1993 force 40 hp Tach issues

[chefblair]

I had my boat out today, and the tach jumped around a lot at full throttle. I know that the engine should run around 4500 - 5500 at WOT, but today it spiked to the 6000s and all around, without any change in engine sound. The last time I was out the engine ran full throttle at 4800. At one point today it sat at 5600 WOT, when I slowed down, the engine revved down, but the needle on the Tach jumped around again.
Any information on how to test, or check the tach, would be greatly helpful. I know very little about how the tach works, or how it is wired in. I only had issues at full throttle ranges.
Thanks

 

[chefblair]

Re: 1993 force 40 hp Tach issues

I was researching this issue, and found a possibility. I may have suffered prop break loose. I had the boat improperly trimmed, that would explain why I was pulling right so bad. In my boat manual, it mentions possible break loose (high rpms) if engine is under load, or improperly trimmed.
Does this sound sane, or possible?
I also checked the tachs wires, and followed the grey wire back. The wires are tight with no visible damage. I have not checked the stator yet.

 

[roscoe]

Re: 1993 force 40 hp Tach issues

The tach gets its pulse signal from the regulator/rectifier (grey wire).

If the rectifier is going bad or is bad, your tach will usually stop working or work erratically.

With the engine stopped, touch the leads of a volt meter to the pos and neg terminals of the battery. Any cheap $10 voltmeter will do. Post the voltage measured.
Now check the voltage with the engine running. Post the voltage.

If the voltage is higher with the engine running, the stator is producing current and the rectifier is working.

I seriously doubt that prop blow-out or a spun prop hub has anything to do with it. If the prop breaks free, the engine will rev high and you would definitely hear it. And it wouldn't stop till you slowed way down.

 

[chefblair]

Re: 1993 force 40 hp Tach issues

Battery was 12.5, with engine running engine is 13.5. Voltage rounded to nearest tenth.
In my manual it refers to this break loose as not necessarally a bad thing. I do not understand exactly what it means though. My prop is on a splined shaft with a shear pin, and the prop cone has a cotter pin. All still intact.
Could my manuals use of the term break loose, mean a sliip in the prop. Some way that the prop slipped the gear allowing a sudden jump in the RPMs?

 

[roscoe]

Re: 1993 force 40 hp Tach issues

The gears cannot slip.
A rubber hubbed prop can spin the hub, but if it does, the rpm will go sky high and you need to have a new rubber hub pressed into the prop at a prop shop.

voltages look pretty good.