WOT RPM 1989 JOHNSON 120HP

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
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
What is the gps speed at 4100?Rpm without the gps speed doesn't mean much. Not unusual for a tach to be off 500 or 1000.
GPS speed can help see if the tach makes sense.
The 120 is rated at 5500. I would guess rpm range is 4500 to 5500.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Most cells can acquire a gps.
At 4100 rpm you have more than a prop problem.While the 17 may prove to be too much.
You aren't likely to get to 5500 with a prop change.
There is a possibility you need top go to a 15" prop. when all is said and done.
I suspect either a boat or a motor issue.
 

misdeal

Seaman
Joined
Jul 14, 2009
Messages
60
Will try and pull up gps on my cell next time im out,,just a little more info,,,boat has new paint on bottom,motor runs like a top, fires right up, don't smoke etc,,,that being said it is old,,and has the worst hole shot you've ever seen, takes for ever and ever and ever and dare I say ever to plain out,,,as far as a new prop was thinking of dropping to a 13
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Taking for ever and ever ??.. to plane sure indicates more trouble than a prop.
Older boats sometimes have absorbed water in the foam under the floor This can turn the boat into a slug.
Raise the bow on the trailer take the plug out and let it drain into something. Once loose water is gone it should stop dripping.
Of course the motor could have issues as well.
Does the throttle open all the way? Good compression?Firing on all cylinders?
Timing set right and advancing as it should.
A 13" will surely help the hole shot but is just masking a problem
 
Top