Re: Idle problems with my Force 120
If you look at all the posts in this forum, you'll see that idle problems are pretty common with the Force engines. There are many causes for poor idle performance. Low compression (this usually causes poor starting and performance issues too), ignition system problems (weak spark), fuel recirculation system problems, to name a few. If the motor is new to you, you should perform some basic checks to see what kind of shape the motor is in mechanically.
1. Compression test. Should see about 120 - 150 lbs evenly across all 4 cylinders.
2. Correct type of plugs. I believe you should be using BUHW or BUZHW NGK's or Champion 5898's.
3. Change the water pump impeller.
4. Change the lower unit oil and check for water intrusion ( milky looking goo instead of black lower unit gear oil ).
5. Inspect the fuel pump diaphragm and check valves. Replace if necessary.
Max RPM may be noted on the model/serial number tag. My 1988 125HP has an RPM range stamped on the tag from 4500 - 5500 RPM's. Your 1994 120HP should be similar. You will find that there is no single prop that will give you good low end AND top end performance. It's either or. Kind of like performance camshafts. If you want top end, then you sacrifice some low end power, and visa versa. I use a 13 1/4 X 17 cupped stainless prop for cruising around with WOT RPM's at 5200 and GPS speed of 45 MPH.
For skiing I switch to a 13 1/2 X 15 cupped stainless. Max RPM's is 5700 at 40 MPH but I generally never run it that fast. My hole shot is way better with the 15 pitch prop and enables me to pull a 200+ LB slalom skier up just like that.
