dfam4

Hi all- I'm new to this forum, but am hopeful someone can help. Here's the story... I saw an add on Craigslist for a 1981 Yamaha xs400s- "$250 FIRM, Bring a truck" I figured since I rode one back in college, this would be a good bike to start on again- it's been a few years. I've put in a carb kit, gone through all the electrical etc. I replaced the stock airbox type air filter with K&N style air filters. Bike will start ONLY with starter fluid. Once it starts, it will run nicely. Will restart when warm on it's own. Float bowls have been adjusted to recomended setting, valves have been adjusted, and I just installed new plugs. Plenty of spark. Things I've noticed- Bike will only run when vacuum tubes on the intake manafold are not hooked up- they are open to the elements and you can feel the air going in and out of the them. When it's running, when I rev it up, sometimes, even though the throttle is back in normal position, it will continue to rev on it's own at about 4,000 RPM, and then gradually come down to normal idle RPM. Thoughts? Comments? HELP! Thanks!