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I have an old yamaha 70 2 stroke outbard engine code 6h3, beleive its 80s early 90s. anyway it came with a boat that hasnt been in the water for 5 years, but it ran sweet when we started it. Before we took it out i chabged the box oil and replaced the impeller. On the first run the boat planed out but it didnt feel quite like it was getting full throttle the last 3rd of throttle travel did nothing. I cleaned the carbs out which were spotless and tried again today, it went up to the same speed as before but gave a few short burst like it was going to go faster but never held the power. I pulled up and topped up the tank and pulled away again and got nothing past a quater throttle it just made no difference opening it right up, it doesnt splutter just seems like there has been a rev limit set on it. I stoppped checked the leads and looked over it but nothing obvious so pulled away again, if I hit the emergancy stop and let go there was a small burst of power after and if i kept doing this i got it to plane out. To me this seems like an electrical issue, something is breaking down under load any ideas? plugs are as new but getting a new set tomorrow to check. The engine is mint for its age having only been used in fresh water