Idle Too Low - 50 HP Johnson

[Sureshot]

Hey everybody,
I rebuilt my Johnson 2000 50 HP (J50PLSSM) over the winter and it's been running great... Almost. I have to push the throttle pretty far forward when I shift into forward gear. The idle RPM's in forward gear should be 800 or so but on my tach they barely register 200 - 500. Carbs were cleaned and rebuilt, mixture screws set to 3.5 (suggested by manual). I've moved the idle set screw all the way forward but doesn't seem to make a difference. Should the engine reach 800 RPM with the carb throttle plates closed? Meaning, should the timing advance be all that controls the idle speed? Do the mixture screws help raise the RPM's? Feel like I'm kind of chasing my tail at this point. Engine runs great mid range and higher speeds. Any suggestions would be appreciated.

 

[boobie]

Re: Idle Too Low - 50 HP Johnson

Is it running on both cyls at idle ? Pull one spark plug wire at a time to find out.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

I will try that next time out. What would cause 1 cylinder to not run at idle but pick-up at faster speeds?

 

[boobie]

Re: Idle Too Low - 50 HP Johnson

Carb prob and sometimes ignition.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

So I checked the engine when I was on the lake. Doesn't appear to be dropping a cylinder. Didn't pull the plugs but used my timing light and was getting flashes from the wires when it was running badly in low speeds. Basically now it will hardly run in neutral or in fwd gear at slow speeds. I have to give it quite a bit of throttle to pick it up and then I'm fine. Any other things I can try to get the RPM's up in idle?

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

Anyway to isolate this issue? Carbs vs. ignition vs. other? I'd hate to go back through the carbs if there was another problem I could cross off 1st.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

My bad!!!! Well when I finally did pull the plug wires like boobie suggested, sure enough it's running on 1 cylinder in low speeds. I'm a noob at this, thought seeing the timing light firing meant the cylinder was running but today in my driveway I pulled the #1 spark wire when the motor was idling. The engine died instantly. Then I pulled the #2 spark wire and there was no difference at all. So clearly #2 is my problem. I also fired it back up and sprayed fuel mix in the #2 carb throat and the RPM's picked up considerably. Now, how to fix? Is this a float thing, dirty idle circuit maybe, etc? I cleaned and rebuild the carbs earlier this year and appear to have done a lousy job at it.

 

[boobie]

Re: Idle Too Low - 50 HP Johnson

Go through carb # 2 again with a fine tooth comb and I'm sure you will find the prob. It's the little things that will always get 'ya.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

So I went through both carbs. They looked good. I adjusted the float a little on #2 to make it consistent and put them back together. Had it idling perfect on muffs. Finally got to the lake this afternoon and same crappy idle. Only registers around 200 RPM or so. Once you give it some throttle it does fine. While idling, pulled plug #2 and it killed the engine instantly. Pulled plug 1 and the RPM's dropped and it stalled out pretty quickly afterwards. Seemed to be just a small bit of fuel spitting out of the top carb from the throat. So is there anything I could be overlooking? RPM's should be much higher based on the service manual specs.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

The motor also shudders pretty bad until I get it to around 2000 RPM. Then it smooths out. I tried spraying some fuel mix into both carb throats when I was sitting around idling but the RPM's still did not pick up. I'm stumped.

 

[Sureshot]

Re: Idle Too Low - 50 HP Johnson

Well I made some progress w/ my idling problems. I guess I had my top cylinder too rich so once I turned the mixture screw in a bit my RPM's increased. Couldn't quite get the 2nd cylinder right. It still shudders a bit in very low speeds. Is there a tell tale way of determining if a cylinder is running rich other than smoke? And in general, will leaning out a cylinder's mixture increase RPM's? Thanks to anyone who can provide insight.