Spark gap question for Joe Reeves

I have been reading your posts concerning setting spark gap at either 0.040 or 0.030. I was wondering if you could elaborate on the differences one would encounter on my E225txcda. I'm trying to improve the idle to run smoother. WOT is fine, but the engine seems to have a slight miss under 2000rpms. It cranks well and runs well at higher rpm's, but I like to slow down and enjoy the scenery sometimes and the engine seems to not like the slower rpms. I thought trying your suggested spark gap of 0.040 might help, but would like to know what changes I need to look for. Thanks!

1986 225hp Evinrude

The .040 gap was the recommend gap for the Magneto Capacitance Discharge ignition when it first arrived on the scene back in 1973 and stayed that way until somewhere in the 1990's when the recommended gap setting was changed to .030 .

Making a long story short... the .040 gap makes for a hotter spark. - - - The .030 results in longer plug life.

If your engine drops a cylinder and it stays out of action while idling... yes, I'd suspect a ignition problem. However, if what you're encountering is a intermittent occasionally, that is usually due to a carburetor running slightly lean.

If you are indeed encountering a slightly lean running problem, the engine will sort of hiccup every so often, spit back through the carburetor, a mild backfires of sorts. To trouble shoot to find out which carburetor is the the offensive one..... have the face plate off, the engine running at idle and the problem occurring. Simply stick two fingers into the carburetor throat acting as a manual choke. When the engine picks up rpms and smooths out... that's the carburetor that needs your attention.

Look in your manual to find out which of those small jets on the left side of the carburetors is the idle "air" bleed jet. The size will be embedded on the face of it. Since it measures air and not fuel, to richen the fuel mixture, you need to install a smaller size jet UNLESS a air leak or some other kind of problem exists with that carburetor or its general area.

So, based on your advice, the plug gap at 0.040 produces a hotter spark. Would that help at idle speed or at top speed to keep plugs from fouling? I'm not getting a hiccup or spit back, but rather a "jerky" motion like one cylinder mis-fires during the revolution. It's not noticeable at higher rpms's, but seems to start when you come off a plane to about 2000 rpm's. I hope that I'm not being to picky about a big V6 idling smoothly, but I've always found when a 2 cycle is not perfect, something is out of order. I'm going to try your method of checking each carb while running to see if there is change. FYI, when I removed the spark plugs last time, they looked equally good, i.e., not too wet and the insulator was a tan color. I re-gapped them at 0.040 and started the engine with the muffs, but it still seemed to have a slight jerky idle. I'll leave it as is and run it this weekend in the water to confirm. I have new plugs ready just in case.

CB, when my (much smaller) outboard experienced that jerking motion it was running too lean.

FYI . . .


CMOS

Hopefully, the "finger test" will reveal if that's the issue. Since it's an old engine, air leaks are a constant problem, but I have replaced most all fuel lines, but I may have a hidden leak somewhere. My older posts indicate that I overhauled the carburetors to get where I am today.

Followup to above posts... Two changes that I made since my last outing were setting the idle speed lower and re-gapping the spark plugs to 0.040. Finally got the idle speed down to 750rpms (manual calls for 650, but I didn't want to go to low, suddenly). Not sure which change helped, but the jerky idle seem to go away and she purred at the low idle speed. That sure made maneuvering much easier! High idle seems to be smoother, too, so I think I've found the fix for my previous rough idle. I burned nearly a tank full of fuel through her on an 90 mile round trip (loaded boat!) from the coast to the lake and back and she never "flinched". I think I'll keep her!

Sounds like a winner to me!