1987 140hp RPM, Speed, and Fuel economy questions

I recently purchased a 96 Playcraft 20' deck boat with a Force 75. It is rated for a 200. After spinning the prop trying to push the boat, I bought a 1987 Johnson 140 off a buddy. The motor runs fantastic. It has a 13 3/8 x 17 prop. I can only get 5100 rpms at 30 MPH and it uses a ton of fuel. I can trim the motor up and get 32MPH and almost 5200RPMs but that is it. I am not too concerned about speed but I do like it. I am more worried about RPMs, isn't there are certian cruise RPM. I have a 23 gallon tank in the boat and after a 2 hour trip I am running on fumes. The throttle cable has some slop and I don't know if that is the issue. I have not checked timing or compression. It has low hours and was owned by a marine machanic friend of mine since new. Starts right up everytime, pulls good, and runs smooth. The plugs look like it may be running rich also. Any advice would be appreciated.

Re: 1987 140hp RPM, Speed, and Fuel economy questions

The 140 Johnson is a great motor although it is not exactly known for fuel economy. You should be turning between 5500-5800 at WOT, so you may be over propped. Get the boat/motor propped correctly so you don't have to run all day at WOT, and fuel economy should improve. Also, make sure you are running a true 50/1 mix. This could cause the plugs to look like they are running rich.

Re: 1987 140hp RPM, Speed, and Fuel economy questions

The only way to check the plugs is to take the mtr for a good hard run and immediatly shut it off. Then pull the plugs and take a reading.

Re: 1987 140hp RPM, Speed, and Fuel economy questions

I am running 50:1. Sometimes when I start the motor it smokes for a second. That could be a needle or float though. I guess tomorrow I will check to see if I am getting true full throttle. I guess at full throttle the linkage should be at its stop.

Re: 1987 140hp RPM, Speed, and Fuel economy questions

If you end up going with another prop I might be able to use yours. It sounds like what I need. Let me know if you decide to sell it.