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1999 Conquest 23 I/O, Mercruiser 5.7L, Bravo III drive.

This problem has happened three times out of the last 30 or so outings.

Symptoms:

Travelling 18mpg @ 2800 rpm. Look down at tach a while later and it now shows 3800 rpm. Engine sound has not changed perceptably and the speed is the same. Slowed down to idle. then gave it gas. Engine sounds normal, but tach shows much higher rpms at slow speeds. I'm afraid to push it because it's showing 3000 rpms and I'm only going about 12mph, but engine doesn't appear to be revving high.

Shut of engine for a minute. Start it back up and everything is fine.

I had the outdrive rebuilt at the end of last year. The problem happened before the rebuild, and again after the rebuild. Could this be caused by a faulty tach or signal?

Bill