I posted something like this in another thread, as it appeared to be the same problem, unfortunately no one came up with a solution, so I'm going to post this hoping that someone can come to my rescue.
The problem started several thousand miles ago in Peru. I noticed that gas was leaking from my petcock & a local mechanic fixed it & I was good to go. A couple of hundred miles later the same/similar problem occurred, again another mechanic did something or other to it & I was good to go for another thousand miles or so. It then happened again as I was climbing up to 7-8000' only this time it wouldn't start. I stripped the carb, though everything seemed spotless, changed the main & the pilot jets, but still no go. Got it towed to a mechanic who replaced the carb, and everything was hunky-dory, up until yesterday. I was at the end of a 300 mile day, as I got into stop & go traffic it started again, coughing & spluttering & eventually dying. I left it for 45' or so to let it cool down, then it started back up again? I would add, that before, the problem mainly occurred (Though not exclusively) when I was at altitude, this time however, I was descending. The weather was hot, but the engine was nowhere near red lining.
I am beyond concerned now, as I am heading into deepest Patagonia shortly, where ditching the bike will probably be the only alternative. Obviously something I do not want to do, as we've been through a nearly 2 year ride down from San Diego. If anyone has any ideas, I surely would appreciate any advice. Cheers Y'all. Tony

Hi,

Did you adjust your pilot screw/fuel mixture screw? This helps with starting the bike.

You can expect the KLR to have problem running at idle at high altitude if no change is made to the carb and your air box is stock. My klr was like that in peru at high altitude. However, i had drilled a few holes on the top of the air box. When riding at low altitude I would cover them and at high altitude I would open them. It did help.

You replaced the carburator: are the jets the same as when you were riding at low altitude? Is it currently jetted for high or low altitude?

Jetting or fuel mix might be a contributing factor, but I doubt it's THE problem, if only because I ran my KLR with stock everything (jets, airbox, the works) up to about 5000 meters/16,500 feet. It had hardly any power, and it was reluctant to start but never refused outright. I think there's something else wrong--alone, or in addition.

OP, I'm not sure you've posted what you've tried in the moment--you know, spark, fuel, the various safety/kill switches (some of which are surprisingly finicky), filters, electrical connections.... KLRs are pretty simple, and when something major goes wrong it's usually not too complicated. I posted some possibilities in the other thread which you just resurrected, but I'm not anybody's idea of an expert.

The two times I can think of when my KLR really would not start (dead batteries, detached connections, and kill switches aside) were due to 1) an exploded doohickey, and 2) a full tank of badly contaminated fuel. Oh, and that time I ran out of gas and for some unknown reason there was none at all in my reserve.

Hedging my bets here, but from personal experience with a gen 1 klr, the problem was the petcock itself. It started misbehaving in the way you describe and after lots of messing about/ attempting to fault diagnose, I eventually borrowed a fellow klr rider's petrol tank and the problem was "solved". I diagnosed that the culprit was to do with the vaccum pipe/ system from the petcock to the motor block that allegedly is some sort of "safety feature" that when the piston isn't going up and down (the engine is off...) no fuel is going to the carburettor.

I solved my problem on my tank by dismantling the petcock assembly completely and making it permanently "on" gravity fed "straight through" with the vaccum pipe/ rubber gasket in the petcock and on/off switch disabled. A leatherman/ lockwire and petrol proof gasket sealant is what I used. Also a search on eBay USA (dot com) of aftermarket klr petcocks (gravity only, without extra/ useless features) showed I wasn't the first with this problem. But being in El Calafate in southern Argentina meant I wasn't buying anything like that from the USA in a hurry. My "bodge" lasted many thousands of miles until I sold the bike.

Good luck sorting the situation.

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