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1990 25hp Johnson runs great, but throttle maxes out at 4000rpm

D

drwambo

New member
Hi all,
I have a 1990 25hp outboard and am having issues with getting rpms beyond 4000. Everything says it should be running around 5000-5500 at WOT. The engine runs perfectly. Starts a tiny bit hard and sneezes a couple of times when cold, but other than that runs fine through the throttle range...it just won't make more RPM. Throttle linkage is correct when the throttle is wide open, so I'm a bit stumped. The prop is new, but the same pitch and diameter as stock (I don't remember what it is off the top of my head) I've tried pitching the motor up a notch (it's manual trim) and that made no difference except to slow down takeoff speed. Any ideas?
 
I have a 91 40hp and I am having a similar issue. Mine maxes out at 3000 rpm in forward when in the water under load. But what is weird is in reverse it will go over 5000 when in the water under load.In neutral it will go over 5000 forward and reverse. When not in the water and I just have the hose and Mickey mouse ears on it I can go over 5000 in forward. I had a starting issue last year and I bought service manual. I found the roller bushings and linkages worn causing the timing to be messed up. Rebuilt everything. New throttle and shift cables. New prop, exact same as old pitch and diameter. Took on the water and had no problems. Put away for winter and took it out for this year and having the problem. It runs beautifully but just won't go over 3000 forward in water. I had the top off while in the water and watched the throttle and it goes wide open.
 
Motors have to work hard to move a boat !!!-----So on that 25 HP post the numbers from a compression test here.
 
Vanman-----A motor will rev up in neutral running on just 1 cylinder.----Your test means nothing.----And on the trailer running in FORWARD is still neutral as the prop does no work moving air !----My bet is the motor is running on 1 cylinder.-----So test for the spark.----Post compression test values.-----Perhaps post your issue in a new thread here.
 
racerone said:
Vanman-----A motor will rev up in neutral running on just 1 cylinder.----Your test means nothing.----And on the trailer running in FORWARD is still neutral as the prop does no work moving air !----My bet is the motor is running on 1 cylinder.-----So test for the spark.----Post compression test values.-----Perhaps post your issue in a new thread here.
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Interesting. I had what I THOUGHT was a problem with my top cylinder before I put it in the water this year. When I purchased the boat, I found that the top plug threads were stripped. Chasing them made it worse and I ended up putting in a sleeve. Last year I had issues at the end of the year. Anyway, I did a compression test, and don't remember the numbers, but they were good and close to each other. I have spark and the plugs both look the same black, but the same. I think I'd know if it was just running on one - especially at low RPM. I'll hook it to a hose and check that it is running. It does seem very smooth through the range.
 
racerone said:
Well----Good luck with your motor.----Sounds like you know how to fix it.
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yeah, just pulled the plugs. both seem about the same - but the top plug is definitely wetter, even on the threads. So I will have to run a test again and pray - or just repower the darned thing. I will put on the muffs and do the obvious and pull a wire to see if things change. That's about the limit of my technical ability. I've never pulled a head etc, so it may just be what it is till I can afford to repower. :-(
 
These things are so easy to trouble shoot and repair.-----But when folks make assumptions and do not follow trouble shooting tips I lose interest.----My trouble shooting is based on tears of experience with motors , starting in 1968 !
 
racerone said:
These things are so easy to trouble shoot and repair.-----But when folks make assumptions and do not follow trouble shooting tips I lose interest.----My trouble shooting is based on tears of experience with motors , starting in 1968 !
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Oh I get it. I'm really hoping it's something dumb, but I'm afraid it may be a bigger problem than I realized. I'm fairly ignorant when it comes to engines.
 
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