Changing air with altitude - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

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post #1 of 19 Old 07-14-2013, 04:52 PM Thread Starter
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Changing air with altitude

Ok, here's a thought for you carb experts. If big elevation changes like going up a mountain results in thinner air a thus a rich mixture, and opening up th air box makes the mix lean, why couldn't you set the carb to be right at lower elevations with a closed air box and then fabricate some sort of slide on the box that you could pull open as you feel the need to lean it out at higher elevations? Example:TAT ride where major elevation changes are going to occur?
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post #2 of 19 Old 07-14-2013, 06:45 PM
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Or use a Kouba mixture screw so you can adjust your mix from the fuel side. When I was running the stock carb I just braised a piece of welding rod bent at a 90 to the screw head.
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post #3 of 19 Old 07-14-2013, 08:39 PM Thread Starter
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Well now that's the part I don't understand. I'm thinking that the pilot mixture balances the mix for the first 25% of the throttle but has a diminishing effect as the throttle opens and first the needle, then the main jet take over. Would the "opened" impact the air mix throughout thr range?
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post #4 of 19 Old 07-14-2013, 09:45 PM
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Originally Posted by Bigryamaha View Post
Well now that's the part I don't understand. I'm thinking that the pilot mixture balances the mix for the first 25% of the throttle but has a diminishing effect as the throttle opens and first the needle, then the main jet take over. Would the "opened" impact the air mix throughout thr range?
Your right it wouldn't help the mid-upper range. A thought on your idea might be to obtain a spare airbox door and make a sliding door (so to speak) to test your theory. Might turn out to be the next great farkle, who knows.
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post #5 of 19 Old 07-14-2013, 09:53 PM
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Interesting concept and theory.... How high are you going? Around here, the 8000 ft. range is about as high as you can ride, give or take. I've read of the guys in Colorado riding above 10,000 ft. with minimal effects when tuning with nothing but the mix screw. I think there comes a point that the density of the atmosphere becomes so thin that the 40mm venturi just won't allow enough volume in to compensate for the altitude.
I am interested in this concept and curious to see some others chime in that actually ride the high country. I am sure they have some first hand accounts to share.
Your "variable airbox intake" is a cool idea, whether it is practical or not has yet to be seen. Maybe you could just drill the 1" holes and then use the rubber plugs to vary intake......?
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post #6 of 19 Old 07-14-2013, 10:13 PM
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I don't do it regularly anymore, but in the past I've taken different thumpers to include the KLR from around 900 feet up to ride in the mountains in Colorado at elevations up to 14,000 feet with no modifications to the air/carburetor system at all. All bikes seemed to me to run the same at 12,000 feet as they did at 900 feet. I noticed they all seemed to start a hair slower up that high, but that's about it.

I'm sure if you lived/rode there all the time, it would make sense to optimize everything for those altitudes.



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post #7 of 19 Old 07-14-2013, 10:24 PM
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That's kinda what I have heard, Planalp. The road over Lassen Peak is only about 8,000 or so and the bike seems to run fine. It is a bit sluggish but nothing that would warrant tuning anything more than the mixture screw.
I think if you want to address the situation properly you want to affect the density of the air, not the volume.
The carb I have now has no jets but is a bit pickier about fuel mix. Good thing all I have to do is turn a metering rod in or out. Takes about 5 minutes but if I know I am going to be doing a lot of higher altitude riding I just tune it for that before I leave or pull over and just do it.

Last edited by SkiBumBrian; 07-14-2013 at 10:27 PM.
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post #8 of 19 Old 07-15-2013, 04:44 AM
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Most tuners reduce intake restriction to enhance performance, seems to me, then make an effort at optimum jetting for the air/fuel ratio they want at their typical operating altitude.

No reason one couldn't have a variable-restriction air intake, I suppose, but . . . why strangle those extra horses (available from a less-restrictive air intake) at lower altitudes?

In the Bad Old Days Of Carbureted Automobiles, tourists drove their family busses from the seashore to Denver and maybe even to Pike's Peak, without mixture adjustment. An inevitable power loss at altitude was present, and--air/fuel ratio not optimal for high-country touring, but--for temporary/incidental travel, motorists muddled through, without carburetor alterations. (The high-country "natives" might make some lasting changes; Ford offered high-compression "Denver" cylinder heads for its flat-head V-8's).

Enough nostalgia!

The CV carburetor performs some limited altitude compensation on its on (a lessened Bernoulli effect from less-dense air at altitude results in less slide/needle lift at the same intake velocity/rpm, with corresponding mixture fuel-leaning). Not a great deal, but--I'd bet you could "make it through the night" with a sea-level tuned KLR in the mountains.

Tweaking the fuel screw might improve starting and idle performance, but the needle/needle jet and main jet dominate mixture beyond a quarter-opened throttle.

Then, the "Dial-A-Jet" might be the answer to travel with high vertical relief. The device permits jet-size adjustments externally to the carburetor. No personal experience, but--an interesting concept.

What the heck is a, "Dial-A-Jet?" Unless you've take tne lifetime blood oath NOT to EVER click on a link on a motorcycle forum, click here:

http://www.thunderproducts.com/dial_a_jet.htm

Last edited by Damocles; 07-15-2013 at 06:00 PM.
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post #9 of 19 Old 07-15-2013, 07:57 AM
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I did have an issue once with a 1982 Cadillac Sedan Deville I bought from an uncle. It had a diesel engine that was kind of a half-ass from-the-factory modified gas engine. I was driving the Beartooth Highway in Montana which I believe tops out at around 11,000 feet.

Halfway up, it felt like I was pulling a 30-foot travel trailer behind me and I actually had to stop and remove the air filter (which was clean) so I could make it over the pass.



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post #10 of 19 Old 07-15-2013, 08:44 AM
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I did have an issue once with a 1982 Cadillac Sedan Deville I bought from an uncle. It had a diesel engine that was kind of a half-ass from-the-factory modified gas engine.
Interesting anecdote, planalp!

Never thought about it before, but . . . since Diesels are fired by COMPRESSION, not electric spark (like gasoline engines), the absolute chamber pressure is critical, and the absolute chamber pressure drops decidedly with altitude. Cramming all the air possible into the cylinders might have made all the difference.

Glad you made it; those were notoriously unsatisfactory engines, as I recall.
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