Bonneville America Forums Forums Technical Forums Performance Mods High altitude?

I live pretty much at sea level or slightly above. I'm planning a trip with 3 Harley friends trailering our bikes to Colorado and riding through the mountains. If my bike runs great now, what problems am I going to have at 5000-10,000 feet above sea level? I haven't been doing carb adjustments and have left that to my dealer. I really hate to mess with it.

The other bikes all have fuel injection. I assume with the thinner air my mixture would become too rich. Will the FI bikes experience the same problem? Is there a simple answer?

On Harleys they often install a an adjustment under the seat for the fuel injection and ignition. I can't remember what it's called anymore. If they don't change anything I'm not sure if thier o2 sensor will compensate or not. If you are jetted correctly for sea level you will be running extremely rich. If you can open up the air box at all it will help. We used to flip the air cleaner lid over on the truck. If you have the air box still might try pulling the cleaner elemant out and putting the lid back on with no element.

Talk to Phil (Bonnyusa). He seems to get by adjusting the mixture screws for this.

Presently, there are no snorkels and a UNI filter in an otherwise stock air box . Are you suggesting running with no filter to increase air flow?

Yes, used to do that with the truck when up in the rockies in Wyoming. I have also done it on an old Triumph , I removed the filters all together and put them in a bag on the back of the bike leaveing the back of the carbs totally open.If you are worried about any debris put a piece of screen over or under the snorkle hole.

EDIT: Keep in mind going from sea level to 10,000 feet you will most likely still be rich.

The highest vehicular tunnel in the Rockies is the Eisenhower tunnel at just a hair over 11K feet above sea level.

I was wondering the other day how our bikes would handle that ascent.

It'd be a scenic ride traveling from Utah across into Colorado and down into Denver. The descent from the tunnel down to Denver is kind of a wild ride. Lots of truck driving schools in the area use that descent as a training tool.

Oh if you decide to use my method and put screen over the hole,simply remove the lid cut screen slightly larger than the lid , poke the lid screws through the screen and bolt lid back on, even better would be to not use the lid at all and just use the bolts and washers to bolt the screen over the hole left by the lid allowing in max air.

After studying up in the shop manual and the Haynes (better description), it would appear I would need to readjust the pilot screws. The Haynes manual suggests warming up and turning off the engine. Then turn both carb screws all the way in and then back out 1.5 turns. Start engine and adjust to correct idle (I do have a tach). Then, adjust one carb at a time adjusting a small amount in or out to achieve the highest idle rpm. Reset idle to correct and then do the other carb.

Sounds fairly easy if I have the correct right angle tool. I just hate to mess up the fact it's been running good. I guess if I make note of exactly how many turns out the screws were, I can get them back very close to that when I get home by doing the same idle adjust process.

Ian, I would think the thin dust filter foam like I use on all my audio amplifier fans would work good for that purpose. It would be fairly easy to cut a piece larger than necessary and even duct tape it on the sides and put the cover screws back in.

That sounds like a very good idea.

As long as the foam doesn't get sucked in the carbs...

I lived up there in Breckenridge for several years, as well as at Lake Tahoe. The rides are exceptional. You will have a lot of power loss. the fuelie boys will have much less of a problem, but will also lose power. It is not uncommon for cars to only be able to muster 45 mph up some of the inclines, especially heading to Buena Vista and Leadville.

If you are just passing through, I'd adjust the mix screws. You'll only be affected for a little while. The elevation drops quite a bit from the pass.

If your going to make a week of riding, I'd bring some jets. there are some magnificent roads that go well over the tree lines.

Try reposting with a Colorado heading.
We have several members there and I know some have ridden highly varied altitudes.

For myself, I live at around 2000 feet, and have ridden from 280 BSL up to 7000 that I'm sure of with no real noticeable difference in performance.
10,000 or 11,000 is a different story though.

I live at close to 2,000 ft, and I've been up at over 8,000 ft on a long weekend more than once. There's noticeable power loss, and the bike is harder to start in the morning. I also have to turn up the idle knob to maintain 1,000 rpms.

Since you don't like to wrench, I'd leave it as is, especially if it's only going to be for a week. If it was 2-3 weeks, I'd change the main jets, but then i don't mind doing the work.

I don't know what your shop has done to measure the performance of your present setup, but according to the jetting calculator on Pat's Website, you should be running 133 mains. If everything is copacetic though, you'd want to drop down 1 jet size for every 1-2,000 feet of elevation increase. And according to this website, you'll lose about 3% of your power for every 1,000 feet of rise in elevation due to the reduction in available oxygen.

When you're in the high country and you feel your bike losing too much power, turn your air/mix screws in a quarter turn. You could go another quarter if you need to. You can also pull the filter when you are really high (in the mountains I mean). Shouldn't hurt anything for a few miles, unless you hit construction, or gravel, or a dust storm. Then I'd put the filter back ASAP. That might be enough. I live at 7200 feet, and I often ride up to 10000 feet. I feel a little power loss, but not much. You'd be surprised what a quarter to half turn on the air/mix screws will do for you. Just keep track of the adjustments you make and set them back when you get back to lower altitudes.

Also, the air is usually cooler up there, and therefore a little denser. That gives you a slight compensation factor, especially in the early morning.

to go from 7,000 to 10,000 a turn on the mixture screw might do the trick but not from sea level to 10,000 feet.

I don't change anything from 7000 to 10000. I'm just making a suggestion.

>Sounds fairly easy if I have the correct right angle tool.

Maybe this is your excuse to get Brent's thumbscrews.

From everything said, I gather going from sea level to 10,000 will be a HUGE difference. I may only have 4 days of riding on the trip which could vary from 5000 feet to over 10,000 when going over mountains. I'm not quite sure what the best plan is. I was hoping to not have to re-jet.

What if I adjust the mixture screws when there (5-6000 feet) and let it go over the mountains that way? It sounds like I have to do something or I could be in trouble.

You won't be in trouble you will just run rich and lack power. I would pull the element as we discussed and maybe the screen under the cooling fan foam to be safe. Do that and the mixture screws at the 5,000' mark and you should be good to go. I'm sure it won't be right but it will help compenate for the high altitude. At 10,000' I doubt anything you do will make it run great.

Thanks for the tip Ed. I was planning to order the small D tool from Brent. Once I figure out how to do more adjusting myself and get more comfortable fooling with it - I'll be really glad I was forced to learn.