98 Johnson 60hp Outboard Shudders at High RPM

[seahawk2000]

I have a 98 Johnson 60hp outboard on my pontoon boat. Last year the engine started to shudder at high RPMs (basically full throttle - 4200 RPM or higher). In the years prior to this - this never occurred. Everything runs fine up until that last moment when you get to full throttle (idle and everything in between). The motor is very responsive to throttle changes as well and holds at a designated RPM without issue. I have never received an overheating alarm.

I have taken the following steps to have this problem remedied (by a local marine shop) - but it still remains. Also worth noting - if I disconnect the thermostat - it runs fine.

Replaced water impeller (every 2 years)
Rebuilt lower unit (new gears and shafts)
Replaced prop (it was chipped anyway)
Replaced thermostat

Any ideas what to do here (spark plugs?) - I would not want to run the motor without the thermostat, but having to worry about the shudder is becoming a pain.

Also of note, I am not going to say I have the utmost confidence in this repair shop - but they are the only one in my area. So if I need to go back and check on a few things to make sure they were done properly (or even done at all) - I am happy to do so - but need some guidance as to what to look for as evidence of replacement. I consider myself fairly handy (can follow directions well) - but have never work on a marine engine before and really have no idea where to start.

Thank you for any help you can provide.

 

[etracer68]

Re: 98 Johnson 60hp Outboard Shudders at High RPM

You say, it runs fine, when you, "disconnect the thermostat". How are you doing that, I would think, its in, or out. Not sure there is a connection, to disconnect.

 

[seahawk2000]

Re: 98 Johnson 60hp Outboard Shudders at High RPM

Sorry - I meant that when the thermostat is out of the unit - it runs fine. But putting it back in - new or otherwise causes the shudder to start again.

 

[Chris1956]

Re: 98 Johnson 60hp Outboard Shudders at High RPM

Gee, it sounds like an overheat condition, causing the SLOW system to activate. Test the overheat sensors. Ground the alarm wires and see if the horn sounds. Do you have the system check system?

 

[etracer68]

Re: 98 Johnson 60hp Outboard Shudders at High RPM

A new thermostat? Sounds to me like you may have a cooling issue. What is the head temp while this shutter is going on? You can pick up a temp gun for about $30. I would be checking the head temp when it happens, or your installing the thermostat wrong.

Chris, has some good suggestions also.