R100gs at altitude....

mavis cruet · Jul 5, 2022

Hi folks,
I’m starting to plan My trip to South America from the UK, and am contemplating taking my r100gs. There will be plenty of riding at serious altitude and I wondered if anyone has any experience with riding an airhead 2 up and loaded at altitude? Any ideas on which jets i will need to carry/change? And by how much? Any thoughts appreciated....
Cheers,
Karl.

DrFarkoff · Jul 6, 2022

If I remember correctly

Theres something about BM Bing carbs self adjusting for altitude due to them being CV carbs

Thinner air = less lift = less fuel

SIM · Jul 6, 2022

When you off to SA and for how long?

Tetley · Jul 6, 2022

High Altitude

Agreed they are CV carbs and will adjust to altitude changes. Last year I did some hi passes on my 860 with 32mm carbs. Once I got above 1500 mtrs the power loss was significant, normally it runs sweet as. If you are two up with luggage and riding continually at hi altitude I would defo recommend re jetting.

ChasF · Jul 7, 2022

We took our R100GS up to 4000metres on mount Kenya, 2 up with too much luggage. Can’t really comment on the power because mostly from 2500 metres up was first and second gear on a steep, rutted track but the bike ran smoothly.

oldnfat · Jul 10, 2022

By weird coincidence I happen to be visiting friends this weekend. My mate is ex BMW U.K. of some 35 years and started work prepping and looking after press bikes…cutting his teeth on airheads. They prepped bikes for a Himalayan expedition and his view was that above 2000m you will have lost c. 50% power with exponential loss of power as you climb.

To cope they re jetted, advanced the timing and put free flow air filter. He reckons that if you contact a BMW dealership that the specs for altitude running are still kicking around and should be able to get hold of them from U.K.

Hope that helps.

wilbjr21 · Jul 10, 2022

I wouldn't worry about it too much. Any change you make will just have to be undone when you return to lower altitude.

I did some high passes in South America two up with luggage on a 2003 XRV 750 Africa Twin. The highest was just short of 5000 metres.

I didn't really notice anything different as the engine was still smooth and it was all low gear work with the dirt and hairpins.

Chris R · Jul 10, 2022

I’ve ridden my R100/7 over many of the high passes in the Alps two up and loaded and you will notice a big power drop. I likened it to having the power of a BSA Bantam over the Col du Galibier. It gave me great respect for the Tour de France riders.
However it will not harm your bike and it may pop a bit on decent ( because of the rich mixture). As others have said you’ll not be going fast anyway.
Go and enjoy it.

Sent from my iPhone using Tapatalk

SteveHop · Jul 10, 2022

Basically the higher you go the richer the mixture gets because the air/fuel ratio changes .There is less air .

Here is Snowbums article , towards the end of it he talks about riding at altitude . https://bmwmotorcycletech.info/bingcv-2.htm

Also if you can download the Bing manual it talks about jetting changes in there

Tetley · Jul 10, 2022

oldnfat said:
By weird coincidence I happen to be visiting friends this weekend. My mate is ex BMW U.K. of some 35 years and started work prepping and looking after press bikes…cutting his teeth on airheads. They prepped bikes for a Himalayan expedition and his view was that above 2000m you will have lost c. 50% power with exponential loss of power as you climb.

To cope they re jetted, advanced the timing and put free flow air filter. He reckons that if you contact a BMW dealership that the specs for altitude running are still kicking around and should be able to get hold of them from U.K.

Hope that helps.

That sounds about right for the power loss.

C

Gelandestrasse · Jul 11, 2022

SteveHop said:
Basically the higher you go the richer the mixture gets because the air/fuel ratio changes .There is less air .

Here is Snowbums article , towards the end of it he talks about riding at altitude . https://bmwmotorcycletech.info/bingcv-2.htm

Also if you can download the Bing manual it talks about jetting changes in there

Interesting article. Reproduction of some text:

"As you go UP in altitude, the carburetor will operate richer. The fuel per amount of air is INcreased. That fuel may not be able to be burned efficiently. This is more so on the slide carburetors, as the CV carburetors tend to compensate SOME for density altitude changes. While a richer mixture might burn more fuel than necessary; & ultimately carbon-up the combustion chamber more, it is SAFE. If you were to use a leaner "jetting" ("jetting" used in its broad sense here), power will be increased some; more or less depending on the density altitude. But, the problem is that if you change the jetting by more than a rather modest amount leaner for riding at high altitude, then as you go down in altitude, say to sea level, you may be running too lean, and if excessive, this could possibly burn a hole in the piston or otherwise cause lean over-heating, if extreme enough. Thus, unless you plan to be exclusively riding at around a mile or more high in density altitude, I recommend you DO NOT change any of the jetting from stock. "Jetting" as used here includes both needle position & main jet size, or even possibly the needle jet; and, rarely, the pilot jet."

So, for R100GS (which has CV Bings as standard), leave alone unless living up at altitude.

Redboots · Jul 11, 2022

I've ridden at 4600m plus at the highest and 3000m plus for some days and yes it was down on power but you always come down again and it would have been a ballache to keep swapping jets. The bike was fine. (R100GS)

Andygbsmith · Jul 18, 2022

As a rule of thumb you lose 3% power for every 1000 ft you climb, the problem is compounded if the mixture becomes very rich.
A friend took his GS all the way to Alaska I’ll ask what he did carb wise.

chasbmw · Jul 23, 2022

Back in the 1980s I rode a R80 two up in the Altiplano, (13k- 15k ft). Yes CV carbs will self adjust for altitude, but you will feel the loss of power from 8k, onwards and it gets more difficult from 12K onwards….. I took with me a selection of jets and reduced used the jet sizes. Given that this was 40 + years ago, I can’t remember exactly what I did, but I would take a set of smaller idle jets and mains, and lower the needle one slot. It’s not difficult to do and if you are spending at least a couple of weeks in the Altiplano, it’s worth doing. Whatever you do the lack of oxygen at altitude means that you will down +- 40% in power.

I went back to Peru about 5 years ago….the roads are very much improved since the 80s, a 13 hour slog through mud and fording streams is now a proper road for Tourist busses.

R100gs at altitude....

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