1979 70hp 3cyl mercury outboard issue

How can I confirm that a stator or a switchbox is bad? The switchbox part number is H332-5524A1. Motor runs great under no load but will not run in the water in gear??? Also have alot of suction on the top carb but very little on the bottom carb. I have switched the carbs around and the problem stays on the bottom. I was told I could possibly have a bad switchbox or stator. Sound correct??? Any input would be greatly appreciated. Thanks.

 

You need to register and become a member at "John's old Merc" site. Folks there are super knowledgeable about these engines and will be able to guide you thru whatever needs to be done to get it right.

 

Compression test. That will tell the tale. I didn't think much of these motors, but I know my opinion don't matter. Its very sensitive to cylinder dammage due to over heating ; or lean fuel mixture ,or advanced timing. Not much tolerance to any of these problems. These motors! Have a four port piston and sleeve which allows about 38% of the piston ring to be unsupported while traveling over port holes in the block. Just not enough ring support especially when excessive heat created severe piston expansion.