If the following rings any bells I would love to hear any suggestions:

It's the worst kind of problem ie intermittent... Basically the bike loses all power for a second or so and then picks up again exactly as if I let go of the throttle and then slammed it on again. It only happens at altitudes above 3000 metres and only after a solid hour or two of continuous riding. If I stop and turn the bike off for 10 minutes it usually goes away but sometimes not. At one point I thought it was the breather tube on the tank being blocked and so I pulled over when the problem was happening and opened the tank cap, thinking that if the breather was blocked, that would clear things up. One time this made no difference and the second time I tried it it killed the engine and I had a nervous few minutes in a desolate place here in the Peruvian Andes unable to start the engine I don't really understand why this would kill the engine but maybe the system is pressurised in a way I don't understand??

I have changed the spark plugs and looked at the injectors (look clean enough to me). I was wondering if it was dodgy fuel as can be common in the Andes but the last time it happened it was fuel from the coast and of a higher octane which is supposed to be much more reliable. At that point I cleaned the air filter but it didn't seem particularly dirty. It's possible that has fixed it as it hasn't happened since but I am dubious as the following day was just descent and I haven't been back up high since.

I will be heading to Bolivia which is all about altitude and lacking in much mechanical support soon and I would like to feel more confident that I won't get stranded somewhere.... I am in Lima for the next day or two which is the nearest thing to civilisation I will see for a while (although the BMW dealer here has not inspired my confidence by messing up a simple valve clearance check..) so here is the place to take any action...

Thanks for any suggestions!

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http://wanderinggoo.blogspot.com
F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Nearly four years ago and not quite the same problem, but stuttering and stammering at altitude...

Turned out to be the side stand switch.

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Thanks Tim. I'll give the sidestand a wiggle next time it happens.

Guy

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F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Guy,
My twin did the same & I thought it was bad fuel (I'm in Africa at the moment) but Chris at Jungle Junction said this "No i have not attended to the Fuel pump problem yet, thank you for the early notice.
We had several BMW F 650/800 Gs with the same symptoms and know wot to do." So may pay to check the fuel pump.
Thanks
Paul

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Thanks for that Paul. Any suggestions as to how I would go about checking the fuel pump?

It would make sense if the pump overheats after a hour or so of constant use and that this only matters when it is working against the low pressure at altitude. Or something like that...

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http://wanderinggoo.blogspot.com
F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Does it occur when the fuel level is low? - pump is cooled by fuel in the tank and some overheat when there is not enough fuel to cool efficiently.

You can test by checking output volume - exact figures can be found on advrider forums.

Possibly fuel pressure sensor also.

Thanks for those ideas Docsherlock. The problem usually occurs after at least an hour of riding - more often 4 or 5, so yes the tank is generally less than half full. Chris at Jungle Junction has kindly sent me some instructions for cleaning the fuel pump so I'm going to try that.

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http://wanderinggoo.blogspot.com
F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Well I guess you don't need his email then LOL
He really seems to know his bikes. Mine did the same tho after about 4 or 5 hrs just goes to cut out like you've run out of petrol & if you idle off a fair bit it kicks in and goes again, but a couple of times mine stopped completely & started again about 10/15 mins later.
Cheers
Paul
Oh yeh I really love the bike !!

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....rather Die Living.....than Live Dying !
www.globetrekkers.net.au

Guy,
I see you have an Enfield ! I just bought one in India about 2 mths ago & is sitting on the docks in Brisbane but I want get to ride it till Xmas when I get home. I think they are fantastic bikes !!!!!
Paul

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....rather Die Living.....than Live Dying !
www.globetrekkers.net.au

Yeah I loved my Enfield! Broke down a lot but sounded like a spitfire when i rode through those Himalayan canyons

As for the GS, this is the first genuine problem in 23,000 tough miles... Have the fuel pump out and have briefly run it backwards (in fuel) Hopefully that will have removed any crud.

Take it easy.

Guy

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F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Hey Goo,

Check out this thread on advrider:

Fuel pump for 800gs - ADVrider

Good luck with it. Might be worth getting a GS911 tool.

Hi Docsherlock

Interesting thread - so are you suggesting I bypass the FPC?

I do in fact have a GS911. What are you suggesting I do with it? Bearing in mind I only have the mobile version with me.

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F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

Firstly, run some techron through the fuel system. Then:

1. Check to see if there are any error codes thrown. If there are, there is likely your problem.

2. Check your fuel pump output - if low, there is your problem.

3. IIRC the fuel pump controller has been less of a problem on the F798GS models; worth disconnecting the fuel pressure sensor to see if that fixes the problem.

Thanks for that! How do I check the pump output?

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F650GS twin - Enfield Bullet 350 - Honda XL250 Degree

You will need to double check the exact procedure in the manual but I believe you need to check the outflow (?90 l/ph) and pressure (??50 psi) for which you will need a container and pressure monitor. I am not certain of those figures - need to cross reference to manual.

I'd take off the fuel pressure sensor first and see if that made a difference as supposedly the bike runs fine without it, albeit the pump is at max pressure all the time.