Big Twin low speed misfire problem

[northcoaster56]

I have an Evinrude 40 HP Big Twin, not sure of the year but it's a model 40352D, serial #E01144.It starts ok but will only run on one cylinder at idle and off idle speeds. To get it to run smoothly I have to lean the mixture and run the engine speed up to clear the misfiring cylinder. Once this is done the motor will run wto and mid range speeds just fine. I can even slow down to almost idle speeds with no problem.The motor will buck and eventually die if I try to go to slower in the leaned out condition. The motor also refuses to start in the leaned out state. I've been making do by richening it up to start and clear the dock, and then leaning it out as soon as the choke opens. I reverse the procedure when making a landing. Plugs are new Champions. Replaced points and condensors a couple of years ago with no improvement.Last compression check showed low 90's for both cylinders. Any ideas?

 

[Joe Reeves]

Re: Big Twin low speed misfire problem

You have a 1963 40hp Evinrude. It sounds like you have a fouled carburetor. I suggest you remove, clean, and rebuild it. Then, adjust it as follows.<br /><br />(Carburetor Adjustment - Single S/S Adjustable Needle Valve)<br /><br />Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.<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 />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.