1977 Evinrude 25hp single plug fouling

Hello, new to the forum! I have been searching here for the last few days for an answer to what has been bugging my motor. Two issues actually. My main problem is the top spark plug fouling. My bottom plug is tan on the side electrode and insulator tip the top plug is oily and slightly fouled. This has been a persistent problem. Yesterday I put new plugs in and adjusted the low speed idle and both plugs came out the same. Today I went out and I was still having problems getting in to idle right. It would sound smooth and then it would just pop and die, so I tried to adjust the low speed again. After I adjusted the low speed again I took it out WOT up and down the river for about 15 minutes and had a 5 minute idle speed trip back to the ramp. When I pulled the plugs at home the original problem showed up again! Top fouled bottom tan. It just doesn't feel like it is running very smooth to me. Here is some background:

-new power pack and both coils new
-rebuilt carbs
-new head gasket
-compression on both cylinders is 105 (+/-) 1psi
-set up both plugs on a homemade tester, both jumped 7/16th of an inch
-new fuel pump (actually, two new ones. I put a new one on and had the issue I described and I was told it was a faulty diaphragm so I returned it on warranty and put a second new one on with no change to the problem)

I guess my question is, with good spark and compression, why is only the top cylinder affected and how can I correct this? Is it a reed issue? Do I need to have it synchronized?

Second: (as if the first wasn't enough) I have what seems like a pretty good amount of steam coming from the exhaust relief ports when I come to a stop and run slowly through the water, there is no evidence of water intrusion on my plugs that I can see, is this something I should be concerned about?


I know I am long winded but I would really like to take care of this on my own, I won't learn anything by paying a shop to do everything! Thank you so much for the help!

Re: 1977 Evinrude 25hp single plug fouling

What plugs are you using, aarond? Have you inspected the coils?

Re: 1977 Evinrude 25hp single plug fouling

Hi JB, I am using BRHS7 NGK plugs, the pair before that were Champion. I went to O'Reily's for both sets, had them look them up and I looked in the book for myself to make sure they were what my model called for. The coils are both new.

I did have it out this morning and I adjusted the idle needle once more the way the book called for. Put it in the water in forward gear after it reached operating temp and richened (not a word) it up until it started to cut and run poor then I leaned it out until it started running poor and backed it out again 1/4 turn to where it ran well. I pulled the plugs after a couple hours of fishing and it looked as though they were much closer in burn conditions. However, while out on the river it doesn't seem to run quite right. At WOT there seems to be what I can only describe as an intermittent miss same thing at mid throttle although 3/4 runs well. I know that RPM's are much more important than speed but, I don't have access to a tach, so, at WOT it goes about 25 top end on smooth water with little wind or current. This seems a bit on the slow side for a 14' flat with 25hp?