Strange Yanmar RPM issue.

[Wotname]

If the "governor lever 2" was replaced, I wonder if the "fuel injector limiter screw " or the "no-load maximum speed limiter screw" has been incorrectly set.

If either was incorrectly adjusted, this might explain the observed symptoms - I think. It would certainly affect the amount of fuel being delivered to the cylinders.

Do you trust the mechanic who did the repair?

[dennismenace111]

Originally Posted by Mick of 42

My Yanmar engine recently did a 'run away'.
It turned out that "Govenerlever 2" had broken and allowed the fuel rack to go to the max open position.
It has been replaced but a new problem has developed where the motor in gear will rev to 2450 plus in reverse but only 1850 if in forward!
Anyone have a clue how this could be? The drive train is free to spin in both directions and my mechanic is at a loss and so far so am I to 3K...

If you have a small Yanmar, like a 2GM20 be aware that the tachomoters are rarely accurate.


It uses a magnetic sensor on the fly wheel. Since the output of that sensor is quite low, any resistance in the 2 wires to the tach will totally throw off your rev indications. Mine shows different RPMs depending on humidity!

[RaymondR]

Originally Posted by Mick of 42

Thanks,
I think the timing shims thing is so steampunk but you may be right. The same shims were refitted but I wonder if they are the problem. Such a dumb way to effect timing adjustment.

Simple, effective and reliable.

The transducer used on the rev counter is a variable reluctance device, small coil and magnet inside, which needs to be adjusted with the end close to the teeth. A Schmidt trigger on the output would boost the signal if you have a long wiring run to the read out.

[Mick of 42]

I don't think it is a tacho issue... I can hear the difference in RPM.
As to adding a 'Schmidt' trigger setup I think this is over kill for the current problem but thanks for the comment. I have worked with electronics and the like for over 30 years and my experience usually shows the simplest cause is the most likely!

[RaymondR]

Originally Posted by Mick of 42

I don't think it is a tacho issue... I can hear the difference in RPM.
As to adding a 'Schmidt' trigger setup I think this is over kill for the current problem but thanks for the comment. I have worked with electronics and the like for over 30 years and my experience usually shows the simplest cause is the most likely!

Since your engine will freely rev to 3,000 without load tells you that the governor is not set too low (Although I think the GM series have a 3,600 limit) This being the case it is either insufficient fuel being pumped or excessive load.

Excessive load for the torque output by the engine is usually indicated by a cloud of black smoke from the exhaust. If their is no black smoke then insufficient fuel is indicated and since it happens both in forward and reverse, where because of the gearbox gearing, there should be a lesser fuel requirement to reach the governed RPMs.

I would suspect something different in the rack setting mechanism.