2000 Johnson 150 throttle pulsing

striperday

Cadet
Joined
Jun 2, 2011
Messages
23
I was able to locate the source of fuel leaking around the gasket located between the carburetor and the carburetor gas bowl on the bottom. This was last June 0f 2013. I parked the boat in the garage and decided to not mess with rebuilding the carburetor until after deer season was out. I decided to practice some preventive maintenance a few weeks ago and went ahead and ordered new kits and rebuilt all 6 of the carburetors which went well. No problems encountered at all.

I had no problems at all with the engine BEFORE I rebuilt the carburetors other than gas was pouring out of one of the carburetors due to a torn gasket but it ran GREAT!

Yesterday for the first time since last June I decided to take the boat for a spin to see if everything was running as it should. The boat started up and idled beautifully!! I eased into the throttle and jumped up on plain and was amazed at how well the boat was raunning. HOWEVER I noticed that the boat would reach around 5000 RPM and the motor would start to throttle on its own up and down ranging between 200 and 500 RPM. It would drop all the way back to around 4000 RPM while the throttle was wide open and run as if I was on the throttle moving in up and down.

Please note the motor never had a miss fire in any way. AS I STATED IT WAS ACTING AS IF I WAS MOVING THE THROTTLE UP AND DOWN BUT NO MOTOR MISSING AL ALL. GOOD POWER TO.

The motor final cleared up and ran like a scalded dog as if nothing had ever happened for about 10 minuts. Then all the sudden after taking off from idle speed, out of no where it dropped back to around 4000 RPM and the throttle started pulsing again.

Any help would be appreciated !!! Thanks a million people!
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 2000 Johnson 150 throttle pulsing

Sounds like it's losing a cylinder. Do a cylinder drop test on it and see if you can find out which cylinder.
 

striperday

Cadet
Joined
Jun 2, 2011
Messages
23
Re: 2000 Johnson 150 throttle pulsing

Sounds like it's losing a cylinder. Do a cylinder drop test on it and see if you can find out which cylinder.

I performed the test this evening and found that the RPM dropped approximately 300 from normal idle on each wire pulled. As I stated I never felt a miss fire when I was running the boat on the water. It was as if I was fiscally moving the throttle up and down in 2 second bursts.
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 2000 Johnson 150 throttle pulsing

Try doing it when the motor is acting up and see if you have dead cylinder.
 
Last edited:

striperday

Cadet
Joined
Jun 2, 2011
Messages
23
Re: 2000 Johnson 150 throttle pulsing

I guess I'm a little slow catching on, but when the boat is runing 4000 RPM it's traveling at 40 MPH. How do I check cylinder fire under those circumstances?
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,033
Re: 2000 Johnson 150 throttle pulsing

Use a timing light !!----It will go dead if you loose spark on one cylinder.---Test all 6 cylinders.
 
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