1977 Johnson 15hp Carb rebuild

[dwoehrer]

I bought a 1977 Johnson 15R77M this past summer and got it to run but had to constantly jack with the choke to keep it running. Lurking around the forum boards I first performed a decarbonization process on the motor. Ran a little better but not great as it still wouldn't run without messing with the choke. I decided then to get a carb kit for it and get into the internals of it. I got an OEM carb kit for it and read up and watched a couple youtube vids. In the kit was a very small needle valve bearing and I thought well those are generally tricky to pull out so I will just leave it. I pulled the needle valve out and shot some Gumout carb cleaner down the throat of where the needle was and out walked onto the red straw of the carb cleaner part of the old needle valve bearing. Oh crap... Ok, I made sure that all of the pieces were out and went to the schematic and found out which way the needle valve bearing went down into the carb. Small end first. I took a 10 penny finishing nail, set the valve bearing on top and proceeded to seat the needle valve bearing with some slight hammer blows. I screwed the needle valve back in till it just bottomed out and backed off 1 1/2 turns. The before and after pictures that I took do not match up. My needle valve in the after pictures show approximately 2 or 3 more threads protruding from the body of the carb than the before pics. I won't be able to start it up for a few more days but I am nervous that I might have done something wrong. Specifically seating the needle valve bearing. It did not go gentle into the night like I thought it would. I didn't have to go gorilla crazy on it but the taps of the hammer seemed excessive to seat such a small plastic part. I don't see how it would come out next time. Any comments?

 

[flyingscott]

By banging it in with a hammer you may have crushed it, never hit those with a hammer. Hopefully you did not damage the needle hole. I would pull the bearing back out using a fine pick or look down down the hole and see if it is crushed.

 

[dwoehrer]

I looked down into the needle seat after inserting the bearing and it looked intact with no visible signs of damage. That being said I could not see the end up against the back wall of the chamber. The needle now does not go in as far as the original setting. Maybe because the original bearing was in pieces... I don't know.

 

[boobie]

You'll find out after you run the motor.

 

[oldboat1]

I agree with Boobie -- try it out and see what you have.

But if you need to revisit the fix, I suggest you pop the domed cap on top and take a peek from inside -- and make sure the passages under there are open, and no debris floating around. I would test fit the needle as well that way, looking for the point to just peek through. It sounds like there is currently a risk of screwing it in too tightly.

If you already used the fitting from the carb kit, you can repair it if you don't do too much damage removing it. I usually drill them carefully, then use a drywall screw to pull them out. To replace, use a dab of epoxy or similar to seal the hole.

 

[dwoehrer]

Thanks for the response. Yeah I thought about removing the welch plug and actually seeing if the tip of the needle can be seen, but it's all reassembled now. In retrospect I should have checked that first before pounding the welch plug back in. I just didn't expect such a tight fit from the needle bearing. Maybe next time a light coating of oil and it would go in easier. I've taken it apart and put back together so if I need to do it again It won't be my first rodeo. Will fire it up in a couple days and post back.

 

[dwoehrer]

FYI: Started up on third pull. I really can't discern any difference in low idle on a 45 degree swing either way on the needle valve. I expected a noticeable cough, rpm change or the like. All in all, pretty happy.

 

[oldboat1]

If the motor is doing what you need it to do, that's fine of course. But IMO it might not be operating through a full range, based on your description. It sounds to me like the idle mix adjustment is not working. Throttle it down as low as it will go, maybe adjusting the throttle stop to let you really idle down low and slow. Then turn the needle clockwise until it's closed (lightly seated). The motor should stall -- assuming it's still running after throttling it way down. Then back off maybe 3/4 turn and try to start and run. If it won't start, open it up slightly and try again. 1/8 turn at a time.

You need to be doing this in gear, engine warm with back pressure (i.e., in a barrel or in the lake). On muffs usually won't let you fully dial it in, but even on muffs the idle needle should be responsive.