Principles Behind Adjusting Idle Speed

[nabeel.alsalam]

I'd like to better understand the principles behind adjusting the idle speed on my 1991 Johnson 40 hp motor. It will not idle with the fast idle lever down. Apparently, I have not achieved Zen with this motor. <br /><br />After reading the service manual, I've learned that the idle speed on this engine is adusted by 1) turning a screw that advances or retards the timing, and 2) adjusting a needle screw which richens and leans the mixture. With the idle lever all the way down, the flaps on the carb are all the way closed, so that the flaps are not a variable in setting the idle speed. <br /><br />I see that the fast idle lever advances the timing and opens the flaps a little. <br /><br />I also see that the timing is 10 degrees BTDC with the fast idle lever up and the engine running at about 1,500 rpm. From the service manual I see that at idle the timeing is supposed to be 4 degree ATDC. So far from where I am!<br /><br />I've followed the procedure to set the timing and the idle mixture screw to their initial default positions according to the link & sync procedure and the carb adjustment procedure. However, the engine dies at about 1,200 rpm when I bring the fast idle lever down.<br /><br />Should I focus on the mixture first or should I vary the idle screw (timing) first?

 

[marinemech1]

Re: Principles Behind Adjusting Idle Speed

make sure carbs are clean is engine ok? compression scored cylinders bad gaskets etc?<br />turn flywheel with a wrench you should be able to feel compression is it ok?<br />remove and examine plugs

 

[OBJ]

Re: Principles Behind Adjusting Idle Speed

Nabeel.....just a question....why are we doing this in the first place?<br /><br />Have we been messing with the original Zen?<br /><br />What did we do to have to readjust the carbs?

 

[nabeel.alsalam]

Re: Principles Behind Adjusting Idle Speed

Compression quickly jumps to 120 to 130 after 3 or 4 strokes. <br /><br />I've rebuilt the carbs including taking the core plug out of the top so that I could make sure there were no restrictions in the idle passages. The orifices in the carbs are the standard ones.<br />I made sure the VRO pump is pumping gas. <br /><br />I bought new plugs. I checked the strength of the spark. <br /><br />I fixed a leaky primer solenoid. I made sure all the fuel lines were in good condition.<br /><br />I removed the leaf plate assembly, checked for damage, and held it up to a light in a dark room -- I did see a tiny bit of light... <br /><br />I replaced a corroded and stuck thermostat. <br /><br />Yes, this engine is wearing me out! <br /><br />I feel I don't understand something that I should understand about getting it to idle. Mixture, timing, etc..

 

[nabeel.alsalam]

Re: Principles Behind Adjusting Idle Speed

OBJ,<br /><br />From the first day I got this boat and motor from a friend, it would not idle. He only took the boat out once himself. So I have no original Zen with this motor. <br /><br />Nabeel

 

[rayjay]

Re: Principles Behind Adjusting Idle Speed

On my 1977 55hp Evinrude the manual goes through all the link & sync stuff and then there is a sentence stuck away somewhere that says to turn the idle screw in until the motor idles at 750rpm in reverse.<br /><br />If you look it the carb butterflies you will probably find that there is a relief to pass air across the idle mixture ports in the carb throat.

 

[Joe Reeves]

Re: Principles Behind Adjusting Idle Speed

If the carburetors have an adjustable needle valve to control the fuel/air mixture, have the engine running at a rpm whereas the engine will simply stay running, then.....<br /><br />(Carburetor Adjustment - Single S/S Adjustable Needle Valve)<br />(J. Reeves)<br /><br />Initial setting is: Slow speed = seat gently, then open 1-1/2 turns. (On your engine, they may need to be backed out further than that).<br /> <br />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.<br /> <br />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. <br /> <br />Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.<br /><br />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.<br /><br />After the above is as it should be, back the mechanical idle stop screw (the one that stops the throttle arm from dropping to a lower setting) away so that it does not hit the stop, then adust the idle rpm via the threaded trunion on the throttle cable (where it clamps to the engine). Then screw in the mechanical screw on the throttle arm until it touches its stop.<br /><br />On a flushette (hose), a 20" shaft engine should idle at 1000 rpm, a 25" shaft engine at 1200 rpm. In the water, the idle rpm will drop to where it should be.

 

[nabeel.alsalam]

Re: Principles Behind Adjusting Idle Speed

Thanks, Joe! <br /><br />I interpret "set the rpms" and "lower the rpms" to mean by using the fast idle lever.

 

[Joe Reeves]

Re: Principles Behind Adjusting Idle Speed

Idle lever..... yep, that'll do it.

 

[nabeel.alsalam]

Re: Principles Behind Adjusting Idle Speed

To start I backed out my S/S needle valve 2.25 turns because that's what the service manual says. <br /><br />Turning the screw in from that position, I cannot get the engine to sneeze or do a mild backfire. It simply slows downs and stops. I tried backing out and that increased the rpm some but not a lot. Not enough that I can drop the lever all the way down.

 

[nabeel.alsalam]

Re: Principles Behind Adjusting Idle Speed

It is idling in a garbage can with the lever all the way down at about 800-850 rpm! The timing light says it is firing at TDC. The S/S idle screws are out about 3.25 turns. Although the timing seems a little advanced and the S/S idle screws seem far out, I can't really argue.<br /><br />Next step is to go to the river and see if it'll idle in gear. <br /><br />Does adding load generally require richer or leaner mixture? Advance or retarded timing?

 

[marinemech1]

Re: Principles Behind Adjusting Idle Speed

yes go to river take boat off trailer and adjust motor for best idle in forward gear not tied to a dock

 

[Johndeere101]

Re: Principles Behind Adjusting Idle Speed

Hi, im having the same problem with my boat, it runs fine until you try and put it in gear, then it stalls out, i was wondering what you did to solve this problem. Please get back to me with any advice. Thanks.

 

[chris abela]

Re: Principles Behind Adjusting Idle Speed

I think its very important to have the oil fuel ratio perfect to have a good idleing speed.