Post by: fossilfuel on April 14, 2025, 12:14:46 AM
As I explore and asses the bike, I noticed this (see pic). I believe that is the "high altitude loop" (HAL) ??? I live in Austin (600 feet above sea level), so if that is the HAL, I should unplug it....?
Correct me if I am wrong. Thanks in advance. Love this forum!!
-mark in austin
Screenshot 2025-04-13 at 10.20.59 PM.png (69.24 kB . 766x576 - viewed 427 times) k751.jpeg (80.9 kB . 768x576 - viewed 503 times)
Post by: Duckbubbles on April 14, 2025, 08:13:00 AM
Frank
Post by: The Mighty Gryphon on April 14, 2025, 11:46:13 AM
I have taken my K75RT as high as 9400 feet without it, and down to sea level with it plugged in. Never noticed a thing; fuel mileage and performance hardly changed, if at all. Last couple years I toured that bike I ignored the plug.
If you understand the operation of the Jetronic system, you realize that it automatically adjusts for altitude. The only actual function I can think of is that it prevents the mixture from being too rich when the engine is being started to avoid flooding, as this is the only time the "barn door" isn't controlling the amount of fuel injected.
I'm not positive, but I think it is supposed to be used above 1000m of altitude or about 4000 ft.
Post by: fossilfuel on April 14, 2025, 02:07:12 PM
I think I will take it off--- the last thing I want in a Texas Summer is a lean condition.
-mark
Post by: daveson on April 14, 2025, 02:25:04 PM
I think it leans out the enrichment mixture, which only comes into play in full throttle situations. At full throttle, with the barn door fully open, the computer has no way of sensing the air density, which is reduced at high altitude. The amount of time spent at full throttle is only a tiny amount of the time spent during a ride, so it would be difficult to measure the reduced fuel efficiency by riding without it at altitude.
An exhaust analyser might be a way to test this.
Post by: The Mighty Gryphon on April 14, 2025, 05:34:21 PM
If you lose the loop, you can always tape a magnet on the fuel line to the rail.
Post by: The Mighty Gryphon on April 14, 2025, 05:38:27 PM
[/quote]
Actually, you would need a Lambda sensor.
Post by: daveson on April 14, 2025, 07:44:31 PM
It really only works with four hole injectors.
Why?
At higher altitudes you want to reduce the enrichment regardless of how many holes are in the injectors. Fossilfuel's brick is a K75 so I'm guessing it has single hole injectors.
Post by: daveson on April 14, 2025, 07:46:58 PM
An exhaust analyser might be a way to test this.
Actually, you would need a Lambda sensor.
Why?
An exhaust analyser can measure a rich mixture. I'm also thinking some exhaust analysers do have a lambda sensor
Post by: frankenduck on April 15, 2025, 12:20:24 AM
That is the loop. I suspect it was something added to appease the EPA.
I have taken my K75RT as high as 9400 feet without it, and down to sea level with it plugged in. Never noticed a thing; fuel mileage and performance hardly changed, if at all. Last couple years I toured that bike I ignored the plug.
If you understand the operation of the Jetronic system, you realize that it automatically adjusts for altitude. The only actual function I can think of is that it prevents the mixture from being too rich when the engine is being started to avoid flooding, as this is the only time the "barn door" isn't controlling the amount of fuel injected.
I'm not positive, but I think it is supposed to be used above 1000m of altitude or about 4000 ft.
False. The L-Jet has no way of sensing altitude or barometric pressure, much less adjusting for it. That's' why that plug exists.
I've taken a K75 up to 10,000 feet . It idles like crap and dies when .stppoed due to the low air apressure.
Post by: daveson on April 15, 2025, 05:56:08 PM
I think Gryph's main point is along the same lines as mine, that the LE Jetronic is capable of compensating for altitude, and does adjust the fuel to suit. For example, two bikes riding along in similar conditions except that one is at higher altitude, the barn door on the brick at altitude would be deflected less, due to the lower air pressure.
But with the barn door fully open, it can't sense the air pressure. The altitude plug is connected to the throttle switch. The throttle switch enriches the mixture at full throttle. A signal at pin 3 of the fuel injection computer from the throttle switch means that the throttle is in the idle position (or that the altitude plug is connected) a signal at pin 2 is a full throttle signal. But what if the computer gets a signal from both pins at the same time? It's impossible for the throttle to be in the idle position and the full throttle position at the same time. I think the only sensible conclusion for the computer is that the bike is at full throttle with the altitude plug connected. Therefore it reduces the enrichment to compensate for high altitude. Also, with a constant idle position signal regardless of the rpm signal, the computer could interpret that as relevant revs in high altitude mode.
Do I know what I'm talking about, or is this just a bunch of guesses? It's just a bunch of guesses. Previously I looked at Bert's guide and electrical drawings and asked, what's the logical explanation, well that's my guess. Previously I had a similar but more refined idea about it, but it escapes me at the moment.
Fossilfuel, welcome to the sometimes meandering Motobrick.
Post by: fossilfuel on April 15, 2025, 08:02:28 PM
In other news, I pulled the loop and found the plug in the little bag of goodies that came with the bike. I didn't know what it was until this conversation, so thanks guys!!! icon_cheers
-mark (fossilfuel)