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Fueling Issues at Elevation?

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Jester13 View Drop Down
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    Posted: 25 Sep 2022 at 21:00
Hello gents, I went for a two day ride over the weekend on the loneliest road in America to do a bit of rock hounding in the Nevada desert. I live at 4500 feet of elevation, and I'm used to a bit of hesitation at low RPM; I figure it's just the personality of the engine. However, while in the mountains between 6500 and 7600 feet I noticed significant power loss at times and unexpected shuddering at low RPM. I initially turned traction control off (even though the light wasn't blinking) because it felt similar to the way the system cuts throttle when the back wheel spins in sand. Since my wheel wasn't actually breaking traction, that didn't help. There were a few hill climbs where I was holding a constant throttle position and I would feel the bike abruptly lose a bit of power, but it would keep pulling.

My gut says it's probably a fueling issue (evap system pulling vacuum maybe) rather than a fuel map problem, but I figured I would check in with you guys before making any changes. My bike is a stock V7iii Rough (no changes to fuel map or any parts other than tires.) I am open to swapping parts and remapping if it will make the bike happy at elevation. I am also open to modifying the evap system if necessary. I love this bike and hope to make it as elevation insensitive as possible.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tris Quote  Post ReplyReply Direct Link To This Post Posted: 25 Sep 2022 at 21:12
IIRC the engine detects the air pressure at each start and uses that to set the fueling until its stopped

If you didn't stop that might be the issue.

However, I think that I'd check that all the sensors  (air temp, engine tem, etc.)  were working within  parameters  just to eliminate them
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Andy M View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy M Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 2022 at 08:00
So you climbed a few thousand feet, or the bike just runs a bit rough? 

There is going to be less power than at sea level . You fill each cylinder with 375 cc's of mixture, that's a fixed amount. At sea level roughly its 24cc's of petrol and the rest air. At 5000 feet there is 84% of the oxygen so 20cc's of petrol to burn cleanly. The energy is all from the petrol, so a 17% drop in power (or increase in fuel consumption because you don't use all the power all the time) . To avoid the basic power drop you need a supercharger.

The FI can deal with this while with carbs you had to fit a smaller jet, but you still lose the power.

These old Bosch style FI systems do indeed typically only set the altitude at ignition on, so keying off and on can improve things. 

If you are just running around at a given altitude you are probably just feeling the difference that these engines have when run to meet the green requirements. They are fine, just not carbed or running over rich. Ditch St. Gretas box then look at fuel maps if it bothers you. 

Andy 


Edited by Andy M - 26 Sep 2022 at 08:14
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Jester13 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jester13 Quote  Post ReplyReply Direct Link To This Post Posted: 26 Sep 2022 at 20:56
Thanks for the information. At one point I did stop and refuel, and when I pulled out onto the road the bike shuddered and almost stalled when shifting into second. I've never ridden this bike below 4500 feet so I don't know what it feels like at sea level, but I've never experienced abrupt changes in power like this before. It's not a gradual loss as the elevation changes, but rather a sudden noticeable change when going up an incline. Typically what happened was while traveling uphill and holding a constant throttle position, I would suddenly feel the bike lose a bit of power and start to slow down. Sometimes I could open the throttle to maintain speed but sometimes I had to downshift even though the bike didn't seem to be struggling moments before. It sort of feels like a carburetor with a piece of debris in the main jet.

What is the best way to verify the sensors are working as expected? Connect to the OBD and refer to a table somewhere?
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Andy M View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy M Quote  Post ReplyReply Direct Link To This Post Posted: 27 Sep 2022 at 10:31
That sounds more like a general fault rather than something related to altitude.

When was the fuel filter last changed?

If the FI knew anything you'd expect it to put the light on, but yes, OBD reader will be useful. Could be coil trouble too.

Andy 
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