Prop pitch/diameter question

[Jimmyb617]

I have a 2000 2 stroke Johnson 70 hp on a 17 foot center console Wellcraft. Wide open throttle only gets me to 3500 RPM and it only pushes the boat around 16 mph. The prop that is on it now is a 13 3/4 x 15. I have no idea if this is even close to being the right prop for this set up because the motor came off of a work boat. Don't know if I need to increase or decrease the pitch or diameter of the prop. I conducted a compression test on all cylinders and it was perfect. Also tested spark one each spark plug and that is also good

 

[Sea Rider]

If OB was found in good working state, before going for other prop, make a hub slip test to check if current prop is doing its homework well, if OK go from there. That OB should run 5500-6000 wot rpm, if so, OB is way low from its ideal rpm range. Will need to drop several pitches to run at least middle to max wot rpm range as loaded.

Happy Boating

 

[jimmbo]

Sounds like one cylinder is dead. with a 12:29 gear ratio, a 15 inch prop is in the ballpark for an 18 footer

 

[ahicks]

^^ My thought as well.

 

[Sea Rider]

Check if tach is doing its homework right, probably is way off from what should read and giving low read outs.

Happy Boating

 

[ondarvr]

What are "perfect" compression numbers, and did you ensure it's running on all cylinders by pulling one plug wire at a time?

 

[Shaun_300]

Man that thing should rocket with that setup. There's gotta be something wrong with the engine not to be able to turn that prop. If anything the diameter might be a touch too big, you could go to a 13 diameter but keep the 15 pitch if you need to fine tune it after you get it running better. If the compression is good you may have a fouled plug or bad coil.

 

[Jimmyb617]

Jimbo, if I did a compression test and each cylinder was 150 PSI, is there any there tests I can do to see is a cylinder is bad? And how would I test each one by pulling the plugs off one by one? Thanks guys

 

[ondarvr]

While it's running pull a plug wire and see what happens, it should run rough, if nothing changes you know that cylinder has a problem. You do that for each cylinder.