evinrude 40hp missfire

1976 evinrude 40604r. i changed the spark plugs and i now have a missfire. when i get it to wot it smooths out and runs well but after i let off it starts to missfire. it ran well before i changed the plugs but i thought they were due. the new plugs are champion ul81c thats what it calls for. the old ones were champion rj8c. they are not even on the list for outboards. thinking about trying new plugs rj8c just to see what happens or just putting back the old ones.maybe theres a breakin period.they did get better but at the end the motor was badly missing at slow speed but it started and idled very well. just wondering if anybody else has heard of this problem. thanks

Re: evinrude 40hp missfire

Put the old plugs back in and see if your problem goes away, if so you know that the new plugs are your problem. The parts breakdown I looked at shows a different plug, #19 below.
http://www.ishopmarine.com/ishop/js...CatFilter=BRP&eCatSupFldr=supplier-100-005-EJ

Re: evinrude 40hp missfire

Oops, withdrawing comments. Wrong year.

EDIT: Now about the correct year, my parts book says the current plug is part number 502189, L82C. I can't comment on that one way or another.

However, the symptoms at hand are typical of a motor that somebody has defeated a burned-up or missing safety switch by grounding the lead. That allows the vacuum cut-out switch to operate at any throttle setting. It is supposed to be operable only at slow throttle settings. Just under full throttle sometimes creates enough vacuum to cause it to operate, causing a miss.

The safety switch serves two functions. The second is it allows running the starter only at slow throttle settings.

Summing up, in other words, if your safety switch is missing, defective, or defeated, fix it correctly.

Re: evinrude 40hp missfire

it says on the chart the j4c is for 1960 -1973 and for 1974 - 1976 ul81c and that plug comes back for the 1980s for the 40 hp

Re: evinrude 40hp missfire

thanks daviet i checked for the l4j like you said and champion says it changed for l82c so now im not sure what i need.

Re: evinrude 40hp missfire

sorry F_R im not sure what you mean about the starter it is a pull start. but i did find the same plug as you l82c.

Re: evinrude 40hp missfire

ok i just got off the phone with brp and they told me i have the right plugs in ul81c.

Re: evinrude 40hp missfire

hardwater fisherman said:

sorry F_R im not sure what you mean about the starter it is a pull start. but i did find the same plug as you l82c.

Click to expand...

A-r-r-rgh. This thing is starting to confuse me more and more. It is sort of an odd-ball motor which was dropped the following year. Anyway, yes you are right, the model # says it is a manual start--a detail that I overlooked. Therefor, it certainly does not have an electric starter motor, so the safety switch does not control it that way.

However, you still should have a safety switch on top of the cylinder block somewhere that is operated by the stator plate as you advance/retard the throttle. That is the switch that I was referring to. It allows the vacuum cut-out switch to operate only at slow throttle settings. If it is defeated by grounding the wire, the vacuum cut-out switch can cause a miss, usually at just under full throttle.

Nobody has yet suggested a bared wire grounding out as you advance/retard the throttle. A possibility

I'll go back to my older motors now.