours. I also had the same intermittent run good, run sorta OK, run good, run sorta OK. It drove me nuts for about 6 months. I too was SURE it was spark but I didn't know if it was plugs, wires, computer or what.

Oh, forgot to mention it is a pontoon boat and I have two 6 gal red gas tanks on the rear.

One day it hit me like a 2X4 in the head, it was only acting up ON ONE GAS TANK!

I sat it aside, bought a new gas tank and have never had the problem since. It was obviously sucking air after it was 1/4 to 1/3 empty. Yeah, I could have bought a new pickup tube but the tank was nearing the end of it's life expectancy anyway.

Do yourself a favor and get an aux tank from a friend, or whoever, and at least try it before you go spending any more bucks researching the problem.

FWIW..

[Gregg927]

Originally Posted by Navicula

had the same problem......I found out that it was the wires...the insulation broke down and I was loosing power to the engine housing......I found this out while the cowling was off and I was playing with the wires moving things around when I recieved this huge jolt of power through my body...

I wrapped it with electrical tape and all worked fine....but I was ready to purchase a new motor....it was driving me nuts

Wow, thanks. I haven't had a good spark plug wire jolt in many years. Your suggestion wound make sense and cause a problem such as I have.

[Gregg927]

Originally Posted by hobopacket

Spark causing rom increase? Don't think so unless a one isn't firing regularly to begin with.

Is it running rough regularly?

I would start with as stated above. The prop. It could be cavatating or better yet the hub is spun.

Hadn't thought of hub spin, but, when the rpm goes up the thrust pushes me backwardly due to the acceleration. Seems like hub spin would work oppositely. I do appreciate your thoughts.

[Gregg927]

Originally Posted by HopCar

When I read your post my first thought was propeller cavitation. It can cause that exact symptom. Air or vacuum getting to the prop will unload it causing the rpm to increase.

Is there anything new on the hull that could be blocking water flow to the prop? Did you raise the engine? Is the engine trimmed all the way in? Is the prop damaged? How about the engine skeg? Growth on the hull or lower unit? Do you have a spare prop you could try?

I think I'd be looking at the lower end of the engine before the power head.

I like your idea, I had not considered it. However, there has been no change in the configuration of the hull or in water part of the motor, the boat is kept on a lift or trailer when not in use. Also, when the RPM kicks up, there is noticeable acceleration g-force.

Thank you for taking the time to reply. I have learned some new logic.