outboard doesnt seem 100%

[MBH]

Whenever I go full throttle it doesn't seem like the motor is putting out 100%. Its a 1998 40 HP mercury on a 14ft aluminum Jon boat. If I would had to guess, it would say I'm running maybe 25mph tops. The motor runs fine, sometimes its difficult to start but once it starts its good to go. I did have the carbs rebuilt and the timing was off so that's fixed now. New plugs, running 100% gas.

It takes forever to plane out and when I go full throttle it doesn't seem like it reacts fast.


2 6 gal gas tanks are in the front with me and a partner in the back with a battery & motor.


Edit : the motor is a short shaft.

 

[muskiemike12]

What are your RPM's at WOT? Sounds like it could be a set up or prop issue.

 

[MBH]

The boat mechanic said it was 5500 rpms

 

[muskiemike12]

Do you know how many RPM's your motor is turning? There is no way to tell unless you have a tach. You should be running 5500, but I'm going to guess you are below that because of the time it takes to plane. Either your motor is mounted too low or your prop is too high of pitch. Your cavitation plate should be even or slightly above the bottom of your boat.

 

[MBH]

I'm running a 15 pitch SS prop

 

[muskiemike12]

You could have a spun hub.

 

[MDCrappie]

"guessing" won't get you very far. Need a tach (wot RPM) and GPS (WOT speed) and size of prop. With these numbers it's will be MUCH easier to diagnose.

 

[MBH]

I dont have a tach but the mechanic said I was turning 5500 at wot... I don't know the exact speed but its slower than my cousin in his 16ft boat with a 40hp

 

[MDCrappie]

MBH » Mon May 20, 2013 2:49 pm[/url]"]I'm running a 15 pitch SS prop

If you're spinning a 15" prop at 5500 RPM with a 1.87 LU and only moving 25mph you are looking at about 59% efficiency.

With a 40hp on a 14' alum I would expect you to be at about 90% or a little over 37 MPH if set up and running correctly.

 

[MBH]

What should I start to diagnosis this problem I'm having?

Btw this is my first boat.

 

[MDCrappie]

MBH » Mon May 20, 2013 3:07 pm[/url]"]What should I start to diagnosis this problem I'm having?

.

Get a tach and a GPS to determine what your RPM and Speed really are. If the speed is down but RPM up (as you suspect) then something is slipping. If this is the case I would remove the prop and mark both the barrel and the hub in a line. Run the boat and then check the prop to see if it's still in line. If not the the prop hub is slipping.

 

[MBH]

Wouldn't a spun hub make the rpms rev higher but not going any faster

 

[cincinnati]

MBH » Today, 14:07[/url]"]What should I start to diagnosis this problem I'm having?

Compression test. Hard starting & loss of top end power are 2 possible symptoms of low compression.

Unless yours is really heavy for 14', a healthy 40 HP should be a pretty zippy ride!

 

[MBH]

I have good compression on all 3 cyclinders..

I'm trying to find a tach to do it myself. Even at WOT the motor doesn't sound like its wide open.

 

[MDCrappie]

MBH » Mon May 20, 2013 5:32 pm[/url]"]
I'm trying to find a tach to do it myself. Even at WOT the motor doesn't sound like its wide open.

You tell us the mechanic says it's spinning 5500, but then you say it doesn't sound like wide open.......this is why you need a tach. If it's spinning 5500 then it should sound wide open.

 

[MBH]

Just order a tinytach... hope it gets here fast.

 

[MBH]

Update..


GPS speed is 21 MPH.


And throttle response is very slow from sitting to wot

 

[MDCrappie]

MBH » Tue May 21, 2013 7:39 pm[/url]"]Update..


GPS speed is 21 MPH.


And throttle response is very slow from sitting to wot

Ok that's a start. Now you need the tach.

Since the speed is down (and feel of power loss) the RPM's are likely down also.

Until the tach arrives here is something to try.
With the motor off. Remove the cowling. Have someone advance the throttle while you watch the linkage movement. If shifter is hard to get into gear DO NOT FORCE it, spin the prop just a little. Watch to be sure the carb throttle plate is opening fully. The plate will be horizontal, and level and you should be able to see straight into the carb throat.

 

[MBH]

Ok. I will try that tomorrow after work and get back to you. I appreciate the advice so far. This is being the first outboard I ever tinkered with.

Whenever I took it out today, with just me in the boat it planed out fairly well. However my partner got in it and it wouldn't plane out until he walked to the front of the boat. Once it planes out, he would walk back to his seat and it'll stay planed out.

 

[MBH]

Quick question ... do you think the fuel line might cause the problem? I ripped the flooring up last night and had a strong odor of gas.