Why is there a power loss when a motorcycle is climbing a mountainous road?

My motorcycle, Honda CG125 (single cylinder) is in perfect order… Runs fine on leveled road.

I have noticed that while climbing a mountainous road, initially it would run fine at lower gear and high rpm (7000+). But after some time, about 30 minutes, it would loose it power. Quite interestingly, I have noticed same behavior with other bikes.

I would like to have some technical reply i.e. How the power is lost at high RPM under load.

Shall appreciate, if links to some technical article and/or some animation is provided.

Your problem is likely due to the rise in altitude… Or over heating.

If you are gaining serious altitude you will find the air is thinner, so combustion is more difficult. Try boiling a kettle as an experiment of revery 1,000 ft you will find that water boils at 1 degree less.

It's NOT just your little 125cc scooter. Pretty much ANY internal combustion engine will lose power when climbing mountains. The air is thinner at higher altitudes which means less oxygen to mix with fuel to create combustion, which means lowered efficiency and less power output. This effect is more noticeable in vehicles with small engines because they don't have a lot of power to begin with. Engines with turbochargers or superchargers CAN compensate a bit for this loss, but even they will eventually "run out of air" and start to lose power.

Increased altitude means thinner air IE: less oxygen. Constant velocity carbs were the first serious attempt at stopping this from happening. EFI works better as the sensors tell the EFI unit how much air fuel mix to make. The old manual slide carbs simply starve your motor for air when the altitude increases.

Fuel mixture ratio is upset when O2 level and pressure less at altitude. The old way to handle this on the Bings and Amals carbs was to lower the needle, raise the clip to get a leaner mix. On BMWs /2s and many of the early limeys, this was a 3 minute job. On the early Harleys- the adjustable high speed/main jet mix wheel on carb was a 1/2 turn while running. 1980s cars with lean run emission for sea level would run much better, respond quicker when the owners got close to Denver. Your cycle- check manual for high altitude run settings.

It may be overheating if you are holding at a very high RPM for 30 minutes. This is why we have gears.

It may also be that you are climbing enough to have the thinner air at altitude start to affect the mixture, effectively leaning out an already lean mixture, which leads to even more overheating.

You should make sure your carburetor is set up for the altitudes you are running.

Heat, like, causes metal to expand, dude, including wires and connections, dude, and increases resistance, dude. Like, there's wires and metal and connections in your bike, dude. Nuttin too technical about it. Also check out Boyle's gas law and adiabatic expansion, dude. Like Wikipedia, dude.

Nothing will be loose if you tighten it down

Same thing happens on my 250 when I go up into the mountains. It is just the altitude messing with it.