1997 25hp johnson chugging after certain RPM

[Dougan]

1997 Johnson 25hp, 2 cyl/2stroke

I bought this motor used last year and it worked fine for a while. However, last summer it started to behave strangely and I can't think of what the cause would be.

When I start it cold, it runs fine. I can go wide open for a while. At some point, however, it starts chugging after a certain RPM level. This RPM level is always the same-- below this threshold it runs perfectly, and above it, it chugs. It's the same RPM level on each outing, too.

By "chug" i mean it seems like it's missing somehow. The motor will jerk around while it's behaving this way. It will consistently behave like this for a while, whether I choke it, turn it off and back on again, etc. After a while (5-30 min) it will stop behaving like this and just work fine again. It's never in-between. It's either 100% perfect or chugging every time i go above that RPM threshold.

I doubt it's anything to do with my carb since it comes on and off randomly. I've tried squeezing the gas bulb while it's doing this and it doesn't seem to have any effect, so I doubt it's having trouble receiving gas. I've also tried running with the cowl off in case it was having trouble getting air, but that didn't have an effect. It may be heat-related since it seems like the only time I can duplicate it is after I've been running it for a while. It's most likely to happen after I've been idling for a few minutes, like waiting in line at a boat landing or something.

Here is a video of it happening. https://www.youtube.com/watch?v=cqRnFO8Onx8

Does anybody have any idea what could be happening? I'm out of ideas and will probably take it to a mechanic, but I'm really nervous about that becuase it only seems to happen intermittently after I've been using it for a while, so I think they'll have trouble duplicating the issue.

 

[fireman57]

It could be a coil breaking down and you lose spark to one cylinder. When it does this stop the boat and unplug one sparkplug wire and restart the engine. If it starts then that one is good. Repeat it for the other one. I'm betting it won't start or won't want to. If it doesn't just replace the offending coil. Sometimes when they get warm they will not work properly, either arcing or not work at all.
This is if you are positive it's not a fuel problem. You have done just about everything to eliminate a fuel problem though. One last thought, you might have some crud in a carb or your gas tank and a small piece of junk can clog a jet. Then when you slow down it falls out, picks back up when you take off again. IF you have stuff in your tank it could be picking it up there too.

 

[tblshur]

you might look at the throttle linkage make sure it isnt pinching a wire or something. good luck:joyous:

 

[Dougan]

It could be a coil breaking down and you lose spark to one cylinder. When it does this stop the boat and unplug one sparkplug wire and restart the engine. If it starts then that one is good. Repeat it for the other one. I'm betting it won't start or won't want to. If it doesn't just replace the offending coil. Sometimes when they get warm they will not work properly, either arcing or not work at all.
This is if you are positive it's not a fuel problem. You have done just about everything to eliminate a fuel problem though. One last thought, you might have some crud in a carb or your gas tank and a small piece of junk can clog a jet. Then when you slow down it falls out, picks back up when you take off again. IF you have stuff in your tank it could be picking it up there too.

Tried two tanks, same effect. I really don't think it's a fuel problem due to how consistent it is. You'd think if it was fuel problem, it would be a little more inconsistent, like the rpms required to duplicate the issue would vary based on how something got stuck in a jet, etc. The way the issue behaves exactly the same every time does have that electrical feel to it.

I never thought to pull plug wires, now i'm a little bummed because that seemed obvious. I bet it'd still start-- when the issue is happening, it runs perfectly below that rpm threshold. But it'd probably tell me what's up. It sucks though because there won't be navigable open water around here until mid-april, so duplicating the issue would be pretty tough on land. It'd be so easy to go give that a shot tonight if it were summer.

 

[nwcove]

does that motor have S.L.O.W. mode?

edit: just watched the vid, my 99 3 cyl 25 didnt sound like that when slow mode kicked in.

 

[kbait]

Video does sound like an ignition issue. Check out the wires that come out from under the armature plate (under flywheel). Those wires flex as you throttle up, and maybe one is chafed and arcing to ground as it flexes. Could also be coil or power pack, but strange symptoms for them.. Good luck!

 

[78 mckee]

Could also be an issue with the stator. Not that unusual. Components within the stator assembly that have broken down over time can begin to fail as they heat up during operation. Could also be a wiring problem with the stator as the stator assembly is rotated as the throttle is advanved and a wire that is broken inside its insulation can interrupt primary voltage to one of the coils and feel like a cylinder is no longer firing, but when you return the throttle to a lower position the contact is made again and the ignition functions as expected.

 

[Dougan]

Just following up in case anyone finds this. I ended up taking it in to mechanic. Turns out it was the engine overheating-- he said there's something in that engine to limit the motor to 3,000 RPM if the engine gets too hot. He replaced the water pump for me. I had a thought in the back of my head that it might be heat-related since it usually happened when idling, and it never happened right off the bat. I'm a little bummed it was something so simple as the water pump though.

I haven't taken it out to verify that the water pump change fixed it. But this all at least makes sense to me, so I have high hopes.