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Old 06-23-2015, 05:48 AM   #1
deeve OP
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Carbs & Altitude vs FI?

This is more of a carb question, but I am curious..I am looking at going smaller riding for ADV style stuff. Going back and forth between a DR650 or a WR250R. l like the idea of FI for easier starts and no worries about riding between different altitudes. In the west when doing multi-day rides I do not think it would be uncommon to go from sea level ( or below) to 6000-7000feet or higher.

I am looking for thoughts on this type of riding with a carbed bike. I do not want to have to rejet in the middle of a ride...and I dont think many folks do that. Are there some carbs that handle this better then others?
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Old 06-23-2015, 06:12 AM   #2
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The BST40 CV carb on the DR650 handles altitude changes pretty well. I did a ride from South Bend, IN (600 ft ASL) to Portland and back on mine, hitting elevations above 10,000 ft ASL in Colorado. The bike, heavily laden as it was, would still accelerate going up hill at altitude.

The Yamaha is a wonderful bike as well, certainly a better bike for when the forest service roads turn to jeep trails and single-track.
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Old 06-23-2015, 06:57 AM   #3
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Quote:
Originally Posted by deeve View Post
I am looking for thoughts on this type of riding with a carbed bike. I do not want to have to rejet in the middle of a ride...and I dont think many folks do that. Are there some carbs that handle this better then others?
A CV carb compensates to some extent for altitude (air density) changes, but . . . think about it: In the Bad Old Days, when automobiles had carburetors, FEW (if any) tourists re-jetted when they took a vacation to Denver, even driving up to the summit of Pike's Peak!

Power loss remains inevitable at altitude, marginally compensated for by fuel-leaning the mixture, but . . . unless you plan on ascending Everest, I think the DR650's CV carb will help you make it through the night.

The WR250R remains a fine machine, and its fuel injection is more elegant than the DR650's carb, but . . . I don't think the high-altitude performance compensation differential reasonably drives the purchase choice, unless it's a "single factor of merit" . . . YMMV!
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Old 06-23-2015, 07:34 AM   #4
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I rode my old KLR from Death Valley at -200 ft up to Colorado peaks over 11k with no noticeable problems.

I now have FI bikes and haven't seen any problems yet either. But I don't think I have been below 3k ft.
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Old 06-23-2015, 07:52 AM   #5
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I've had a DR650 and presently have a WR250r.
I have ridden the DR at altitudes of 1200 foot to 13,000 foot. It handled the altitude changes pretty good to about 10,000 then started getting "Altitude Sickness", that being poor throttle response dictating the use of lower gears. Other then that, the bike performed admirably.
I went to the WR250r because I was looking for a bike that was a little lighter and easier to handle. The WRr experiences no symtoms of "Altitude Sickness" at any altitude. It, however does not have the brute low end torque the DR has. The WRr likes RPMs, it's almost like a 4 stroke motor with some 2 stroke tendencies.
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Old 06-23-2015, 08:12 AM   #6
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Thanks everyone. Doesn't sound like the carbs make toooo big of a difference on rides that don't get crazy high.

CNRED, I sent you an off topic pm
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Old 06-23-2015, 09:00 AM   #7
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Originally Posted by deeve View Post
Thanks everyone. Doesn't sound like the carbs make toooo big of a difference on rides that don't get crazy high.

CNRED, I sent you an off topic pm
It makes a much bigger (more noticeable) difference on a 250 than it does a 650.
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Old 06-23-2015, 09:34 AM   #8
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The CV carb (BST33) on my 350 does well. At 12'K it is a bit boggy, but rideable. Never taken my bike to sea level, 1'K was the lowest I have ever been on this stead.
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Old 06-23-2015, 10:01 AM   #9
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It's interesting, but I would think that a pumper type carb would not like the altitude much, but some reading I have done suggests otherwise.
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Old 06-23-2015, 10:16 AM   #10
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Of some interest is the loss of fuel pumps in some strange parts of the
world. Case in point is the rider presently stranded in Albania, on his
Dakar. Junya lost a pump in south america, but anticipated the issue
and carried a spare. Give me carbed and air cooled. But then again
I am an old fart.
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Old 06-23-2015, 10:21 AM   #11
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Originally Posted by deeve View Post
It's interesting, but I would think that a pumper type carb would not like the altitude much, but some reading I have done suggests otherwise.
That is correct. Pumper carbs do not do well with altitude change. That is why most road going bikes seem to have CV type carbs.
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Old 06-23-2015, 12:20 PM   #12
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Quote:
Originally Posted by MrPulldown View Post
That is correct. Pumper carbs do not do well with altitude change. That is why most road going bikes seem to have CV type carbs.
All non-FI Harleys between 1989 and 2006 were equipped w/ a 40 mm Keihin CV carb that had... an accelerator pump. The Keihin 40 mm CV carb on the Vulcan 1500 Classic was virtually the same carb w/ a different air cleaner mounting.

I'm guessing that the Boyesen Twinshot accel pump was developed to tune the squirt to conditions rather than the factory-supplied one-size-fits-all.

Here are a Keihin 42 mm CV (XR650L) and a Keihin 40 mm CV (Harley) side-by-side.



But I have heard that flat-slide pumpers like the FCR41 and Mikuni TM40 are sensitive to changes in altitude.
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Old 06-23-2015, 06:52 PM   #13
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Late to this thread, but noting that I had some issues with starts >9K ft with my newish WR250R (FI). I did nothing other than put 300 road miles on it and the problem went away. Is there an engine break-in period for altitude adjustment?
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Old 06-23-2015, 08:20 PM   #14
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Originally Posted by XDragRacer View Post
think about it: In the Bad Old Days, when automobiles had carburetors, FEW (if any) tourists re-jetted when they took a vacation to Denver, even driving up to the summit of Pike's Peak!
Yeah, and overheating, shitty mileage and stalling were business as usual, rather than something old people scare us around the campfire with
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Old 06-23-2015, 08:43 PM   #15
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Quote:
Originally Posted by MrPulldown View Post
That is correct. Pumper carbs do not do well with altitude change. That is why most road going bikes seem to have CV type carbs.
It has nothing to do with altitude and everything to do with manufacturers liability.
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