20 hp Johnson Carb problem

[Relic68]

Hello I have two questions,
1. I have a early 80's 20 hp Johnson engine, it starts fine idles ok but when I give it gas it boggs down and has very little power, I've cleaned the carb really good(I think) and if I play around with the throttle plate linkage it seems to help a little. Does it sound like a carb or linkage problem.
2. I here that the only difference between a 20hp and 35hp is the carb, if I replace the carb with a 35hp will it work and will it make it a 35, sounds too easy but it all over the web.
Thanks

 

[F_R]

Re: 20 hp Johnson Carb problem

Did you inspect the high speed fixed jet in the bottom of the carb bowl? Do you know if your motor is perhaps running on one cylinder? Or perhaps one and a half cylinders, so to speak?

As for the other question, maybe and maybe not. Go ahead and buy a carb and try it and let us know the (true) results. If you can tell the difference between 25 and 30 horsepower with the same exact motor and boat combination, you are sharper than I am.

 

[tashasdaddy]

Re: 20 hp Johnson Carb problem

did you do all this. http://forums.iboats.com/showthread.php?t=244013

it also has an adjustable, slow speed jet.


(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.

Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.

Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.

When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.

 

[HighTrim]

Re: 20 hp Johnson Carb problem

Couple of other things to look at if the carbs check out. Check to see if the timer base moves smoothly without sticking when you advance the throttle. Take the airbox cover off & observe whether the throttle plates are parallel.
Also, the timer base should start it's travel before the throttle plates start to open, otherwise the old girl will die when you give her the juice.

 

[bgbass.1]

Re: 20 hp Johnson Carb problem

You have to change the intake to a 35 intake to acomadate the 35 carb.

 

[Tinman57]

Re: 20 hp Johnson Carb problem

I had a mechanic change a 1987 20 hp Evinrude to a 25 hp, which required a new intake manifold. The carb remained the same.

However, the mechanic said it is much more difficult to change a 20 to a 35. That would require a new carb, a new prop and a new intake manifold. There may be more, but I don't remember.

 

[Relic68]

Re: 20 hp Johnson Carb problem

Thanks for the replys looks like I have some work, once I find a carb kit.