Limited RPM (No Power)

[buffdoc]

I have a 1990 120 hp Evinrude that will not run above 3400 RPM under a load. It had sat up for 5 months while deployed so I started with rebuilding the carbs and new plugs. While I had the carbs off, I went on and pulled the reed block and cleaned them. Pulled the intake, cleaned out all the vacuum ports, went back together with all new gaskets and blew out all the lines; fuel and air. Pulled and sprayed carb cleaner thru all four check valves. Idles great but it will still not get on plane, so I bought a book and started troubleshooting. I read up on the S.L.O.W. system, so I was thinking that it was somehow the problem, although it is not overheating! I checked thermostats and found one was stuck open, replaced them both, with new gaskets. Following on down the line I ended up with Ohm'n out the Power Pack, and replacing it. The motor idles great, follows the throttle lever and sounds really good but under a load it will not get over 3400 RPM and at that RPM it is not cutting out or missing, sounds great just no power. I have previously disconnected the VRO system so I am running an electric fuel pump with a pressure regulator and mixing my oil/gas. My next thought was to check the fuel pressure under a load.

Has anyone out there had this problem?

Thanks in Advance

 

[Chris1956]

Re: Limited RPM (No Power)

Test compression and Spark to all cylinders. If that checks out, you have a mystery on your hands. Is the prop new? Did the boat gain weight?

 

[Dhadley]

Re: Limited RPM (No Power)

As mentioned, spark & compression first. Then get back to the stock fuel delivery system. Then we'll look at the carbs again. I see you mentioned you sprayed out the carb passages but we'll need to soak them this time. Also look for damage to the emuslsion tubes and leaks around the side plates.

The SLOW system will limit the rpm but as you it'll be around 2700 or so.

Tell us more about the set up. What boat, propeller etc? What's the wot timing set at? Are the throttle plates going past center? Is the timer base moving with the throttle?

 

[buffdoc]

Re: Limited RPM (No Power)

The compression is good and the plugs are new as well as gapped per the manual.

The prop is a one year old 21 Raker and the boat is a Champion Mean 15.

The timing is something I am also curious about. When I returned from deployment, I took it to a local Mechanic that kept it a month to clean carbs and set timing. When I picked it up it had a hard miss that would stall the motor. He had my 1 and 4 plug wires crossed. So this led me to taking matters in my own hands. With that said, I don't have a tank or test wheel, like the book calls for. Nor can I get the RPM to reach the 4500 requirement to check it properly. I tried this for a short time at the Ramp. Also there are two screw labled spark advance, but look only like stops and then there is a plastic handle that turns labled dwell adjustment. Any help there is appreciated!!


I will pull my carbs and soak them, since that was something I trusted the mechanic did properly.

Thanks for the input, I will update after my carb cleaning.

 

[Chris1956]

Re: Limited RPM (No Power)

A 21" pitch prop is pretty tall. Has it ever run to WOT with that prop? What kind of boat do you have. I run a 21"pitch prop on my 21 footer, with a 150HP Johnny, and it will hit 5K RPM, but no more.

 

[buffdoc]

Re: Limited RPM (No Power)

I was mistaken, I'm running a 19 pitch raker.

Good news is that a good carb soaking fixed my problem!

Thanks for the inputs!

 

[Dhadley]

Re: Limited RPM (No Power)

Raker props are in even pitches, not odd.