Motor kills at idle, top end is ok

[Bensalvage]

I have a '75 40HP evinrude that is driving me nuts! It won't idle, it just dies. If you keep it just above idle it sounds like its miss firing, it just spits and kicks. The top end is fine, runs pretty smooth but I have a feeling it could run better. This is what I have tried so far- correct new plugs gapped at .030 - pulled the flywheel and set the points which were basically fine to begin with - took apart the carb and sprayed everything I could think of - toyed with the mixture setting - fixed a switch that wasn't working which is located under the flywheel and is depressed when the motor is running at slow speeds only, (not sure what thats purpose was but it made the motor run a whole lot better at slow speeds then it did when I got it). The spark jumps about 1/2" thick and blue, fuel pump kicks out fuel in spurts while cranking the engine. Thats all I know, I'm stumped. Any help would be greatly appreciated!

 

[tashasdaddy]

Re: Motor kills at idle, top end is ok

suggest you try this, fiddling doesn't cut it.

(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)

Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.

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.

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.

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.

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.

 

[Bensalvage]

Re: Motor kills at idle, top end is ok

I assume when you said "Single S/S Adjustable Needle Valve" you mean the screw/knob right on top of the air intake? I detached it from the knob on the outside of the motor and adjusted it as you said but it did not seem to help a whole lot. Is that for sure the screw you are talking about? What rpm is normal for a 2 cyl engine of this size?

 

[tashasdaddy]

Re: Motor kills at idle, top end is ok

idle should be 500-650 in gear. 4500-5000 wide open, but this depends on the boat and the prop. i think you are due for a carb cleaning and rebuild kit.