I recently was having issues with my 1984 XT600 (LC) dying at highway speeds. If I let off the throttle and tried to cruise at 55mph it would sputter and die. After a weekend of troubleshooting I found out my XT600 had a black screw in valve under the tank. OK, so California models suck. Literally, the clogged "roll over valve" was causing a hardcore vacuum in the tank and making gas and air evacuate the carb bowl and travel backwards up the fuel line. This "roll over valve" is listed as #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.  I just removed the roll over valve 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 the 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 acting up 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.
Just thought I'd share since I was sooooooo pissed off at it this weekend and there wasn't a ton of California Emissions stuff linked to sputtering out at speeds. And I know the roll over valve was bad because air could not be sucked or blown through it and if tapped on a hard surface debris would come out of it.
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Perpetually fixing an XT600 as old as I am...
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