WOT RPM 1989 Johnson

misdeal

Seaman
Joined
Jul 14, 2009
Messages
60
Hey folks,,i have a 1989 120hp Johnson model # j120txcem I have a 13.5 x 17 prop on a 20' bow rider and can only get 4100 rpm out of it with just me in the boat, as of yet I don't have a engine manual and was wondering if anyone could tell me what the factory wot rpm might be for this motor,,,,thanks for the help
 

TD_Maker

Chief Petty Officer
Joined
Aug 31, 2005
Messages
564
That motor should be able to turn 5500-5800 RPMs all day long. At 4100 RPMs you are seriously lugging the motor and this is not good.

1. I would make sure your tachometer is working correctly. Make sure the correct pole setting is selected on the back of your tach.
2. Are you sure you are hittting on all 4 cylinders? A motor running on two or three cylinders will also cause this.
3. If you are running a 17 inch pitch prop, for every size down, you will increase your RPM's about 200 RPMs. So my gut feeling is there is something else wrong.

Finally...are you sure you don't have a water-logged hull? A big ole heavy boat will never allow you to turn enough RPMs. I once had a 24ft Robalo Center Console... the foam filled liner adsorbed water and made the hull hundreds of pounds heavier than it was supposed to be. My boat was just a big pig.

Just a few thoughts coming from a little experience. I hope I have helped a little bit.
 
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