Mercury 125 saltwater jumping from 2100 to 4000 rpm

Hello,

I have just bought a second hand 2001 boat with a Mercury 125 Saltwater engine. I had it serviced before the first use, and it's only got 70 hours on it.

The engine runs fine, however when I ease the throttle up the motor will jump from around 2100 rpm to 4000 without me moving the throttle any further. I have tried taking the throttle up faster and slower, but it makes no difference. The slightest feather of the throttle above 2100 rpm will jump it straight to 4000 rpm in about 5 seconds, a feather back down and it'll drop back to 2100 again.

It does this every time and its making it hard to use. I want to run around 2300 rpm with the kids on a tube and it's impossible to get the motor to do that.

The motor runs smooth, it's just the 2100 to 4000 rpm band that's a problem, lower and higher than that range and the throttle works well.

Anyone able to give me an idea on what to check?

Many Thanks
Bern

Does the boat pick up speed when this happens?

Certainly does, the boat takes off as though i was intentionally pushing it to 4000 rpm quickly.

Bernn said:

Certainly does, the boat takes off as though i was intentionally pushing it to 4000 rpm quickly.

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May be time to update to a Gen2 control box and cables. Sounds like the throttle cable is binding.

I am going to assume you have a 2 strk 4 cylinder motor. That is a 2+2 motor, what that means is the motor idles on the top 2 cylinders until about 1800 rpm and then the other 2 cylinders start working. The transition point can be a abrupt if the link and sync is off. Your best bet is find a mechanic or dealer who is familiar with that particular motor. They are tricky to link and sync a good dealer mechanic can smooth that out some but the transition will always be there. If you want to do it yourself a FACTORY manual is your ONLY option. I had the same motor and it is still one of my favorites.