Found the name of the questionable part. It was apparently a California model and it is called the "roll over valve" and is part # 20 on the California tank diagram. WTF I hate California stuff. I;m off to research how people bypass this garbage.

OK, so California models suck. Literally, the clogged "roll over valve" that goes from the tank to the canister was causing a hardcore vacuum in the tank and making gas and air evacuate the carb bowl. This "roll over Valve is listed at #26 on this diagram:
1984 Yamaha XT600 (CA) XT600LC Parts, 1984 Yamaha XT600 (CA) XT600LC OEM Parts - BikeBandit.com
The previous owner just removed the canister and plugged a long piece of fuel line from the broken roll over valve to a nipple on the carb just above the Fuel line nipple. "Thar's yer problem..." I just removed the roll over valve entirely and it will just act like a vented tank now I'll eventually try to put a one way valve on there like they sell for MX bikes to let air in but no gas fumes out. in the winter when I store it I will WD40 an empty tank or fill it with gas and add stabil to prevent rust from condensation.

The scoot will run with that nipple on the carb plugged or unplugged. It just makes a sucking noise like "ffuff fuff ffuff" as you go down the road. This leads me to believe that dirt could get sucked into my carb bowl... so I'm going to cap it off. pretty sure thats a safe bet as the parts fiche doesn't even show the nipple on the carb.

So if you have stumbling issues at highway speeds make sure its not due to crappy California emissions gear that is faulty/improperly removed. I made sure this fix was repeatable and sound advice. I put the parts back on and the fuel line to connect them and it caused stumbling EVERY TIME. Take it off and it stopped EVERY TIME. Also make sure you're not lean due to your airbox snorkel coming off under your seat as I'm sure that was not helping the troubleshooting process. doh

fuel problem
 

Hi GUYS MY BIKE IS CUTTING OUT AT HIGH SPEED WHO CAN HELP

Decompresion
 

Hi guys what are the chance that it is the decompressing lever in the cam that can cause the bike to cut out at high speed

Clean out the filter in carb. Under the fuel cut off valve.

I have an 86 xt600s and when I first got it I had the same issue the cutting out at high speed and was also rough to start the previous owner had installed the wrong spark plug! The plug installed wasn't the recommended plug and had the wrong heat rating (not hot enough) not saying for sure that you guys have the same issue just wanted to throw it out there

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1990 XT 600 dying
 

Hello, I realize this is an old thread.

I have a 1990 XT600. I've had it for 3 years and it's never given me any problems. I am not sure if it's a California model. I think its just an EA.
Yesterday morning it started acting like it wanted to die while on the freeway connecting into town. I was going 55mph at the time but as I got into town the speed limit was only 35 and it continued acting as it wanted to die. I would pull the clutch in and it would die, I'd pop the clutch out and it would start but keep cutting in and out. I coasted into a parking lot and it died. It started right back up and I made it to work. On the way home it did the same thing. So where would I look for something that would cause the engine to die intermittently? Coil? Some sort of sensor? Kill switch making intermittent contact with handlebar? I've searched the Internet for other having this issue and this was the only information I've found so far.

I suspected the kick stand sensor so I kicked it a few times while riding and nothing happened.

Thanks for your help.

Yes you can check the switch on the sidestand first, but i think its time to do a clean of carb. Iam thinking the filter inside carb is clugging up from rust, check tank and see if filter inside is still there.

Thanks xtrock
 

I will check those things. Hope that's it. :-)

Hi if it depends on the throttle mosition its carborator. I had simular problems enging shuting down at 50 km/h caused by a broken regulator. At too high voltage the CDI switched of for emergancy: Engine of at 50 km/h.
Good luck!

Thanks TA-Rider
 

Thank you TA-Rider. That's good info.

Just injecting some new life into this thread....

I had this problem - bogging at 4500rpm - and limped home some 600km from Aachan to London a few weeks back, in the slow lane.

After cleaning everything else, switching inlet manifold rubbers etc I finally got to underneath the valve seat as above today and found a small nest of s**t under there.

Cleaned out, problem solved and celebratory fish and chips consumed.

Hat tip to Ted