Stumped on how to fine tune idle jet on 15R75C Johnson

[Xeenan]

I recently rebuilt the carb in my 15R75C Johnson. I did like it said in my shop manual to lightly seat the idle jet and back it out 1.5 turns. I then installed the idle screw knob back on with the arrow facing straight down, so I would have room to fine tune. Now I read on a web page that I needed to lean it out in small amounts till the motor either tried to die or heard a mild backfire. Then Rich it and within 1/4 of a turn would be my sweet spot.

lol well the damn thing runs so good after the new fuel pump and carb rebuild that it wont even fart to tell me when to back it out. It runs great on the reset idle placement, so should I just leave it there or am I even doing it right.

 

[sutor623]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

Great!! Sounds like you did a great job. When you tighten that lean knob all the way, the motor doesn't try and die on you? If it doesn't then I would back it out 1/4 turn from being tightened all the way and call it a day.

 

[F_R]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

Surely it will lean out and sneeze if you turn it all the way in. If not, something is wrong and getting gas from where it isn't supposed to.

I think the OP is saying it won't sneeze when turned to the limit of the knob (??). If that is the case, pull the knob off and reposition it to where you can turn the needle a bit more. You are doing it correctly, just putting too much credibility on that 1.5 turns rule. That's just a place to begin.

 

[HighTrim]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

Did you give it plenty of time to respond to your needle adjustments, like 30 secs between?

I would not leave it at 1/4 out from seated, much too lean. Maybe you are not actually seated? Is it richening up from another source?

You pretty much had the procedure down pat. Start at 1/5 turns out from lightly seated. Lean it out (clockwise) 1/8 of a turn at a time, giving the motor time to respond, until you hear the mild backfire. Then back it out 1/4 turn. WITHIN that 1/4 turn is the smoothest idle setting. I do all of this with the knob off so as not to limit the travel of the needle.

 

[raczekp1]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

did you install small bushing on the slow speed needle end.
this bushing goes into bottom of slow speed needle chamber?
its part no 10

 

[Xeenan]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

Yes I installed a new bushing that came with kit.

I have to admit one thing, lol I didnt richen (clockwise) the knob any from idle reset only leaned (counter clockwise) it. I guess I need to try that lol

 

[Joe Reeves]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

NOTE... Turning the slow speed needle valve "Clockwise" leans the mixture, "Counter Clockwise" richens it.

(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 2: If the engine should be a three (3) cylinder engine with three (3) carburetors, start the adjustment sequence with the center carburetor.

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.

 

[Xeenan]

Re: Stumped on how to fine tune idle jet on 15R75C Johnson

Ok I just realized I was definatly turning it the wrong way. Ill have to use your advice next time im out on the lake. thx