RPM change

G

Grumpy

Active member
Had an odball thing happen on last trip. Cruising along at 4000 rpm on twin Honda 40's, suddenly starboard engine revved to about 4800 rpm.
I checked throttle position and it was same as port engine. Rapidly pulled it back and gradually increased it back to 4000 rpm. That was OK but it seems that it only required a very small throttle increase from there to get it to jump to 4800 again.

Didn't pursue the issue any further due to nervous glances from passengers and just went home at 4000.

First thought was a spun prop but it looks and feels OK although of course I have no way of putting enough torque on it to be sure.
I know I didn't hit anything solid but there was plenty of kelp around.

Anyone out there have any knowledge/advice ?? If I change the prop (have spare) is there any way it can be tested. (other than the obvious one of going back out and doing some hard accelerations)

Merv
 
Look for little bits of rubber around the prop hub. If they are spun , sometimes you will see some evidence there . If you can pull it look on the inside surface [ the "engine side"] as well .
Also if in the water , run on one engine [the "slipping" one] . Try to accelerate and if its spun you will know right away .
Marc
 
Marc, sounds like you are confirming my fears that it can be spun without it being obvious. Thanks for info. I will try it.

Merv
 
Possibly cavitating due to small amount of kelp or debris on that prop? Then fell off when stopped. Let's hope so, a lot less $$$ than a new hub or prop.
 
Recently the tach on my 1997 90 hp Honda has intermittently started to increase from my cruise rpm of 4000. It will jump up randomly sometimes pegging the needle. All it takes is a slight tap on the gauge face to bring it back to normal. This is not prop slip for sure as the engine sound or speed does not change.
Any ideas on the cause of this.?? It is a worry and I would like to stop it.
Other than that and a stuck thermostat last month the Honda has been flawless for 1200+ hours.
 
I've had exactly the same experience as Jack with my 2002 Honda 90. The tach suddenly shows 1000 to 1500 rpms faster than I know the engine is turning. A tap on the tachometer brings the needle back.
 
Going back to my original post, that was definitely not an indication issue. Everyone on the boat heard the rpm change, so we might have two things going on here.
I never did find out if the prop was "spun". I know it still works up to 4000 rpm so I just put the spare on and will try the old one out again when I have some spare time on the water.

Merv
 
Last night while trolling for salmon the tach quit completely and will not respond with the gauge thumping trick.
Anyone have any ideas???
It is going to the motor Dr. tomorrow.
 
check all connections, for both tightness and bright metal. Generally we keep connections clean with an anticorrosion agent. Also if the engine/tach are 20+ years old, there could be corrosion in the wire, to the point where it is not conducting.

Is the alternator working? Batteries charging?
 
Typically the selector switch on the back of the instrument can get a small amount of corrosion on the wiper that makes electrical contact with the different positions for the settings on different engine applications. You can try pushing in gently and rotating the switch to a different location and back to the original setting. Sometimes this "cleans" the contact and all is happy again.

Generally there are two outboard settings and three sterndrive/inboard settings on the instrument. If that does not work and the wire connections are good then it is time for a new gauge.

Hope this helps, Randy.
 
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