prop change or re-jet a force 85 at altitude?

[capnvinny]

I have an 86 Force 85 and live at about 7000 ft. I've been told I would get much better preformance if I re-jetted the carbs,but then I had another mechanic tell me to change the prop to another pitch and it would be cheaper and if I wanted to drag the boat to a lower altitude and use it I could just change the prop back(made sense)Trouble is no one can seem to tell me the pitch change I should make???Anyone??

 

[fishrdan]

Re: prop change or re-jet a force 85 at altitude?

If the boat is going to live at 7000' and not go down to sealevel I would re-jet the carbs,,, AND swap to a lower pitched prop. If it's going to be bouncing around in elevation, just swap props.

For prop suggestions, what is the current speed, RPM, etc (look at the stickies at the top of the forum).

I run 4" less pitch going from sealevel to 9000', but it's a different engine combo.

 

[emoney]

Re: prop change or re-jet a force 85 at altitude?

You should start by sharing what type performance you're now getting, and what is your current prop size? I've always heard it's best to rejet. Max RPM? What type boat? What's the ultimate goal?

 

[Frank Acampora]

Re: prop change or re-jet a force 85 at altitude?

At 7000 feet elevation, the air is less dense so the engine will ingest less oxygen for each piston. Therefore it will run rich and be a bit sluggish.

Changing jets will correct this and get the mixture back to correct proportions. Changing jets to a smaller size for altitude is easy and quick. Just unscrew the fuel bowls and the brass jets will show on the bottom of the carb casting. Unscrew them and screw in new ones.

Of course, the high speed jet only LIMITS the fuel delivery at full throttle, therefore if the boat will be used at cruise almost all the time, a jet change may not be necessary.

However, since the engine will be making less horsepower it is likely that the prop will now be too large for correct performance.

So: BOTH mechanics were wrong and both were right: If you intend to use full throttle, change both the jets and the prop to a smaller size. Otherwise, a lower pitched prop to account for reduced horsepower will probably suffice.

Usually you go down one jet size for every 5000 feet of altitude so you would step down two jet sizes. And, a Guesstimate of pitch reduction would be between 2 and 4 pitches less.

Understand that the engine will be making less horsepower: Acceleration, load carrying capacity, and top speed will all be less.


What in hell did they do to the forum now? Every last or partial sentence is centered. That's not me.