Quote:
Originally Posted by mollydog
This is typical at high altitude. I don't know your bike ... but if it has a
Pilot fuel screw (usually on bottom of carb) the fuel screw needs to be turned IN (clockwise) to LEAN out the low speed mixture.
If you can ... OPEN UP the Air box. I would leave the air filter ON. But try to remove any thing impeding air entrance into the Carb. On my DR650 I cut a big hole in top of air box to increase air flow.
Some air boxes will have a removable side panel. If you have this ... remove it.
If you can LEAN out the fuel screw (it may be hidden and sealed on your bike) then you will also need to raise up the idle speed. I'm sure you have this adjustment on your bike.
But a 250 is going to suffer at 3500 meters no matter what. But leaning out
pilot fuel adjustment, opening the air box and increasing idle speed will :
1. allow smoother running
2. better fuel economy
3. better starting/idling
MORE AIR is what you need!
Good luck! 
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Molly,
Cheers! Bike is a Honda 250XR Tornado. I have learnt about a lot of stuff on the bike, but the carb still looks a mystery to me. I`ll have a look for this fuel screw - and good advice about filter, my bike does indeed have a plastic panel so will remove this when I am finally reunited with bike (next week I hope!).
On a side-note, do you think when this sputtering is occurring it is better to gas it or close off the throttle? Better to drop down a gear and rev it or try to slunk along? Does this cause any damage to the engine or other components of the bike? I did a pass from Argentina to Chile and hit terrible headwinds, throttle fully open in second gear and probably doing 30-40kps!!!
Someone told me about getting the jets changed, but wouldn`t that affect performance when you come back to sea-level?
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