Shimmy and Shake at mid-RPM's

[Ralphy]

Long time reader, 1st time poster. What a great forum this is!!! The way you pro's help us neophytes is truly amazing.. Thanx in advance<br />Had my Pontoon out on the lake today. Powered by a 98 50 HP Johnson 2 cylinder. Cruising along all day at 5200 RPM's smooth as silk when all of a sudden, felt like a hiccup and motor started to shimmy and shake pretty violently. Immediately backed throttle to idle and it smoothed right out. Turned off motor and raised it to check prop and skeg even tho I knew i was cruising in 40+ feet of water. Everything fine on drive line. Restarted motor and slowly inched throttle up. Fine at 1000 RPM, still good at 2000, right about 2800 it starts with the shaking. Any ideas what happened here? Seems to be a miss of some sort, but clear out of the blue like that? Any help is most appreciated.

 

[Solittle]

Re: Shimmy and Shake at mid-RPM's

Ralph - A big welcome to you sir.<br /><br />Do you have the OMC Shop Manual (+ Parts Manual)? If not they are a must if you are going to do your own work.<br /><br />Sounds like you are dropping a cylinder. You might first check the condition of the plugs followed by checking the spark. Both cylinders should have a bright blue spark that snaps and will jump at least a 1/2" gap. If they don't I would swap plugs, wires & coil from one cyl to another to try to isolate the problem. Use the manual to troubleshoot & pop back in from time to time with questions or to let us know how you are doing. We learn from your feedback.

 

[rickdb1boat]

Re: Shimmy and Shake at mid-RPM's

Could be going into S.L.O.W mode... But it does sound like one cylinder is not firing correctly. what does it do if you hit the choke real quick? Pick up or slow down? Plug wires on tight? I'd bet on a bad plug...

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Thanx for the responses SoLittle and rickdb. Dont think it is S.L.O.W rick. Engine is quite cool to the touch. Just put new plugs in 2 weeks ago (Champion QL78YC). I gapped them at .040 instead of the recommended .030 as I do a lot of trolling at slow idle. Has run fine like that until now. Any chance that could have any affect? Will try to get down to the Marina today and check plugs back to coil, if the girls can keep Momma busy on her day. Could be risky tho hehe.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Ok...was able to check plugs. Getting good spark on both plugs. Got a manual on order, but looks like Im gonna miss some good boating time til it gets here. Anything else to try while Im waiting??? Will it hurt it to run it a low rpms?

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Does anyone have any idea what could be the problem? Never had any trouble with the engine before. Have had all the maintainence done. Do you think it is a serious problem?

 

[twista_02]

Re: Shimmy and Shake at mid-RPM's

Does it do it all the time or only when it is in gear?

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

I put it in neutral and using the fast idle lever it did the same thing. When it hits the shaking RPMs it doesnt rev any higher regardless of throttle. Just goes into a pretty violent side to side shake.

 

[Johnson70]

Re: Shimmy and Shake at mid-RPM's

Long shot....Had this issue on another motor once. Possible that you have water in the gas? Mine would be fine up till about 2000 then would bog down....just a thought

 

[twista_02]

Re: Shimmy and Shake at mid-RPM's

Does it sound like it is missing when it starts to shake.

 

[Walker]

Re: Shimmy and Shake at mid-RPM's

Sure sounds like S.L.O.W. system is kicking in. I'd check spark at the rpms where it starts to act up. The S.L.O.W. system is in the power pack and can go bad just like any other ignition component.

 

[ob]

Re: Shimmy and Shake at mid-RPM's

Just to rule out fuel retsricted carbs or failing fuel pump,next time you run it and try to fast idle it to the point of it not responding ,as Rick mentioned,bump the primer button and see if you get an increase in rpms.If so ,you likely have one or more fouled carbs.Then ,during the same test run ,try pumping the primer bulb and see if it responds. If so,the fuel pump may be the culprit.<br /><br />Sure sounds like an ignition issue though.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Thanx for the quick responses. Will check out your suggestions and report back as soon as I can.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Ok...finally got to go work on it for a while today. You were absolutely correct rickdb, it is going into S.L.O.W. mode. It cranked right up and ran nicely for about 25 mins. then went back into S.L.O.W. It doesnt seem to be getting hot. No alarms are going off and the system passes the self-check when i turn the key to on.<br />I let it rest for 5 mins and it resets. I was able to run for a good 45 mins at about 45-4800 RPMs but if I bumped up to 5200 or more it would go back into S.L.O.W. Also if I slowed down to 3500 same thing. Manual should be here 1st of the week so hopefully I can find something that makes sense. If anyone has any ideas in the meantime, I would be most thankful.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

I did the fingers on the head thing, and no problem holding for a count of three.

 

[ob]

Re: Shimmy and Shake at mid-RPM's

The first item that I would suspect is the temp sensor if it is indeed going into the s.l.o.w. mode.However if this is the case ,the constant alarm should sound.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Ok, I finally got the OMC manual today, but now I am even more confused. The manual states that when S.L.O.W. occurs, the engine will gradually drop down to a maximum of 2500 RPM,s. When my problem occurs, I am running at around 5000 rpms and it starts running extremely rough and drops to about 3300 rpms. No alarms, no warning lights.<br />I drop the throttle to idle and the engine smooths out and will run smooth up to 2500 rpms. If i give it more throttle, it goes into the rough running and will not go past 3300 rpms. <br />I tried pumping the primer bulb, and hitting the primer button, no effect. What baffles me is the inconsistency at which it occurs. I ran last weekend for almost 2 hours before it happened. Let her sit for 15 minutes and ran great again for about 10 minutes. Another 15 minute rest and ran for 45 minutes. No pattern that I can find. Any ideas or places to start will be most apprectiated.

 

[ob]

Re: Shimmy and Shake at mid-RPM's

First thing you absolutely have to verify is whether the engine is overheating when it drops back in RPMs.For this you will need either a thermo melt stik of 163 degree to mark or touch check the engine or a temp gun that you can buy or rent.If you can establish that it is not overheating(excess of 160 degrees) during a water test,then you can try disconnecting the temp sensor for a spell and see if your symptom clears up.Touch checks are fine but by the time you can get your fingers on the head covers after a higher speed run the engine has had an endless supply of cool water pumped thru it.Temp stiks will tell you what the temp 'was' if it overheats ,not just what it is when checking.<br />The S.L.O.W. system gets its signal from the temp sensor ,so that's where I would start.<br /><br />If this test doesn't confirm overheat or disconnecting the sensor has no effect ,it may be another ignition issue such as an intermittent spark loss on one coil or a failing powerpack.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Thanx ob. Will check the temp out this weekend. Will have to find a place to rent a temp gun as nobody at the parts stores know what im talkin about when I ask for thermo melt stiks. Dang young uns lol.

 

[Ralphy]

Re: Shimmy and Shake at mid-RPM's

Ok got an ifrared thermometer today. Ran for 45 min. and went into chughachug mode. Ran over entire motor and hottest spot was 153*. Disconnected Temp sensor (Tan/blue wire) and it started right up, but only went about 15 mins be4 going back into chughachug mode. So I disconnected Solid Tan wire. Ran without a problem for a good 45 mins, but I ran out of time so not sure if its that. Dont think so as it still connects to Temp sensor, but goes behind some sort of terminal bank it looks like. Since it does not appear to be heat related, I'm guessing must be ignition system? Any help is most most most appreciated.