Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

[mike460]

Ok we have this new engine in the boat and can only achieve about 2000 RPM under load. Timing and valve lash was set by mechanic. Fuel system has been flushed with new filters. 17p prop. Carb linkage has been checked and carb is fully open. We are running out of ideas? Any help to get this thing out of its own way is appreciated.

 

[mike460]

Re: Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

Does anyone have any ideas?

 

[Bondo]

Re: Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

Does anyone have any ideas?

Ayuh,.... Welcome Aboard,... Try a compression test, 'n a 15" prop....

Either it's over-propped, or Under-powered....

Does it have the Right drive ratio on it,..??

 

[mike460]

Re: Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

In my past experience a prop change of only 2 pitch will not warrant 2000 RPM. The engine is brand new and compression is excellent on all cylinders. How do I determine the drive ratio?

 

[series60]

Re: Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

How do I determine the drive ratio?

For the ratio, put her in gear. Take your plugs out, and rotate the engine 10 times by hand. Count the number of times prop goes around.

10/1.5 = Prop goes around 6.7 times

10/1.81 = Prop goes around 5.5 times etc.

There is usually a number stamped on the upper half of the drive on the port side of the drive.

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[Bondo]

Re: Mercruiser 120 new rebuild in a 76 starcraft Islander low RPM

For the ratio, put her in gear. Take your plugs out, and rotate the engine 10 times by hand. Count the number of times prop goes around.

10/1.5 = Prop goes around 6.7 times

10/1.81 = Prop goes around 5.5 times etc.

There is usually a number stamped on the upper half of the drive on the port side of the drive.

View attachment 160673​

Ayuh,... 'n Mike should have a 1.98:1...