1987 evinrude 150xp - odd rpms
I bought the boat with 1987 evinrude 150 last fall. the tach was reading 4200 rpms at wot. I believe that the prop was way over-pitched as the boat was very fast (65mph), had a hard time getting on plane, only 4200 rpm's, and had oily plugs. I switched to a 19" aluminum prop (the old ss prop wasn't labeled as to the pitch, but i suspect 24"+). That seemed to fixed my problems: rpm's around 5500, good low end, 52mph, plugs dried up. That's the history, now the problem:
the last few times out, i noticed that *sometimes* my rpm's would seem way too high. They'd hit 6000rpm at 3/4 throttle. After a while, it seems the tach would fix itself. It would be back to 5500 or so at WOT. It would work for a while, then go back to being high. I was fishing and I only fired up the engine 5 or 6 times each time I went out. Maybe 1/2 the time, the tach was reading high.
Any guesses as to what the problem might be? Perhaps just a bad tach? It is the one that came with the boat and seemed consistent last fall and earlier this spring.
I should also add that the performance (speed and power) of the engine didn't seem to change either way. I don't think the prop is slipping (or whatever).
tia,
don
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