I went for a trip to the Pyrenees yesterday. Absolutely fantastic trip, avoiding highways and running in local roads.

However once I reached certain altitude the engine started to fail and at some stage refused to run with the throttle open. It would only run at part throttle. At the same time, I lost the temp gauge. I thought that the ground cable had become loose.

On the way down, yes, the temp gauge came back to life and, of course, engine response was recovered.

I was so surprised on the effect of altitude on jetting, I could not believe it. Now I know what "running rich" is.

This is a std, wire spliced MC21. Max altitude was 1,500 m and I was 200 km away from sea level. Air temperature could be +/-20ºC
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Where's the photos of the trip?

Well, now you know how sensitive the jetting is to changes, having said that, that is a big range of variables you went through. When you say your temp gauge stopped working, did it max out of min out? That's interesting why it did that.

I did not take any pictures during the trip! I Had to meet a guy up there so I didn't stop except for gas (and was not necessary). But you are right, these trips are worth some pictures.

The temp gouge went down to zero. And I think it did that becouse the engine was running cold. My engine will only reach 60 when in the city or running on primary roads. It is true that a lot of heat is produced, as it really warm my legs, but the gouge will only indicate 60 or a little more.

When climbing the mountain, It wouldn't rev past 8k plus the rich condition plus air temp probably at 20C...

I had checked the gouge in the past earthing it and it does indicate up to max so I believe it works OK.
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i wonder if we have a science boffin on the forum that can explain why the temp gauge would drop at altitude, i don't believe the coolant temp woild have dropped that much

The temperature guage would no drop as a function of altitude. It will drop as a function of temperature. My thoughts are, do you have a thermostat installed? Is it closing or stuck open? Rich running shouldn't cool the engine that much.

Yes, you must be right about the thermostat. There's obviously not one in there, that's why it fell so low in its indication. This would also explain why it heats up so slowly and doesn't go over 60+ºC.

Should I install one?
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Took my CR500 up the passes on the way to Andorra once, it ran like a bag of spanners.

The look of suprise on the faces of the BMW GS owners, who were having a break at the top of the pass was priceless when they saw my 500 come past.
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Lemon owner and lover