3 ms·

Lower air density means old carburetted engines run leaner - which since most such engines are tuned to run slightly rich for reliability in favour of slightly

Lower air density means old carburetted engines run leaner - which since most such engines are tuned to run slightly rich for reliability in favour of slightly lean, gives you more power up until the point they overheat and if they're a 2 stroke motor relying on the oil mixture in for lubrication the fuel, seize. One of the old traditional racetracks here in Australia is Mt Panorama at Bathurst. It's got about a 180m elevation change between the start/finish/pit straight and the top of the mountain. Back when they were allowed to race motorcycles there, old 2 stroke racebikes were notorious for seizing at the top of the hill if they'd been jetted down at the bottom and hadn't been set "rich enough" to allow for the altitude change. Many many riders crashed and said "It was running really strong, til it locked up on me and spat me down the road..." (No apologies at all for my off topic response to pedantry...)

mikestew 6y ago
Mmm, pretty sure sea level carbs run rich at altitude. Less air means a need for less fuel for the same ratio. But the fuel side didn’t change because we didn’t rejet, so now too much gets pumped in. If I’m guessing the bikes might have seized at the bottom of the hill instead? Or it just took until the top before the damage was done.
- bigiain 6y ago
  Ahhh, yeah - you're right. I've got it back to front there. In my defence it's been a long time singe they've been allowed to race motorcycles there. At least 20 years. And now you mention it I do recall bikes seizing at the end of pit straight, right down the bottom of the hill.
  - mikestew 6y ago
    In my defence.. In your defense, anyone old enough to remember carbs how work, even if only slightly, is forgiven the occasional lapse of memory. ;-)
    - bigiain 6y ago
      Uhhh, thanks. I think. (And get off my lawn!)
- foobarian 6y ago
  Could the lower air pressure at altitude still end up pushing less fuel in through the jets? It's not obvious to me how this would behave when deviating from normal conditions.
  - bigiain 6y ago
    Carbs are half black magic and half voodoo. If you find someone who _really_ knows how they work, pay them what they ask, and keep going back to them...
KMag 6y ago
Some Rotax two-stroke engines running pre-mix have been used in ultralight aircraft, so I think appropriately sized altitude-compensating carburetors designed to run a petrol-lubricant mixture may be commercially available. However, I imagine it's pretty expensive to put aircraft parts (even ultralight parts) on a racing motorcycle. These carbs use an aneroid to move a needle valve at the base of the jet as the atmospheric pressure changes, effectively dynamically re-jetting the carb on the fly.
- bigiain 6y ago
  Neat! Got any keywords that'd lead me to something like that on Google quicker than me randomly searching? (Or even a link to such a carb?) Edit: Found it. "High Altitude Compensator". Thanks anyway.

mikestew 6y ago

Mmm, pretty sure sea level carbs run rich at altitude. Less air means a need for less fuel for the same ratio. But the fuel side didn’t change because we didn’t rejet, so now too much gets pumped in. If I’m guessing the bikes might have seized at the bottom of the hill instead? Or it just took until the top before the damage was done.

bigiain 6y ago
Ahhh, yeah - you're right. I've got it back to front there. In my defence it's been a long time singe they've been allowed to race motorcycles there. At least 20 years. And now you mention it I do recall bikes seizing at the end of pit straight, right down the bottom of the hill.
- mikestew 6y ago
  In my defence.. In your defense, anyone old enough to remember carbs how work, even if only slightly, is forgiven the occasional lapse of memory. ;-)
  - bigiain 6y ago
    Uhhh, thanks. I think. (And get off my lawn!)
foobarian 6y ago
Could the lower air pressure at altitude still end up pushing less fuel in through the jets? It's not obvious to me how this would behave when deviating from normal conditions.
- bigiain 6y ago
  Carbs are half black magic and half voodoo. If you find someone who _really_ knows how they work, pay them what they ask, and keep going back to them...

KMag 6y ago

Some Rotax two-stroke engines running pre-mix have been used in ultralight aircraft, so I think appropriately sized altitude-compensating carburetors designed to run a petrol-lubricant mixture may be commercially available. However, I imagine it's pretty expensive to put aircraft parts (even ultralight parts) on a racing motorcycle. These carbs use an aneroid to move a needle valve at the base of the jet as the atmospheric pressure changes, effectively dynamically re-jetting the carb on the fly.

bigiain 6y ago
Neat! Got any keywords that'd lead me to something like that on Google quicker than me randomly searching? (Or even a link to such a carb?) Edit: Found it. "High Altitude Compensator". Thanks anyway.