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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 11-07-2012, 06:02 PM   #1 (permalink)
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Rejetting for altitude: any tips?

I've recently moved from SC to Prescott AZ. It's sort of a funny area to ride in: Prescott itself is at 5300 ft elevation, while Phoenix, about an hour south is at maybe 1500(?). About 1/2 hour north of here, Flagstaff is over 7000.

I've done a rough rejet on my FCR carbs from their SC settings (approx 800ft elev), leaning the fuel screws 1/4 turn, dropping the needles one slot and going one size smaller on the mains (142 from 145). The bike runs great here, Phoenix and up in higher elevations. I have an Innovate that I haven't hooked up yet, but I have no intentions of making it a permanent fixture, or of carrying around a rejet kit.

My question is simple: is there some rule-of-thumb for fuel mixtures and altitude? I'd like to find a comfortable middle ground so the bike doesn't go mega-rich in Flagstaff or dangerously lean in Phoenix.
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Old 11-07-2012, 06:38 PM   #2 (permalink)
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I would get all your settings alittle on the rich side in the high country that way atlest you wont lean out to much on the lower ground maybe set a/f at 12.5to 12.7 in the high country .It should run ok with that in the high country and be close to right in the low country.Maybe to be on the safe side run the a/f gauge both places to see what the change is.You will find a happy middle ground.As good as your bike runs once you get the setting I dont think you will feel the change in a/f much.
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Old 11-07-2012, 06:46 PM   #3 (permalink)
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+1 on Mike's advice. I'm at about 2000 ft here in Madrid and depending if I ride to the Sierra de Madrid, or the Sierra de Gredos I can go up to between 5 and 6000 ft. It's best to decrease your jet size by 0.5 for every 1,000 feet (300 meters). Find a mid point and you can't go wrong.
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Old 11-07-2012, 10:03 PM   #4 (permalink)
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Thanks, gents, I was figuring on around 12.6 here at 5300, hoping it wouldn't go bad lean down low. I guess it's a good thing these engines were designed to run lean from the factory. I'll try to get it to Phoenix with the Innovate on it to confirm.
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Old 11-08-2012, 06:40 PM   #5 (permalink)
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Welcome to Arizona! I never actually thought of this issue while living here, and the guys I ride with also seem to ignore it.
Fairly often we'll start somewhere South of Tucson and ride to Mt. Lemmon on the North side in the Santa Rita mountains. It has some peak elevations of 9-10K feet and I can't remember anyone complaining of poor running or loss of power.
That includes my carbed Bonnie and their electronic BMW's.
Great riding here - Enjoy!
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Old 11-09-2012, 07:52 AM   #6 (permalink)
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Originally Posted by LapinAgile View Post
...I can't remember anyone complaining of poor running or loss of power.
That includes my carbed Bonnie and their electronic BMW's.
Great riding here - Enjoy!
If you still have the CVKs, then you have some level of jetting compensation via the diapragm's control of the slide position. Eric has slide carbs, which don't have that 'luxury'...

Regards,

--Rich
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Old 11-09-2012, 09:15 AM   #7 (permalink)
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If you still have the CVKs, then you have some level of jetting compensation via the diapragm's control of the slide position. Eric has slide carbs, which don't have that 'luxury'...

Regards,

--Rich
Wow...Thanks beemerich, I sure hadn't realized the difference. It's still so lucky for people that I'm not yet their highly paid mechanic.
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