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Engine missing over 10k feet Engine missing over 10k feet
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Photo by Igor Djokovic,
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  #1  
Old 28 Oct 2015
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Engine missing over 10k feet
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
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  #2  
Old 28 Oct 2015
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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
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Old 28 Oct 2015
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Edited due to brain fade!
Last edited by Nuttynick; 30 Oct 2015 at 10:32.
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  #4  
Old 28 Oct 2015
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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
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  #5  
Old 28 Oct 2015
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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!
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  #6  
Old 28 Oct 2015
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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.
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Old 6 Nov 2015
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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.
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Old 27 Nov 2015
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You can also set your bike up to run lean 'standard' at low altitude, then fit a sock or restrictor over the air-intake that'll rich it out again. Run with the sock at low altitude and without the sock at high altitude. Quick fix if you're constantly running up and down mountains and not practical to change jets etc. and you can check the mixture when you're testing it.
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