Help with 78 Johnson 25hp revs up and down while at constant throttle

[edstanley]

I have a 78 Johnson 25hp that I'm trying to wake from the dead. It's been sitting for 15 years. I've replaced the wiring harness, fuel pump, plugs and rebuilt the carb. I can get it to start then it revs, acts like it starves for fuel then revs again. This is all with the throttle at half or more. With the throttle closer to idle, it cranks, revs and chokes down. If I bump the choke when it chokes it will rev again then choke down. As long as I keep bumping the choke it will keep running but it's up and down. Has anybody got any advice for me. This thing is about to drive me nuts. Thanks!

 

[racerone]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

Having to use the choke clearly points to a fuel problem.----------A lack of fuel due to blockage in the high speed jet Or possibly just too lean on the low speed adjustment needle.---Did you remove the high speed jet from the carburetor bowl to clean it ?

 

[edstanley]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

I didn't remove the high speed jet because I couldn't get it out but I did get it clean by flushing it with carb cleaner. I got it clean enough where I could hold a flashlight up to the drain hole and see light passing through the jet at the bottom of the bowl. I have adjust the low speed needle from a half turn out to 3 turns and it doesn't seem to make much difference.

 

[James R]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

This sounds typical of fuel starvation caused by poor fuel flow from the tank to the pump. A clogged filter in the fuel line, a clogged pick up tube, a bad fuel line and bulb. You can check all of these by using a new fuel line and bulb connecting directly to the fuel pump with the input end of the fuel hose into a jug of fuel mix. I you did not replace the motor fuel lines then the carb is probably blocked again. The old lines will be perished allowing lining particles to block the carb. Always use Stabil Marine to keep your fuel fresh, your carb clean and minimize the damage done by Ethanol.

 

[wrench 3]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

Did you remove the small core plug on top of the carb closer to the engine, when you rebuilt it? The slow running circuit goes through there and is impossible to clean out properly without removing and replacing the plug.

 

[edstanley]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

Yeah wrench 3 I had to take it back apart and remove it because the tip of the low speed needle got stuck and actually came out of the brass and stuck on the hole. I took the plug out so I could get the needle tip free and replaced with a new needle. While I had it out, I blew everything out with carb cleaner. All fuel lines are new. The bulb is a couple of years old but it has only been used to crank the boat a few times. I think I'll try running a line straight to the pump like James R suggested. James R, I'm a believer in Stabil also. I run it in everything I own except our daily drivers.

 

[oldboat1]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

have had the same experience with a broken low speed needle -- assume you got the plug out, then pushed the tip of the needle out. That is, you didn't have to drill it out, which can create problems. It sounds like you have done what you should have with the carb, except the high speed jet. The fact that it is stuck in there indicates to me that there is some varnish in there that could still contribute to problems (would have to soak it, then try to remove it). If adjusting the low speed needle doesn't affect operation, you might be back to square one. Given the work you did so far, I would go with James R's recommendations, and if there is still a problem go back in for that high speed jet. You may want to be sure that reassembly was correct (float installed correctly -- possible to put it in upside down), and may need float replacement if you haven't done that. The old cork ones lose their coating.

 

[boobie]

Re: Help with 78 Johnson 25hp revs up and down while at constant throttle

Two things. Take the carb bowl off and run a packing wire through the high speed jet. Next pump the primer bulb once and see what happens.