15 Johnson stop idle, any ideas ?

[Sylva-Ranger]

1994 15 hp Johnson 2 strokes, start good, run ok a hi or medium speed........But idle, run good for a short period (1-3 minutes) and stop sudently.

​Need choke to restart, It' like lean mixture. after restart run good at all speed but stop again if idle for 1-3 mins

​Carburetor is very clean , try many diferents adjustements for idle mixture screw and/or float setting and always the same........... ......Synchro for pick-up point is ok.

​fuel pump pressure is good, when stop the carburetor not empty of gaz. the primer bulb always stay normal (hard).

​Try another fuel-tank/hose whit fresh gaz and do the same.........



​Thinking it's air leak someting........Not make pressure/vacuum test but i Removed the powerhead, put new gaskest,crankshaft seals and piston rings and no change, exept maybe little bit more smoth idle than before, but stop again after 1-3 mns of trolling.

........Reeds valves, cranshaft bearings etc are good. Exhaust is clean (no carbon).



Do the same whit the hood on it or not.

​It's like the carb itself have someting .........It's the carb whit plastic cover on the top.

 

[kbait]

Look for hairline crack(s) in the carb body cover (not the air silencer box, but the plastic carb top that has the low speed mixture needle threaded into it). A crack that leaks air into idle circuit will cause a lean condition at idle or slow speeds..fairly common issue. You can try to seal crack w/JB weld, or replace it part #436762. If no cracks, check plug bolt/seal on front of that cover, as it needs to be sealed.
Good luck!

 

[AlTn]

making a couple of assumptions...the adjustments for the '94 and '92 models are the same and you are using QL77JC4 gapped at .030"...run the engine at 1/2 throttle to reach normal operating temp...turn the idle speed adjusting knob < on the end of the tiller handle > CCW to full SLOW position...run the engine at slow speed and adjust the slow speed MIXTURE needle until you obtain the highest consistent RPM..after you obtain this setting, turn the idle MIXTURE needle CCW 1/8 th turn to prevent a too lean mixture...now, remove the mixture knob and reposition it so the pointer is facing down < don't disturb the setting while doing this >...reduce the engine speed and turn the IDLE SPEED ADJUSTMENT KNOB to run at ~650 rpm with the engine in FWD...run the engine near WOT for 1 minute then quickly reduce the engine speed to SLOW and shift into NEU..the engine should continue to run smoothly...if it stalls or pops the mixture is too LEAN..turn the idle mixture needle 1/8 turn CCW..wait 15 secs or so..and keep adjusting 1/8 turn as necessary to get a smooth idle in neutral...repeat wot run for 1 min..quickly reduce the throttle and shift into neutral..adjust idle mixture as above until you get a smooth idle...in effect, you are trying to achieve the highest idle rpm w/o being too lean with a warm engine in neutral and the idle speed control adjustment to its OFF position..don't be too surprised if the idle mixture needle is 4 or more turns out from a lightly seated starting point...as you increase the rpm for trolling, you may find you need to adjust the mixture too to find a sweet spot..sorta common dilemma with 2 strokes in general in my experience

 

[ondarvr]

Primer bulb shouldn't stay hard, after just a few seconds it will be under a slight vacuum and will become softer.

 

[oldboat1]

How about temp? If it's running cold, will be hard to maintain idle or dial in the mix. Nozzle well gaskets in place? (Probably would affect full range, but maybe more pronounced at idle.)

 

[racerone]

There is no nozzle well gasket on a 1994 model carburetor.

 

[Sylva-Ranger]

making a couple of assumptions...the adjustments for the '94 and '92 models are the same and you are using QL77JC4 gapped at .030"...run the engine at 1/2 throttle to reach normal operating temp...turn the idle speed adjusting knob < on the end of the tiller handle > CCW to full SLOW position...run the engine at slow speed and adjust the slow speed MIXTURE needle until you obtain the highest consistent RPM..after you obtain this setting, turn the idle MIXTURE needle CCW 1/8 th turn to prevent a too lean mixture...now, remove the mixture knob and reposition it so the pointer is facing down < don't disturb the setting while doing this >...reduce the engine speed and turn the IDLE SPEED ADJUSTMENT KNOB to run at ~650 rpm with the engine in FWD...run the engine near WOT for 1 minute then quickly reduce the engine speed to SLOW and shift into NEU..the engine should continue to run smoothly...if it stalls or pops the mixture is too LEAN..turn the idle mixture needle 1/8 turn CCW..wait 15 secs or so..and keep adjusting 1/8 turn as necessary to get a smooth idle in neutral...repeat wot run for 1 min..quickly reduce the throttle and shift into neutral..adjust idle mixture as above until you get a smooth idle...in effect, you are trying to achieve the highest idle rpm w/o being too lean with a warm engine in neutral and the idle speed control adjustment to its OFF position..don't be too surprised if the idle mixture needle is 4 or more turns out from a lightly seated starting point...as you increase the rpm for trolling, you may find you need to adjust the mixture too to find a sweet spot..sorta common dilemma with 2 strokes in general in my experience

​Thats the way i ajusted the carb,. and i try many times little bit more rich or lean and dont thinkit just a question of adjustment.......

 

[Sylva-Ranger]

How about temp? If it's running cold, will be hard to maintain idle or dial in the mix. Nozzle well gaskets in place? (Probably would affect full range, but maybe more pronounced at idle.)

​The temps is ok, the thermostat keep the engine running at the right temperature....

 

[Sylva-Ranger]

Primer bulb shouldn't stay hard, after just a few seconds it will be under a slight vacuum and will become softer.

Thats what i mean by "normal".

 

[Sylva-Ranger]

Look for hairline crack(s) in the carb body cover (not the air silencer box, but the plastic carb top that has the low speed mixture needle threaded into it). A crack that leaks air into idle circuit will cause a lean condition at idle or slow speeds..fairly common issue. You can try to seal crack w/JB weld, or replace it part #436762. If no cracks, check plug bolt/seal on front of that cover, as it needs to be sealed.
Good luck!

​Not seem to have cracks on that cover but me to i think my problem mabe somewere in that way..........The cover is NLA .....

​A old former OMC dealer told me that a kind of update kit whit a gasket and a more thick plastic cover was available some years ago, but now it's NLA to.........

 

[Crosbyman]

if you suspect a hairline crack try "painting" the cab body with crazy glue to seal the whole outer surface .. if any improvements show up start looking closer for cracks

 

[racerone]

Maybe you just need to increase the idle speed a bit.---What sealer did you use when you installed the crankcase cover and did you make sure all old sealer was cleaned off ?--What type and octane fuel is available where you are ?

 

[oldboat1]

A throttle friction clamp is available on some models (sailboat models, I think), and they are effective for holding throttle speeds. I used one on a smaller trolling motor and liked it -- prefer the clamp with its thumbscrew over the control on the end of the tiller arm, but just personal preference.

You also might consider a different prop. Compared to the standard prop, a five or seven pitch model should let you slow troll at higher rpms, and might be a good choice. I currently use a 5-pitch high thrust prop with a 9.9, and find it works well.

 

[Sylva-Ranger]

Maybe you just need to increase the idle speed a bit.---What sealer did you use when you installed the crankcase cover and did you make sure all old sealer was cleaned off ?--What type and octane fuel is available where you are ?

​It's a the specified RPM , i try to increase a little bit, it may run a little bit longer but the problem is stil

​I cleaned the old sealer and used a good anar?obic sealent like y use on every other 2 strokes engine like snowmobile or outboard for years.

​I use 87 regular or 91 super like other comparable engine in my area.

 

[racerone]

From my many years of experience the motor is running too lean.---Or there are issues with crankcase compression.--Perhaps you are loosing spark on one cylinder.---Put a timing light on it and see what spark does when it stops suddenly.

 

[Sylva-Ranger]

I made test whit starting fluid (ether) around crankcase, intake etc and not seem to have any air leak below wrist-pin.......For the carb itself it hard to test air leak.....And of course it's hard to test the lower crankshaft seal but it's new....

​Engine run in my test tank whit ignition neon testers on each spark plugs and all seem ok.

​When it stop, i need chose to restart like if the carb run progressively dry but is not the case..........Carb's bowl stay full of gaz...

And when i restart, the engine run about the same time before stop. Warm or cold is about the same

​At hi or medium speed it run perfectly.

​I don't have another carb whit a plastic cover to make test............Do you think an older 9.9 hp carbs from late 70's can be use just for testing purpose ?

​

 

[Sylva-Ranger]

If not the carb, the next step is removing the powerhead and make a pressure/vacuum test

 

[boobie]

Pressure/vacuum test where ??

 

[Sylva-Ranger]

Pressure/vacuum test where ??

​You put special plates to plug the exaust and behind the carburetor, you put a certain amount of air pressure by a feeting and see if leaking , for that you can use a pressure/vacumm tester like you use for checking outboards lower unit seals .

​This is a good test when you suspect air leak in a 2 strokes engine. Popular test to check seals or other leak on snowmobile engines , in this case this test is easy to do because on a snowmobile you dont have to remove the engine.

 

[boobie]

Now I understand what you mean. I did the same tests years ago and they found the problems. Only PIA was making the plates.