RPM's appear to be about 50%

[Sailbike]

I recently had water in my fuel. We determined the fill cap seal was bad. I drained all of fuel and replaced with new.. A mechanic replaced fuel filters and at the same time I had the 2 hoses, engine to water heater and return, replaced. Since then the engine starts and runs fine but the tach will only run up to about 1200 rpm. The engine sounds normal at about 6 knots, which before would have registered about 2000 rpm. It is a 1984 Hunter 34 with 3gm Yanmar with 680 hours. ??

[a64pilot]

Verify your tach, if your running the same speed as before, it's very unlikely you are doing so at half the RPM

[sailorboy1]

Tachs are just such a waste of time (you have a tach problem). Both my boats have had tach problems, but I don't really care too much and run the engine based on speed and sound.

Just a question, but do watch the tach in your car or just your speed?

[Sailbike]

Thanks for the thoughts. I do think I have a tach or wiring problem. Since I had work done just prior to noticing this, I was thinking a wire could have been changed. ?? (Or its just gave up after 30 years.)

[a64pilot]

I believe there are a couple of different ways to pick a tach signal off and engine, first you need to determine the source of your tach signal.
I prefer my tach work, but it isn't an absolute requirement, I set cruise by RPM, speed varies significantly depending on how clean the bottom is, sea state and lord only knows, I don't

[Blue Stocking]

If it an alternator driven tach, you need to know if its 12 or 14 pole. Makes a difference to the brand of tach.

[phstol]

Yup, its the tach. But I can't agree with a blanket statement that you don't need a tac of reasonable accuracy. It depends on your application and situation ... having said that, by all means use the engine sound and other seat of the pants inputs to judge your speed vs RPM and general well being of your boat.... but be careful... On a planned, or unplanned/unexpected long period of motoring you can easily get lulled into thinking that a reduction in boat speed is the result of the RPM reducing b/c of who knows why. Then you push on the throttle to regain boatspeed and all is well.... except that now your gal/hour fuel use has gone way up. If your application is putt-putting across the bay to the bakery, no big deal. If your application is several days along a potential lee shore, it cd become a big deal. Get it fixed, and may you always sail with fair winds and empty holding tanks!.

[Neo]

Pretty sure the 3GM tach signal comes from the alternator.

Make sure the alternator belt isn't slipping.