2021 Tohatsu 6hp on 12’ aluminum Jon boat running @ 6500 ft elevation

[rcspott]

Is it more important to rejet the carb or get the right pitch prop?

 

[km1125]

Probably both. What prop do you have on there now?

To expand a little bit... They run as lean as possible for emissions sake, so adding altitude only complicates that. They also need to get up in RPM to deliver that rated HP anyway, so if you can't get the RPM up with the load of the boat, you'd need a prop change to accomplish that. The HP is also directly impacted by the elevation change, so you're probably taking a 15% hit there so if the current prop is not getting you up in RPMs at sea level, it's going to have a much harder time at 6,000 ft.

Now, if you're lugging the boat because you can't get to the RPMs, then it's really critical that you don't also go lean, because that could have devastating consequences. So, to answer your question o which is "more important", I'd point to the rejet if I could only do one or the other. The rejet might not get you to the performance you need/want, but it might save the engine from destroying itself.

 

[pvanv]

They do not run lean at higher elevation; they run too rich. There is available a 68 main jet and an idle jet 35 for high altitudes.

 

[km1125]

They do not run lean at higher elevation; they run too rich. There is available a 68 main jet and an idle jet 35 for high altitudes.

DOH! You're right. I was thinking air density was dropping the fuel flow. I'll go back and fix my post (if I can) or repost.

 

[km1125]

Looks like I can't edit my previous post, so I'll just repost it here with the fixes:

Probably both. What prop do you have on there now?

To expand a little bit... They run as lean as possible for emissions sake, so adding altitude offsets that to some degree. They also need to get up in RPM to deliver that rated HP anyway, so if you can't get the RPM up with the load of the boat, you'd need a prop change to accomplish that. The HP is also directly impacted by the elevation change, so you're probably taking a 15% hit there so if the current prop is not getting you up in RPMs at sea level, it's going to have a much harder time at 6,000 ft.

So, to answer your question o which is "more important", I'd (now) suggest getting the prop correct first, as even at sea level that can be an issue with getting the engine up into it's operating RPM with a stock prop.