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Old 09-03-2007, 08:12 PM   #1
ewalenza
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Default (very) high altitude alterations

We are soon leaving for Tibet and will be passing over peaks higher than 15,000ft. The bikes we're riding are single piston, 125cc Hondas. We've been told that the jets will need to be changed according to altitude but we don't know which size to buy. To complicate matters we are in China and must content with the language barrier. If you have any experience with a similar trip, or are simply a motorcycle guru, please help us prepare for this journey.

Many thanks,

Erik and Chris
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Old 09-03-2007, 08:46 PM   #2
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As every engine is different, it will be nearly impossible to say "you need X-size jets for Y-altitude". I don't know much about 125 Hondas in general, am not a guru, etc. BUT - here's my suggestion.

If it's possible, find out ahead of time what exact model the bikes are, what make carb(s) are on the bikes, enquire/look to see what jets are in there now (if other than stock), and someone will chime-in with a jet size-suggestion for you to carry with and swap-out once the bike(s) start to sputter. It's unlikely to be "spot on", but if you have a range of sizes (jets are relatively cheap) you might be able to continue under the bike's own power, rather than push it up the side of one of the Himalayas.

Easy enough if you're riding the bike(s) right now, not necessarily-so if you're planning on renting/buying just before your trip, I know.
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Old 09-04-2007, 05:20 AM   #3
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To go from sea level to 9000 feet I had to go three jets leaner on my IT175. But it's a 2 stroke and thus more touchy than a 4 stroke.

You don't mention at what altitude the bikes are currently set for. Air pressure will drop from about 14.7 psi at sea level to 8.3 psi at 15K. since pressure at 15K is 56% of pressure at sea level then your 11HP CB125s are only going to put out about 6hp. Don't forget to consider that as well.

I'd guess that you probably should just get several jets for each bike leaner than stock. They are pretty cheap. As you ascend and notice rich conditions change to smaller jets. Just keep changing to smaller ones as needed. At least they are easy to change on the CB125.

Don't forget to change back on the way down.
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Old 09-04-2007, 06:43 AM   #4
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Quote:
Originally Posted by seruzawa View Post
To go from sea level to 9000 feet I had to go three jets leaner on my IT175. But it's a 2 stroke and thus more touchy than a 4 stroke.

You don't mention at what altitude the bikes are currently set for. Air pressure will drop from about 14.7 psi at sea level to 8.3 psi at 15K. since pressure at 15K is 56% of pressure at sea level then your 11HP CB125s are only going to put out about 6hp. Don't forget to consider that as well.

I'd guess that you probably should just get several jets for each bike leaner than stock. They are pretty cheap. As you ascend and notice rich conditions change to smaller jets. Just keep changing to smaller ones as needed. At least they are easy to change on the CB125.

Don't forget to change back on the way down.
I hear Benny has a wide range of jets.

Sincerely,

EJ
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Old 09-04-2007, 08:23 AM   #5
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OUCH!

>Rimshot!<
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Old 09-04-2007, 10:08 AM   #6
sarnali2
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Quote:
Originally Posted by Buzglyd View Post
I hear Benny has a wide range of jets.

Sincerely,

EJ


sheesh, there's one in every crowd.....


On the plus side you could probably trde your decedant western subversive Goodbye Yellow brick Road album for whatever jets you need plus a healthy supply of curds and yak butter to munch on...
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Old 09-04-2007, 11:02 AM   #7
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...since a 125 honda 4 stroke single only goes anywhere at full throttle, just hold that sucker wide open to you reach the crest, and coast down the other side. You can also engage in a fine game of "turtle chasing" where the Honda 125 rider chases a turtle until he runs out of gas, the turtle becomes exhausted, or the rider is able to flip the turtle on it's back with the regulation turtle chasing spatula that's avaialble at WalMart. Have fun!
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Old 09-04-2007, 05:48 PM   #8
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Sounds like an epic ride. Wish I was going!
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