Hello, I am fairly new to boat engines, but know cars real good. I have a 73' Merc 1500 inline 6, I bought it from someone I knew well, he said it ran great and very fast on the little boat it was on before, now I have had it in the garage for 6 years, and installed it on a 20' trihull. Before I ran it I installed new plugs and installed new impeller, which the passages seemed a little clogged with some what appeared to be old mud, nothing major. Some wires were dry rotted I sectioned in new wires where it seemed real necessary. dumped out old gas and rinsed a gallon or two out in the old tank, that had old gas, replaced the fuel lines and filter. Replaced the fuel pump diaphrams and check valves, also some fuel lines on the motor from fuel pump to carbs when ball was pumped, so we replaced those too. Then replaced a gasket kit for the carbs and one float that had bubbles on it. Started the motor up and it warmed up and seemed fine even in gear throttled up fine in gear. Took it out and it bogged then cleared up a little and would only get up to about 25mph then a few seconds or couple of minutes would bog down again. We thought right away, fuel.... so we bypassed the tank outlet itself, by running a fuel line right into the tank, with no results, we then ran it in the dark, and splashed some water on the plug wires to look for spark jump, but seemed fine. Fuel ratio we used was 50:1. the water pump seems to be pumping fine, but I looked at some newer motors on the lake and they seemed to be pumping a much better stream at idle. can there be another gasket somewhere along the way leaking water , but if the motor was overheating why would it as you are going at full throttle only run at like 5mph only like 1/4 its rpm, then slowly clearout and run at about 25mph and almost full throttle, but not get up there like it should. Could I have the lower unit installed wrong? causing some kind of binding. I have read that there could be some corroded wires not connecting properly, bad grounds or of the such, but it doesn't seem to be the problem, we also went through a lot of gas as if we were running full throttle all the time. should that motor use that much gas? 30 gallons and I would say we only went about 20-30 miles.Everyone say it should push the boat about 45-50 mph. I am dissappointed, and the usual, we don't work on older motors. Any Ideas? 