I took my 1967 Evinrude 6hp out for a short run today. Before the test run I put in new plugs, and made sure that the high speed orifice was clean and clear. The motor as usual started up on the first pull and appeared to run rather steady with no loss of rpms which is a positive change, but it did not sound like it was running at full throttle. Could the fuel tank be causing a problem? I have an older metal fuel tank that is in great shape, but it does not appear to have a vent on it that I can find anyway. When you open the cap you clearly hear the tank release a lot of pressure. I do not know whether or not that can causes a decrease in rpm's.