First off, I am new to this forum and pretty new to outboards and working in them. Most of my mechanical background comes from working on cars and trucks. So here is some background on what I have and my problem.... I recently acquired a 1978 dynatrak bass boat with a 1979 100hp evinrude outboard. The boat had sat for three years outside. The gas tank was fuel and stored with stabil in it. I was told the impeller and lower unit oil had been changed previous to it being stored. I installed a new battery, checked the fluids, cleaned the battery connections and putt the muffs on it. It started right up with no problem. The only thing I could find that didn't work was the tach. I put the boat in the water and it ran perfect. Verified 42 mph with the speedo and my gps. Since then I had ran 5 tanks of gas through it with no problems. A couple weeks ago the clip that he's the spark advance rod broke and would only idle. I put a zip tie on it to get back to my dock. It still ran great this way. The problem I am having is every time I take the boat out it goes slower and slower, i lose 3-4 mph every week. I am down to a top speed of 31 now. The motor still sounds fine. There is no misfire, it seems to rev pretty close to the same Rpms although I can't verify this because the tach is inop. It seems like it is running as it should I just lost top end speed. I marked the prop to see if it had spun or was slipping and it hasn't moved. Sorry for the long post. I'm going to try some sea foam in the gas and sprayed they the carbs, replace the clip and probably to plugs and a compression check same time.... Just wondering if there us something I should be looking for or somewhere to start first. Thanks for any input!