I use 150 and 250 cc carbureted bikes for high mountains due to their weight. Whenever I climb over 10k feet they start missing/coughing and lose power which is abundantly needed here.

What can I do to make it run smoothly at this altitude?
Help would be appreciated.

Regards
Omar

rejecting is the norm, or try removing air filter if its for a short trip (this isn't good but sometimes one doesn't have a choice.)
notice I didn't mention onions in the air box....thats an old fashioned remedy that some people are convinced works

Edited due to brain fade!

NN if you remove the filter more air gets into the engine..engines run rich at altitude as air is rarified.
The OP said carbureted
Omie; it just takes a few goes at google and you tube, or do a search here

It's true, removing air filter allows more air which leans mixture, but you risk ruination of the engine due to engross of dirt. Best leave air filter on .... but somehow OPEN up air box to allow more air flow. (most are restricted to reduce noise)
More air will also help with More POWER overall!

High altitude makes mixture TOO RICH! More air means it's LESS RICH (leaner) ... but there is more to do.

Re-Jetting (not REJECTING ) is the real answer for better performance at altitude. Go ONE (or two) SIZE smaller on your main jet, leave Pilot Jet standard. Try to open air box up for more air flow. If you have a Fuel screw (Idle Fuel screw), turn this screw adjuster IN (clockwise) to lean idle mixture out. Helps with starting, idle and saves fuel.

All these steps will help the motor run better. But REMEMBER ...all engines lose power at high altitude, even Fuel Injected ones. A 150cc engine has very little power to begin with ... not an ideal bike for high altitude riding but GREAT fuel economy!

Air is thinner at altitude.. It has less Oxygen.

Oxygen is needed to ignite fuel.

So when you're at altitude, you need more air or less fuel. If you have the choice, both..


If the carb has an adjustable Jet needle then lowering the jet will lean the mixture up, and getting you closer to the correct ratio of air/fuel (15:1) to run properly. If the jet isn't adjustable you can run a smaller Main Jet. Go down one size.


I would never remove my air filter. Especially in dusty conditions. You will f**k your cylinder and dirty up your carb internals.


Final point.... Rich engines run cooler than lean ones. So if you go down in altitude and don't re-adjust your carb then you will be running a very lean mixture.

Lean engines overheat. Overheating is bad. Things go bang, rattle and pop.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

God bless fuel injection is what I say.

In this respect. It's hard to argue against fuel injection.

Honestly though. It's only the really small engined bikes that you really feel it.

The times you're going to ride that high are few and far between and most people just ride through it. Most don't even notice.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

Thanks gentlemen for your replies. This is what i understand,
At high altitudes air is thinner so less oxygen is being supplied to the engine.
Despite lesser oxygen supply, the fuel supply stays the same making the mixture too rich.
This imbalance affects combustion making engine lose power

The following protocol appears to be suitable,
1. Remove the air filter box for easy breathing while leaving the air filter connected. I can do it on all my bikes.
2. If still the problem persists make the mixture leaner by adjusting the screw.
3. Go back to the original settings once back to lower altitudes.

In Pakistan with the world's most enchanting mountains this kind of riding is not rare. I go there often, rather I mostly ride on the mountains and drive my MB in city and on highways. Biking takes you to the far off places no jeep can.
In my experience the big bikes are not very suitable due to their weight as falls are common, and also unavailability of spare parts. We had many problems with them previously. A few of our foreigner friends didn't believe me and had tough time on their GS's and Africat twins too.
I counter the power/torque issue with changed gearing. My 150 with 13x52 sprockets is strong enough to take me anywhere although it gets unsuitable for highways. However my favorite is water cooled 250cc Honda weighing 115kgs which I can lift singlehandedly.

Cheers
Omar

Hi Omie, you've got it.
but just to clarify
Remove the air filter box for easy breathing while leaving the air filter connected." I can do it on all my bikes.

You can't remove the air box (the filter would have nowhere to go) the suggestion MD and TT are making is just to take the LID off, so you would first take the lid off or somehow let more air into the airbox, (on one of my bikes there is a snorkel taking air into the air box and I took that off)

Secondly and only for emergencies take the air filter out.

best solution is to adjust jetting Not forgetting to go back to original setting when you get off the mountains, here you would need to do some trial and error to see what works best.

Other things that may work , adjusting ignition if it has manual points (Mollydog help me on this one!), riding when it is cool in the morning (air is denser), changing air filter to a less restrictive one.

Best of luck..just remember you can't really damage an engine if its running too rich, but you can totally **** it up if you run it too lean, hence readjust jetting after getting off mountain

I used to live in a place located at nearly 7000 feet above sea level and, therefore, it wasn't rare to ride at altitude.
My carburetted 250cc (big in other words!) Chinese bike, that I bought there new, worked just fine and I always assumed that it was jetted for that area.

__________________
Dave

Most new bikes these days, if using carbs, are jetted very very lean from the factory.

So no surprise your machine runs fine at 7k ft. My DR650 runs fine there too with stock jetting/air box in place. But 7K is not high by Himalayan standards. Even in the Andes of S. America, you are frequently riding well above 8K ft. to 12K ft.

My DR starts to suffer over 8K ft. but fine below that. By 12K ft. power really drops off badly, its running too rich, won't idle and hard starting. Fuel injected bikes run fine up high but also lose A LOT of power, but will still start, idle and run quite well, albeit down on HP.

Correct. What is commonly done is to cut holes in air box ... but DO NOT remove air box or filter. I cut open the top on my air box on my DR650, BIG improvement but requires different needle and different main jet.

Main thing is to change out your MAIN JET when running up high, then put bigger Main jet back in when at lower elevations.
Most modern CDI ignitions are NON adjustable.

My Chinese bike was purchased at 7K ft asl, in the foothills of the Hindu Kush.

My main point is that not all bikes start their life at sea level.

__________________
Dave

My XR250 tornado ran like garbage above 2,600m. Removing the air filter grill made a huge difference. I also jetted down to 128 from 132s. Bike purrs along, even over 4,500m can maintain 90kph and never worry about not making steep gradients. This was impossible without mods.