Great Lakes Bass Fishing Forum

Buddy of mine has a 91 Force 120 4cyl 2 stroke which he's been having problems with, main problem is as described in title, the bottom two cylinders are as clean as can be. The motor was rebuilt a few years ago and never really saw much use, always been very temperamental and we finally had to start taking things off trying to figure out what is wrong. Well we took off the cylinder head and right away it was obvious that the top two cylinders were the only ones being used from the beginning. We figure it is a vacuum leak somewhere and have narrowed it down but neither of us are familiar with these motors.

We think its either the Crankshaft oil seal at the bottom, or it's the crankcase seal at the bottom which is causing fuel to not get to the cylinders. We already checked the carbs, switched them around, as well as reeds, so we are now at the fuel delivery part for those two cylinders. The cylinder heads look good, no scoring or burn marks thank goodness...this motor wasn't ran much over the past few years and when it was, it was for very little time.

We are hoping to get compression numbers as well to make sure it isn't a bad ring, but I'm really convinced it is one of the bad seals. Just wondering if anyone can confirm my thoughts or possibly offer any other insight. I have left a link to the expanded picture of the crankshaft parts, the part I am thinking is bad is at the bottom of the crankshaft #5




http://www.crowleymarine.com/mercury-outboard/parts/1455_130.cfm

I missed this post but I'm not help. My mechanics are only slight above duct tape and tapping in things. Sounds like you were very thorough. Did you ever get it solved?