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1998 Johnson 40HP low RPM

S

stereosympathy

New member
I am new to outboard motors (and motors in general) and I just acquired an 18 foot pontoon with a 40 HP Johnson on it.

The motor starts up first turn, shifts fine and generally seems to be running well. The only issue I have noticed is it doesn't seem to throttle up all the way. Thinking maybe the cable has stretched and is preventing it from fully throttling wide open. I am getting about 3000RPM with the throttle fully forward and only me (180lbs person) in the boat. I found a chart showing this engines RPMs in the range of 4500 to 5500RPM depending on load.

I am not sure where to begin on this engine when it comes to adjusting the throttle. If anyone could point me in the right direction it would be very appreciated. I have attached a picture of the throttle for reference. Really just not sure what to adjust here, and I don't want to screw up a currently working engine. Picture shows engine in neutral with wide open throttle.
IMG_2682.jpg
 
Do some trouble shooting before fiddling.----Do a compression test and report the numbers.----Does spark jump a gap of 3/8" or better , yes or no ?----What prop is on it , post those numbers.-------Oil injection in service and working properly ?-----Water pump impeller replaced when you bought this boat ?
 
Thanks for the reply! I am not at all a mechanic, but I will pick up a compression tester and run a compression test. Spark plugs were replaced, and the spark is strong, over 3/8. Impeller was replaced when I got the boat and it does not have an oil injection system. I am learning as I go, and would like to gain more knowledge of how this motor works.

To my non-mechanic eyeballs it looks like there is more travel to the throttle. It also feels a bit loose when moving the throttle forward and backward. I have heard that throttle cables are prone to stretching and I don't think this one has been serviced or touched in many years.
 
?---Well ,----- I believe you need to review on this.-----The cable pushes the throttle open.----So how could a stretched cable be " bad "-----Inspection and trouble shooting is what you need to do !----Post #1 says ---" neutral with wide open throttle "-----Well, most gearboxes will only allow part throttle when in neutral.-----Sorry, there is lots to learn about operating an outboard.
 
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I should've realized the throttle cable was pushing, not pulling. My mistake. The cable pushes the same distance whether the engine is in neutral or in gear. The picture I took just happened to be when the engine was in neutral. It seems to me that the travel of the cable is not moving as far as it could, which makes me think this is the cause of the low rpm when wide open. When I am in neutral and the throttle is set to idle position the idling seems a bit low as well.

Perhaps I am looking in the wrong direction. Can you give me an idea of what else might cause low rpm at full throttle as a starting point for diagnosing the issue?
 
racerone said:
danny----Post all the information you can.----Model # is ?
Click to expand...
Its a 1998 40hp Johnson 2 stroke with the VRO deleted. I cannot find the model number anywhere. Motor runs great at idle and up to probably 3000 rpm, but when i give it full throttle it wont go any further. Doesn't quite get on a plane, it has electric trim and iv spend hours on the water moving weight around but it wont get up. Prop is 11-3/4x17. Boat is a 14ft Aluminium hull. The motor will rev further with no load on, iv pulled the carbs off and cleaned the jets that sit in the bottom of the float bowls as one of them was a bit blocked, but didnt solve the issue. Im going to take it out again tomorrow and check the butterflys are opening fully, and iv just put new spark plugs in too. Iv been told things along the lines of checking spark advance, but have no idea how to check for it. As a diesel mechanic, im thinking its a fueling issue. Primer bulb primes good and stays firm, but im not fully happy with the lines and fittings from the tank, so will change those out when i can make it to a boating shop. Im just slowly working my way through the motor bit by bit. Any suggestions will get greatly appreciated and will definitely be looking into them.
 
Perhaps you bought a problem?----Post compression values FIRST.----Does spark jump a gap of 3/8" on both leads ?----Check flywheel key.----The lower units on these are NOT robust.---Drain / change gear oil using best factory oil.----Perhaps fuel pump diaphragm is ruptured and easy to check / test that.
 
Just had it out on the water again. Theres no air filter, should have noticed that before, regardless... when i removed the filter cover and exposed the carbs and ran it. I managed to get it to rev a bit more and got an extra 2 knots of speed. Jets are spraying evenly. Spark is good too. Im thinking the carb mixture is too rich and is bogging due to to much fuel. Plus the old spark plugs were very dark. I tried to tweak the carbs while on the water but didnt have a screw driver that would fit. Hoping thats my issue. Would make sense, as the extra air would lean out the mixture a bit and make it run a bit better. Does that sound at all possible/correct?
 
Are the nozzle gaskets in place in the carburetors?-----And what number is on the high speed jets?----The carburetors only have low speed mixture screws.----These motors have NEVER used an air filter either.----Checked the flywheel key?----Post the compression values here !
 
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