One lean, one rich

[mattjeanes]

I've just rebuilt my 1974 15hp Evinrude. During the first hour or so it ran real well but then started to run on one cylider. I've done all kinds of checks and found that the top cylinder is running a lot leaner than the bottom. (I found this out by disconnecting one plug at a time and adjusting the idle screw - there was more than 180 degree difference between the optimum settings for the two cylinders. What do you suggest I do next?

 

[JB]

Re: One lean, one rich

Howdy, Matt 3.<br /><br />I think either your "lean" cylinder is sucking air from somewhere. . . . reed plate gasket, bypass cover gasket. . . .or has a leaky reed.<br /><br />OR. . . . . . . . <br /><br />Your "rich" cylinder is getting extra fuel from somewhere. . . like a leaky fuel pump diaphram.<br /><br />Good luck.

 

[OBJ]

Re: One lean, one rich

Hi Matt.....do ya' still have a good spark to both cylinders? The spark should jump a 7/16" gap with a hot thick white spark.<br /><br />If the spark is good, remove the carb cover and start the engine. Check the mouth of the carb for blow back which would indicate a bad reed. If it's blowing back, you will be able to feel it at the mouth of the carb.<br /><br />A good test for leaking gaskets is to dribble some carb cleaner along the seams. If the engine speeds up, you found the leaking gasket.<br /><br />Keep us posted as to what ya' find.

 

[mattjeanes]

Re: One lean, one rich

Progress so far...<br />I've eliminated the fuel pump by running the engine with it removed (hole plugged with 1/4 screw and fibre washer). <br />The spark appears to be good so I'll look for leaks next (when it stops raining!)

 

[mattjeanes]

Re: One lean, one rich

Thanks for the suggestions - the rain has stopped but I've just spent several hours on this one and I've still got a problem.<br />I checked for blowback as suggested and as far as I can tell there isn't any (is it usually easy to detect?)<br />I tightened a few bolts and checked everywhere I could get at for leaks and found none.<br />I checked the float level, points gap, spark strength and all are OK.<br />I removed the fuel pump again, put nuts on the mounting bolts to hold it together and squeezed the primer bulb. As I did so some fuel came out of the central hole which I assume is not meant to happen. I'm guessing that there is a leak in the fuel pump diaphragm but this doean't really solve my problem as the hole for the fuel pump goes into the top cylinder which is the one running lean - surely a fuel leak would make it run rich!<br />To be dead sure that the fuel pump was not the issue I filled the carb using the primer bulb and ran the engine with the pump removed having made doubly sure that the hole into the crankcase was well sealed - ran just the same as before.<br />This is just beginning to get really frustarting - has any body got any more ideas please?

 

[Paul Moir]

Re: One lean, one rich

Did you ever get a chance to look at the top crank seal? It seems unlikely to me though since it would likely foul the points rapidly enough.<br />Another idea - carefully go over the recirculation lines looking for cracks & making sure they function. I had one recently that was giving me fits and that was the cause.<br /><br />How's compression?<br /><br />EDIT: I overlooked that you had checked the points gap. You would surely have noticed a top seal leak if it was present, as there would be oil all over the place up there.<br /><br />EDIT 2: And then there was that time I found a wire tie ('hose clamp') that had been sucked into the carb holding a reed ever so slightly open. Might be worth checking visually for the sake of a manifold gasket.

 

[nordy]

Re: One lean, one rich

Hello MattJ,<br /><br />You only have tested idle performance of the two cylinders.<br />It's quite normal for idle, that each cylinder needs a different fuel/air-ratio.<br />For optimum performance idle screw has to be adjusted in a middle position between lean and rich which leaves one cylinder too rich and one not rich enough, just a compromise.<br /><br />You'd better have a look at the two spark plugs after heavy load.<br />They have to look equally brown both.<br /><br />If one was white and the other brown something would be definitively wrong.<br /><br />If you have no problems with top speed forget about the "problems" with idle, they aren't any.<br /><br />Greetings from Germany <br />Nordy

 

[mattjeanes]

Re: One lean, one rich

Thanks Nordy. As I'm still running in I have not exposed the motor to heavy load yet and as it's winter I'm inclined to spend a bit of time getting it running as well as possible at idle.<br /><br />Question for the reed experts:<br />I managed to get the reed plate out today (quite a sruggle with powerhead in place).<br />The top cylinder (lean) reeds do not sit completely flat on the plate - the gap is approx 5 thous at the wide end - is this enough to cause problems?