R100GS at Altitude

[IanJ]

Greetings All

Has anyone had any experience with running A R100GS at altitude? We are heading down to SA and even now while we are travelling in the US at around 6000 ft the idle needs to be upped. (I never appreciated how flat Australia is)

Did you change jets? What sort of adjustment was needed? Tweaking carbs is a skill I haven't had a lot of experience with.

I did a search of HU and there is one other post relating to this, just looking for extra info.

Thanks for your help

Ian

[Redboots]

Originally Posted by IanJ

Greetings All

Has anyone had any experience with running A R100GS at altitude?

Hi Ian,

rode through western China and down through the Pamirs, Karakorams and Himalayas over 5 week period.
Hight was up to 4600 metres and the only problem was the idle went to rats.
And a bit down on power but that's no big deal.
I did not adjust anything. I didn't feel the need to. How often do you leave the bike on idle?

Cheers,
John

[AliBaba]

As you know you will loose some power and your idle speed drops. For me the power has never been a real issue but at 10.000 feet it is very noticeable. It might help to drop the needle one notch but I have never bothered.

I usually adjust my idling speed, but nothing else.

[beddhist]

We are in Tibet and there are a couple of these bikes with me. They didn't change anything. Lots of passes 4500 - 5200 m high. No problems.

[petefromberkeley]

Originally Posted by Redboots

Hi Ian,

rode through western China and down through the Pamirs, Karakorams and Himalayas over 5 week period.
Hight was up to 4600 metres and the only problem was the idle went to rats.
And a bit down on power but that's no big deal.
I did not adjust anything. I didn't feel the need to. How often do you leave the bike on idle?

Cheers,
John

Yep, it does not run very well at altitude, but I never did anything about it. No problem.

[IanJ]

Thanks for the replies,

I will need to up the idle as it just stalls immediately when I stop which is a pain while trying to manoeuvre at low speed two up and fully loaded but that would seem to be all.

I did check the plugs today and they are still in good nick, no carbon build up.

Ian J

[seanh]

I took my R65 over 5600 metres in Northern India, same experience had to adjust the idle but that's it. You can put in smaller jets and drop the needles, but if you have enough power to get up, there's no need really. As the altitude changes constantly in mountainous areas, you'll never get it exactly right anyway. Also, if you neglect to change it back at lower altitude, you will be running lean, which could cause some problems.
Sean

[stipej]

At stella alpina meeting last summer my 100gspd also stalled idling, but I used my throttle screw as a help to maintain idle rpm. No other problems, at sea level everything was ok again.

Cheers, Stipe

[gsworkshop]

Dropping the needle one notch will only have an effect at over 4000rpm. If you want to lean out the idle mixture for better idling you will need to adjust the screw directly in front of the float bowl. You can reach this from the bottom where you will find two screws, the small one is plugging a hole for connecting a vacuum gauge to help with balancing the carbs, and the larger one recessed in a small tower is the idle mixture screw. The idle mixture screw is normally set at a specific setting for different models but by turning it out a half turn or so you will supply more air to lean out the over rich idle mixture.

To make an adjustment to the idling speed is real simple. You need a flat screw driver an adjust the two hex screws with screwdriver slots in them directly in front of the carburettor cover from above.
If you are worried about the balance between the two carbs make a note of the amount you turn the screw in. For example start by making on quarter turn on each side and if that is not enough do another quarter turn till you are happy it idles fine. Remember to add the quarter turns and make a note to reset when at lower altitude again.(this technique can be used for both idle speed and idle mixture adjustments.)

IMPORTANT: do this once the bike is at the normal operating temperature. This will take about 10 minutes of driving under normal load.

As one member commented though you don't spend lots of time at idle but mostly around the 4000rpm mark. Dropping the nozzle needle one notch will have a dramatic effect on the performance and fuel consumption. The loss of power is partly on account of the smaller amount of air that mixes with the fuel, and as we know that to burn the fuel well we need lots of air. The problem is now compounded because the over rich mixture will cause the fuel to burn slower resulting in a loss of power.
Leaning out the mixture by reducing the amount of fuel supplied (this is the opposite from adjusting the idle mixture where you are increasing the amount of air to lean out the mixture.) will result in better burning of the mixture.
To drop the needle you do not need to remove the carbs. With the cables attached remove both the springs for the throttle and choke cables behind the carbs. You should be able to move both throttle and choke levers in an upward position to remove the cable nipples from the mechanism. Loosen the two screws on the cover and you can remove the cover with both cables attached.
Pull out the piston and by gripping the needle firmly turn it one quarter turn and pull on the needle. It should drop down about one millimeter and clip into the next notch firmly.
Refit the lot and remember to change the needle position back when you are at lower altitudes again.
The most common position is the third notch from the top and on some models the fourth, you will have to get the specs for your particular model though and remember that BMW changed the size of the jets and needles from some year models so it will differ on some year models as well.

[IanJ]

Thanks for that,

I'll digest all that and have a gentle tweak at a later stage,

Ian