25 hp Johnson Rough Idle

[teewhy]

Looking for help for why my 1954 Johnson 25 idles rough then quits. I've installed new coils, new condensors, new points (gapped to .020), new plugs (gapped to .030), newly rebuilt carb (installed a new float, needle and seat, gaskets, high/low speed needle packing, and cleaned with carb cleaner). The engine has great spark and compression (although don't know the exact reading).<br /><br />The carb's idle speed adjustment needle was gently seated then backed out one turn. The high speed was backed out 1 3/4 turn. The pressure tank supplies the fuel filter with plenty of fuel and has no apparent leaks in the hoses.<br /><br />The engine starts after two or three pulls but, the power head seems to vibrate excessively. When I slow the idle speed, it begins to cough and then quit.<br /><br />Have I missed something? Any suggestions? Thanks very much for any help.<br /><br />teewhy

 

[alcan]

Re: 25 hp Johnson Rough Idle

Hi Teewhy<br /> It sounds like you got it pretty close. I will usually take the low speed needle out 1 3/4 turns. You may be a little lean. I like to set the high speed first. Run at full throtle and lean it out as far as you can. Then back it of a little (1/8 turn) maybe. Then adjust your low speed. Be sure to adjust slowly, give the motor time to clear the previous mixture. Also when you went through the carb did you replace the little round gasket on the main nozzle? Make sure your new needle packing is snug. Make sure the o-rings in the fuel connector are good (not leaking air). No other air leaks. Check the link and sync, wear on the throtle roller,(cam follower). You may have to fudge a little on the point adjustment one way or the other, usually wider .022 on new points. Take your time with this, you'll get it tuned. One last thing, very last thing, replace top and bottom main seals.

 

[teewhy]

Re: 25 hp Johnson Rough Idle

Thanks for the speedy response, alcan . . . much oblidged.<br /><br />I think you may have hit on a couple of things. First, I didn't take very much time in setting the idle adjustments (letting them clear out previous mixutre) - I'll try this again.<br /><br />As for the 'round gasket on the main nozzle', not sure which one you mean. The gaskets I replaced were the one for the glass fuel filter bowl, the needle packings, the small one under the needle seat, the large one between the float chamber and carb body, and the one between the carb and the powerhead. Are there more? Thought I got 'em all.<br /><br />Tonight I'm going to check for air leaks anywhere and everywhere and spread the points just a little bit more (this is proving to be tedious). Last thing, I know these older motors burn pretty dirty and some oil in the test tank is expected. However, I seem to get a slick after just a few minutes. Is this normal? Could this be a result of my carb's needle settings?<br /><br />Thanks again!

 

[alcan]

Re: 25 hp Johnson Rough Idle

Hi Teewhy<br /> Yes this can get a little tedious, be cool, it will come. Work on one area at a time. Fuel adjustment first. Work through the entire deal. If you start jumping around it going to be hard to tell when you get it right. When you get tired or frustrated or both leave it and go watch TV. I wish I could follow that advice. The main nozzel is what hangs down in the bowl (center) and draws fuel from the high speed needle. There should be a small gasket (washer like) between the bowl and the nozzle. With out this gasket you will nor get the motor to respond to needle adjustment very well if at all. Yes your test tank will look like the wreck of the exxon valdez. Usually the situation you describe is fuel related.

 

[DJ]

Re: 25 hp Johnson Rough Idle

Ditto Alcan.<br /><br />I think you have a "lean sneeze". Not uncommon on big twins.<br /><br />A little more fiddling (fattenning up) of the low speed jet should help out.