understanding idle on 1971 Johnson 50 hp

[fatlenny]

Hello, New here to the Forums and the boating world.

I have always considered myself fairly mechanically inclined in small engines and old auto engines. From a child I have always been able to tear apart my toys to see how they worked and put them back in working order.

That being said, I am baffled by this motor that I recently purchased and how it idles. At home I had got it seemingly in great running order (on muffs). It started right up with a bump of the key even after a couple days of sitting with no prime or choke. I was able to do this by allowing the throttle plate to be slightly opened by the roller that rests on the cam. (I did this prior to to finding the whole sync thing) but after taking it to the water she seemed to idle poorly, and ended up with a fuel line leak. (old lines needing replaced) after getting out a small fire on the water that I thought was a backfire issue at the time, I took her home to investigate further, repaired the leaking fuel line, put new plugs in it, (kind of confused on the plug that should go it is as depending on the place you look it is champion UL-77V or QL77JC4. the later is what I purchased as that was what was available) checked the compression (high 130s) and fired her back up in a 55 gal drum of water. Same issue. And it seems to smoke quite a bit. (50 to 1 on 89octain)

I am going to check how well the spark is but had to help the wife out and stop for the night but here is my first question. How do do you set the idle rpm on these motors when the throttle is not suppose to be touching the cam at idle and there is no adjustment screw to open or close the carburetor butterfly at idle?

This is something alien to to me. Is there some old Johnson witch craft you have to know to understand this?

Leo P.

 

[Crosbyman]

common old style carb with a LS needle on top. have you tried adjusting the needle with small increments

have you serviced the carb at all ?? easy to clean. see carb issues pictures. old carbs but same issues

Yours has no HS needle on the bottom just a fix jet on the bottom.

btw they do smoke in a bucket /drum generate lots of oily goo and inhale their own fumes
so try to adjust it on the water

 

[Crosbyman]

to idle properly the idle circuit of the carb must be clean to allow proper fuel vapors to be generated by the upper part of the carb.

If you feel adventurous pull the dime size plug on top (drill a small hole and use a nail to pop it out do not drill deep !! inspect /clean the small holes and check to see if you do see the LS needle point sticking out the side hole. Remove the needle and blow passages with carb cleaner As you reassemble the carb blow ALL passages including side channel leading up to the top idle drip chamber) . Install a new dime size plug and punch it down just a bit to seal the rim. I usually apply some clear fingernail polish

 

[Faztbullet]

This model looper was bad about skirt wear on pistons(side thrust) and crankcase case leakage. They will idle great on muffs but not in water. One tell-tell sign is if you can keep it running with a lot of timing advance when in gear.

 

[fatlenny]

Thank for the feedback, on the carbs it was helpful to see what is there on these carbs, but my question remains unanswered. let me try this.

All other carbs I have worked on use a set screw that moves the throttle butterfly opened more or less to a position that makes the engine idle at the desired rpm. These carbs seem to have a fixed position the butterfly sits at idle with only a low speed needle to adjust how much fuel is allowed at the set amount of air due to the fixed butterfly at idle.

My experience is this needle is to lean or richen the air fuel mixture in combination with slightly opening or closing the butterfly at an idle, not to set an idle RPM. Is the idle RPM set somehow by the timing advance?

An explanation on this subject is what I am looking for while I am trying to find my timing light. thanks again on the feedback and looking forward for more.

 

[fatlenny]

This model looper was bad about skirt wear on pistons(side thrust) and crankcase case leakage. They will idle great on muffs but not in water. One tell-tell sign is if you can keep it running with a lot of timing advance when in gear.

I can tell you that it ran great mid range and WOT on first pass. Came back to dock to set the trim on the motor down a notch and seemed still fine but noticed idling poor. took the hood off and made an on the water adjustment to the slow speed needles and seemed to improve while in neutral. I dont know if it was just a coincidence, but when I put the hood back on, it idled bad again. Made a second pass and the boat almost instantly jumped up on plane and ran good again at WOT. while idling in gear waiting for my buddy to get the trailer back in the water she started having issues. (Fuel leak, stalling at idle, small fire, no damage, FUN TIMES) Ever since then I have had issues with idle in all situations.

 

[racerone]

On that motor the throttle plates must be closed at idle.-----Repeat CLOSED.-----Idle speed is controlled by the timing.----Need to accept the facts.

 

[fatlenny]

On that motor the throttle plates must be closed at idle.-----Repeat CLOSED.-----Idle speed is controlled by the timing.----Need to accept the facts.

Thanks for the response and the half of an explanation, but no need to be so cocky "Need to accept the facts." I accepted the fact on how the throttle plates needed to be closed, and was asking for an explanation, not a scolding.

Let me report my own findings when pulling the carbs and breaking them down for cleaning and explain for anyone else who may want to know my understanding on how this system works.

The throttle butterfly does completely close (as stated above) when at idle unlike others I have been more familiar with. The throttle plate has a cut out in the top of the plate that allows a factory set amount of air through to pass the low speed orifice and pick up the amount of fuel set by the low speed screw to maintain a smooth idle. Although timing will effect the speed of idle a little bit, I dont feel this is really the idle adjustment as the research I have found requires a timing of 3 to 4 degrees of timing at idle, I believe the idle adjustment screw is only to set timing at idle, not idle rpm.

Any friendly feedback if I am missing anything is appreciated.

 

[fatlenny]

I seem to have found the issues with this motor. The idle would start having issues when the motor cover was placed on it, is seemed to be starving for air.

The bolts connecting the power head to the exhaust housing looks like they had been replaced with studs and a couple of the nuts was loose. The gasket (A) had failed allowing a large amount of exhaust fumes to enter the enclosed powerhead through the hole the shift wires pass through (B).

I have made a quick repair with some high temp Permatex until my ordered parts arrive. Under testing, this has eliminated all issues.

 

[racerone]

Good luck with your motor !----Post #1 indicated you were struggling.-----But obviously you are well versed on your motor now.

 

[tphoyt]

Those fumes will indeed cause you problems.
As for your idle as you know the throttle plate remains closed until about 1/4 throttle maybe a bit less at which time timing start to advance as well. Under the welch plug there are 3 tiny holes that let fuel in to mix with a preset amount of air that passes the butterfly.
The amount of fuel the tiny holes allow through is determined by the slow speed needle. The idle can be adjusted at the throttle linkage but it’s set at the factory and really shouldn’t need to be messed with unless someone already has in which case it will then need a link and sync.
Hope that helps

 

[McGR]

Sounds like you were sucking in some exhaust.

 

[country00g]

Thanks for the response and the half of an explanation, but no need to be so cocky "Need to accept the facts." I accepted the fact on how the throttle plates needed to be closed, and was asking for an explanation, not a scolding.

Let me report my own findings when pulling the carbs and breaking them down for cleaning and explain for anyone else who may want to know my understanding on how this system works.

The throttle butterfly does completely close (as stated above) when at idle unlike others I have been more familiar with. The throttle plate has a cut out in the top of the plate that allows a factory set amount of air through to pass the low speed orifice and pick up the amount of fuel set by the low speed screw to maintain a smooth idle. Although timing will effect the speed of idle a little bit, I dont feel this is really the idle adjustment as the research I have found requires a timing of 3 to 4 degrees of timing at idle, I believe the idle adjustment screw is only to set timing at idle, not idle rpm.

Any friendly feedback if I am missing anything is appreciated.

View attachment 351302

any idea how many turns the set screw is from all the way in

 

[fatlenny]

any idea how many turns the set screw is from all the way in

I followed something I found online something like described here. https://blog.marineengine.com/2018/09/06/johnson-evinrude-carburetor-adjustment-by-joe-reeves/