I'll be the first to admit my technical skills may be lacking...

I'm from Missouri and recently had my ATV's jetted to ride the Silverton-Ouray area of Colorado.

I've returned now and my ATV's appear to run great with the jet's (don't have the sizes off hand) for the higher altitude. I'd just as soon not rejet as I travel between Missouri and Colorado often.

Now that I'm back in MO, am I most likely running lean or rich on the CO altitude jets?

I assume that if I'm running rich, but ok, I'm in no danger of harming my ATV's. I also assume that I am potentially posing them to more harm if I am running lean. Are these assumptions correct?

Thanks in advance for any expert knowledge on this subject!

 

I would just rejet. It only takes 10min max and your quad will be running the way it is supposed to. To answer your question though, it is better to run rich than to run lean. Neither is ideal in my opinion. Just my $0.02

 

I would guess lean....at altitude the air is less dense and contains less O2....lower altitudes the reverse is true....if you are jetted for 5000ft and then go to 1000ft you'r jet won't be putting out the fuel needed to properly mix with the increase in air density.

Here's a good article http://www.hondaxr.com/tech/howtojet.asp

Altitude Compensation

For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!

Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!