jets and altitude

[rccook16]

Newby question I can' t seem to get an answer on. Would like to take my 2006 17' Lund with 90 2 stroke from Michigan to a lake at 6300' for a weeks fishing. Last time I took a 50 Hp to lake it would hardly get on plane at WOT. Planned to change main jets before the trip but can't get any info on what size from Yamaha. They did say to try a prop change. Pretty expense "trial". Any info would be appreciated.

[rodbolt17]

your best bet would be to contact a Yamaha dealer in that area.
some motors require rejetting,reproping and a different gear ratio set.

[rccook16]

Thanks for quick reply. Closest dealer was in Salt Lake and wasn't much help. I will try marina at lake and see what they do.

[boscoe99]

To properly rejet the carburetors takes time and money. It could conceivably take an entire weeks time for you to test and determine the specific jets that would prove to be most beneficial. Yamaha is very proud of their jets and charges accordingly for them. However, if you can find out the specific type of jet that it is used you can get them much cheaper over the internet.

If and when the motor is rejetted it is still going to produce less horsepower at altitude than it does down below. So, a different propeller that will allow the motor to reach at or near its redline when the boat is loaded will be needed. More testing and money.

As rodbolt suggests, call dealers in the area where you will be boating and see what they can recommend. Does not have to be a Yamaha dealer, it could just as well be a Mercury, Honda or Suzuki dealer. Maybe they can suggest going down XYZ in jet sizing and/or changing the pitch of the propeller by ABC inches based upon the motor with either (1) the original jets or (2) altitude jets. Most Yam dealers that I have dealt with won't have much, if any, experience with jet changes. Yamaha US itself offers very little guidance on this matter. But then it is more of an art than an exact science.

Wouldn't it be nice if our motors had a variable mixture control and an adjustable pitch propeller like some airplanes have?

Here is a link to some information by Mercury Marine on this subject in case you have not seen it. Maybe give them a call as well. Ask for Joe in their customer support department. He is very helpful.

General Propeller FAQs | Storage & Maintenance Propellers FAQs | Mercury Marine

[rodbolt17]

biggest difference between aircraft and boats?


boats don't tend to change altitude much.

I have mostly lived on the salt pond and never in the mountains.

I understand high altitude engine issues but have no real practical experience with it.

[rccook16]

Ok. Sounds like I'm buying a new prop since no info on jets. Probably should have a spare anyway. What is on my 90 is an aluminum 3 blade, with these numbers 6E5, 17K, Y-3. I think it's about 13" dia. Now would you gentlemen care to recommend a prop for higher altitude and less power? When I called the marina at Flaming Gorge they said to bring an assortment of props?

[boscoe99]

A 6E5 K17 aluminum propeller has a diameter of 13 and 1/4 inches and a pitch of 17 inches.

Without knowing what the current WOT RPM is with the boat loaded at sea level it is all but impossible to offer a suggestion to you. But, I will do so anyway.

I would try something with about a 13, if not 11, pitch. Better to be under propped than over propped. Just mind the tachometer.

Since the boat is only going to be used for a short period of time, and if it were me, I would go with the cheapest aluminum propeller that I could find.

Here are many different types and sizes of propellers that will fit your model. Solas seems to be the cheapest of the lot.

OEM Yamaha Propellers and Aftermarket Solas Props - 90 2-Stroke (15 Spline) 1984 - Present

[rccook16]

Thanks much for the rec. I trust you more than the dealers. I will watch the tach. Going to be doing more *****ing than running alot. Thanks again. You guys are a great resource for a novice like me!

[99yam40]

higher altitude means less O2, so motor will be running rich.
At least you should not have to worry about running lean and burning a piston.

Did the marina you spoke with give any info on re-jetting your 90

[BQUICK]

If you are not running ethanol gas then switching to it would lean it out some.

[rccook16]

The mechanic at marina said they were on the edge of elevation for jet changes. Most people change props around 4 pitch sizes. So a 13 pitch sounds right. He said bigger boats running 150-200 HP change from 23's to 17.