Temperature vs Altitude question

I have noticed my bike rides very well in the altitude range of 2500-5000ft. No problems. With temperature, totally different. Runs great at 75-100 degree temps but once the temp drops in the 60s or below, it starts to sputter and run like jetting is off. Anyone else have similar experience? Seems to me the temp plays more of a role than altitude.

My jetting is unrestricted bike, 168 main, 48 pilot, 4th clip position on one size richer yamaha needle, 2.5 turns out on fuel screw, rest is stock.

I have noticed my bike rides very well in the altitude range of 2500-5000ft. No problems. With temperature, totally different. Runs great at 75-100 degree temps but once the temp drops in the 60s or below, it starts to sputter and run like jetting is off. Anyone else have similar experience? Seems to me the temp plays more of a role than altitude.

My jetting is unrestricted bike, 168 main, 48 pilot, 4th clip position on one size richer yamaha needle, 2.5 turns out on fuel screw, rest is stock.

i noticed the same thing one time down in baja...its cold and it was bogging bad. i just turned the fuel screw and that seemed to fix the problem. oh and im running my clip on 5:excuseme:

Thanks - I may drop the clip one position. I also get good results with simply turning the fuel screw out when its colder.

Thanks - I may drop the clip one position. I also get good results with simply turning the fuel screw out when its colder.

i believe the fuel screw it to help blind the pilot jet with the needle (i could be wrong though!)

http://www.atlanticmx.com/archives/00000135.shtml

here is a good article about the fuel screw

none of the links work

I have noticed my bike rides very well in the altitude range of 2500-5000ft. No problems. With temperature, totally different. Runs great at 75-100 degree temps but once the temp drops in the 60s or below, it starts to sputter and run like jetting is off. Anyone else have similar experience? Seems to me the temp plays more of a role than altitude.

My jetting is unrestricted bike, 168 main, 48 pilot, 4th clip position on one size richer yamaha needle, 2.5 turns out on fuel screw, rest is stock.

Your jetting has about a 30 degree temp range and about a 4000' elevation range. Once you start exceeding these ranges, re-jetting is probably in order.
I have noticed my bike rides very well in the altitude range of 2500-5000ft. No problems. With temperature, totally different. Runs great at 75-100 degree temps but once the temp drops in the 60s or below, it starts to sputter and run like jetting is off. Anyone else have similar experience? Seems to me the temp plays more of a role than altitude.

My jetting is unrestricted bike, 168 main, 48 pilot, 4th clip position on one size richer yamaha needle, 2.5 turns out on fuel screw, rest is stock.

Your bike sounds like it's on the lean side.

As altitude gets higher the air gets thinner, so your bike would automatically be running richer - same fuel, less air. Normal correction would be to lean out, so if your bike is already working good, it must be a bit lean.

As temperature gets colder the air gets denser, so your bike would automatically be running leaner - same fuel, more air. Normal correction would be to richen the mixture, so if your bike is not working good, it must've been a bit lean to start with.

Since you work better at high altitudes and warm temperatures, you must be running close to the lean limit for your normal temp and altitude. I'm not saying that the lean jetting you have now is bad, in fact you may be jetted close to perfect for your normal operating conditions. If you were running on the richer side, your bike would act the opposite, better at cold and worse at high altitude.

I change my bike jetting more based on temp, and my changes are fairly basic since I don't want to be changing jets every week. My snowmobile was a different story. Lean and it would blow up (2 stroke), so I changed jets a lot, and they were much easier to get at. I installed exhaust gas temperature probes into my exhaust pipes (twin cylinder) and a gauge in the dash which displayed live tempertures while the engine was running. The leaner the engine runs, the higher the exhaust gas temperature climbs toward the melting point of aluminum.

I kept it running as close to the danger point as possible (lean) for better power and response, but going from the mountains to sea level, or temperature dropping 5 to10 degrees, my exhaust gas temp would hit the danger point and have to I'd stop and swap. I had a small jet kit and tool kit under the hood so I could swap on the trail in just a few minutes. The bike won't blow up, just run a bit worse, so I ride and just deal with it if I don't feel like swapping jets.

I agree with you Frostbite. So what I did was enrichen the pilot circuit with a 48 pilot, from the 45. The other day I was riding it was in the high 40s low 50s. I rode the same area with the 48 pilot (everything else same) and the temp today was in the high 70s low 80s - lots warmer. Well I ended up getting to ride okay but needed the fuel screw only 0.5 turns out.

Anyone know if its a bad idea to keep it in that far?

Soon it will be warming up and I will switch back to 45 pilot.

Thanks for all the help guys. Oh, yeah - put a new Maxxis IT desert on rear and I love that tire. I am a convert! Fourth gear wheelies for the first time!

none of the links work

Sorry. Been offline for awhile. If this link works you'll end up with a good primer on jetting your bike. If it doesn't, run a search on "Jetting Guide: The Yamaha / Keihin FCR Carburetor.":thumbsup:

http://www.thumperfaq.com/jetting.htm

Cairn

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