I have a 120 Johnson that seems to have an odd problem. When the black plastic cover is on the carbs it will only run on 3 cylinders. But if the cover is removed it runs on all four perfectly. I took it to the lake with the cover removed and it wouldn't get up on plane unless the electric choke/primer was pressed. If I pressed it down it hopped up on plane and would fly as long as I held the key in. With the cover on it will lag to get on plane to the point that somone would have to stand on the bow to get it to slowly fall over on plane. Compression is good as is spark on all cylinders. Any help would be greatly appreciated
