High altitude

[Captain Dick]

Just returned from Montana. Had trouble with a 1985 75 HP loading up and fouling plugs at idol. Had carbs leaned out as much as I could and still run at top end. Any suggestions for a Hoosier.

 

[fishrdan]

Re: High altitude

Just returned from Montana. Had trouble with a 1985 75 HP loading up and fouling plugs at idol. Had carbs leaned out as much as I could and still run at top end. Any suggestions for a Hoosier.

Did you only lean out the carb with the idle mixture screws, didn't lean it out by swapping jets in the carb?

Running at high altitude is a pain as any changes made for high altitude, must be reversed when going back down in elevation,,, or you'll toast the engine. High altitude adjustments from easiest to hardest:

- swap to a prop with less pitch, helps regain lost power for planing and top end
- re-jet the carb(s) and adjust idle mixture, corrects the fuel ratio
- bump timing a smidge, probably don't want to do this.....
- change gear ratio for drive if possible, only do this if running at high altitude all the time

When going up to 9000' I re-jet the carb leaner, drop from an 18P prop to a 15P and bump timing 2*. I have an I/O so the changes are relatively easy, 1-2 hours work before the trip, then 1-2 hours after the trip reversing the changes...

 

[bajaunderground]

Re: High altitude

What are you calling 'high altitude'? I know the jets should be changed depending on altitude? You lose roughly 10% HP for every 1,000' The factory manual should have 'high altitude' instructions? I never bothered on my Mercury (range 2,000'-6,000')? YMMV?

~Brett

 

[Chris1956]

Re: High altitude

Normally, standard carb jets are OK up to 5000 feet altitude. After that, you need to rejet the mains and bowl vent jets (backdrag jets). idle mixture is reset manually as well.

How high did you go?

 

[Captain Dick]

Re: High altitude

I was at 6700 feet. I am new to the larger horsepower outboards, 75 HP in my case. I have done most of my boating with a 15 HP Johnson. Still on the learning curve.

 

[Captain Dick]

Re: High altitude

What's the track record on the 1985 75 HP Mercury 4 cylinder? The motor had very low hours and runs strong on the top end. I can't seem to tune down the idol speed. It idols at over 2.5 MPH.

 

[Chris1956]

Re: High altitude

That should be a standard inline 4 Merc. A good design, and should have the modern ign system. The idle speed is controlled by the idle stop screw on the motor and the adjustment of the throttle cable. After you back off the screw, adjust the throttle cable to push the carbs closed.