Tricky spark issue 83 40hp Mariner maf by Yamaha

[caliber]

Very random issue losing spark to single cylinder. Very random happens maybe twice a day. It has a pulser Yamaha type ignition. It seems to not do it at a idle or at high throttle seems to always do it right above no wake speed. Also have never been able to duplicate this on a hose or in a tank only under load in the lake and very randomly. It has new plugs I have been told to replace the pulser by one guy but am gun sly parts for these motors are not to cheap. Any ideas I would love to hear.

 

[Faztbullet]

If it has the single ignition coil it fires both cylinders at same time, so if this is the case look for a leaking head gasket.

 

[Star]

Hey Caliber, I had the same kind of thing with a 05 15hp mercury four stroke made by tohatsu. It used a yammy coil that measured good on a meter. If I ran it at WTO for 45 minutes it would stutter and buck. If I turned it off for a minute it would start right up and run fine. My 4 stroke used the lost spark system which fired both plugs off of a coil with a single primary and dual secondary. I lived with it for two summers until one cylinder went out solid for 15 seconds. I purchased a new coil (Yammy parts are cheaper) and the problem has not occurred. In my case if it were the trigger all spark would disappear. The motor always got me home though!!!.

 

[caliber]

My first thought was a coil. But what makes me second guess that is. I have had bad coils they tend to act up when they are warm. This seems to be totally random some times cold some times after running a hour. You do not have to restart the motor it will simply do it 15 sec to a min then snap back and be fine.