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apologies in advance for the long post. i have a 2004 yamaha 4 stroke f40/30 with a jet on a 16' john boat. has always been a little hard to start but ran well. i use it to run up and down a shallow rocky alaska river. have had usual bumps on gravel bars and rocks but only run it about 25hrs year. i change plugs/fuel filter yearly, fog for storage in winter. about a month ago it stopped developing full power. it is also a little more difficult to start now. it makes just enough power to get the boat where it is about to tip over onto the step but just cant make it even with just one person on board. after pushing in this position it will begin to "chug" every couple seconds. i havent been able to figure out if the engine is missing or if the jet is cavitating. impeller was in bad shape so replaced that and wear ring and no change. talked to local dealer and was told it is almost always water in fuel. tried new gas tank with fresh fuel, new gas line, drained carbs, checked fuel filter still no solution. checked plugs and all seem OK, if disconnect one plug at a time similar additional loss power. in neutral the engine will rev up to full rpm and not miss. local dealers mechanics advice now is to lower the engine one bolt hole on the theory that the bottom of the boat is dinged up enough to affect water flow to jet. have asked around and have gotten other suggestions such as bent driveshaft, faulty cdi, faulty fuel pump. problem is that boat is at remote alaska river. to bring engine to town would require bringing it up a 30 ft steep bank, carrying 1/4 mile then stuffing into small plane for trip back to town. would really like to fix it where it lays. any one have any other ideas on fixing or diagnosing the problem? thanks in advance
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