40HP Johnson Rough Idle

I have a 1989 40 HP Johnson that runs great at anything over an idle speed. When slowing down to an idle to approach a dock/pier or trying to troll, it runs rougher than a cob. I have changed out the plugs, gas tank and gas with no improvement. On a side note, some time back I had a 1984 40 HP Johnson with the same problem, do these motors have a design flaw that causes them to be rough idling motors?? Looking for someone who can get it running right. I live in SE WI, however, I'm willing to travel to get the job done correctly. Thanks in advance

I have a 1966 Johnson 40 hp and had the same problem last year & all summer long this year, Around the end of and, the beginning of Sept, I was out fishing and on the trip back to the dock ai refused to idle/run at low rpm. I opened the frond access cover & found fuel running out of the carburetor throat. It turned out to be the carb float was "water logged". i spent $35 for a carb kit and replace j=the float, needle & seat & gaskets....it runs like a clock again. It doesn't even 'backfire' when first staring when cold anymore.

Sounds like you need to adjust the carbs. I'd do that after running some sea foam through it.