90HP Evinrude and altitude help!

[tunes]

New to boating. I can fix cars and planes do not know squat about an outboard. We brought a boat home yesterday that the seller swears runs great. Do not doubt that but we drug it home from Kansas to Colorado. Instead of 800 feet elevation it now lives at 5500 and runs like doo doo. The plugs are wet and wont turn up. I am sure it is running really fat. Please how do you fix that. I have also heard that a pitch change on the prop will probably be in order. How do I find that info.
Thanks Mark

 

[clanton]

Re: 90HP Evinrude and altitude help!

You have to change the carb jets, Dealer should have service bulletin with chart with jet sizes by model number. Two weeks before I can get to my chart, if you are not in a hurry, and you provide modle number

 

[jimmbo]

Re: 90HP Evinrude and altitude help!

You will also have to reprop it with a prop with less pitch 3" to 4 inches less.

 

[tunes]

Re: 90HP Evinrude and altitude help!

Thanks with that. It has a 13 3/4 x 15 on it now which I assume is standard for sea level. Since we will be boating from 5500 to 7000 MSL would a 11 be OK.
Thanks Mark

 

[jimmbo]

Re: 90HP Evinrude and altitude help!

I was assuming it was running a 17" or 19" presently. Those are most commonly used on boats in the 15' to 17' range. How big is your boat?

 

[tunes]

Re: 90HP Evinrude and altitude help!

The boat is a 17' Monark Mcfast FS. The motor is a 1984 90 Evinrude. I checked the prop for size and pitch 13 3/4 x 15. How much trouble is it to rejet the carbs?
Mark

 

[jimmbo]

Re: 90HP Evinrude and altitude help!

If the parts are availible, not too difficult. Remove the carb drain plugs. Using a narrlow blade screwdriver remove the old jet and install new one. As for the idle mixture, Some OMC motors of that era did not have an adjustment needle for the low speed. There are other persons on this board that know the OMC line much better than I.

15" prop on that boat @ 800 ft above sea level sounds odd to me. Unless it is very heavy.

 

[tunes]

Re: 90HP Evinrude and altitude help!

The boat is a fish and ski. Configured like a bass boat with extra seating forward. The boat doesn't pull or act heavy. Some one may have chewed a prop at some point and had that too put on. The prop on it is chewed and needs replaced anyway. I have read that the prop is more important than the carb jetting in getting the boat to run out. Has yet to be seen.

 

[SKEETR]

Re: 90HP Evinrude and altitude help!

I agree that changing the prop is a better way to maintain rpm levels at high altitude. My dealer told me that decreasing the jets one size for every 2000' elevation is the way they guage the jets. So if your carb is running 56c high speed jets you would want 53c's to get your fuel mix leaned down for 6000'.