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"Jeff, normally it is somewhat

"Jeff, normally it is somewhat cost prohibitive to "upgrade" a motor.

And I will start by saying that I don't work on Honda's, but looking over the parts lists between the 75/90 models, the only immediate difference I see is the the jets in the carb - so could be a very "cheap" upgrade.

Honda Dude would probably know for sure..."
 
"Jets in the carbs, or get new

"Jets in the carbs, or get new carbs. Cam shaft and ECM are the same. You could do it! Modifying the engine will void the warranty (if there is any warranty left)."
 
"I have a Honda 4 stroke, carb

"I have a Honda 4 stroke, carbureted. I typically run at sea level on the ocean. I have heard conflicting reports that if I want to use at mountain lake at altitudes of 6,500 ft., that I either need to get the carb re-jetted, or else I just need to swap out props (after recording RPM during a test run.)

Which solution is best, and why?"
 
"Jay, changing the prop is onl

"Jay, changing the prop is only really masking the problem. The motor wouldn't breathe properly, so it would be working harder to try and turn a smaller prop (at a lower speed with less thrust).

Swapping out the jets would allow the engine to breathe in a way it was designed to. It would however, still burn a little more gas because of the altitude.

Swapping jets is a very simple job and you would have them if you used the motor again at that altitude - besides, the jets cost a few bucks (typically, under 20 bucks for a set), the prop could be quite expensive in comparison..."
 
"Graham,

Thanks for the adv

"Graham,

Thanks for the advice. My father was an auto mechanic, so although that is not my forte, I at least picked up enough residual knowledge to know that carbureted engines perform differently at various altitudes. I couldn't really figure out how a different prop would resolve the altitude issue.

Is the jet swap-out something that could be done by the average Joe following a repair manual?"
 
"Jay, yes, a very easy job - t

"Jay, yes, a very easy job - the jets are really nothing more than a (screw) with a pointed end, for lack of a better way to explain them.

A manual will give you any particular set-up or adjustment requirement (if necessary) - but basically you simply unscrew one and replace it with the new one.

On a couple of the Merc models they had a "high altitude" version - the difference was the jets and the gears.

Lower gears also help to compensate for the poor breathing of the motor - hence the prop change (trying to make the motor act as if the gearing was lower).

However, for occassional use, simply re-jetting should do the trick for you. If you are going to make running it at high altitudes a habit, you may also wish to re-prop. But I wouldn't immediately go out and buy one.

Try it with the jets changed and determine, by rpm, if another prop is warranted."
 
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