if you think it's your valves you could have some really bad deposits on them... you could do a trick that is old school....
put about a pint of transmission fluid in your intake (slowly), then mist water in there from your hose... don't do a lot, just use a wide spray and only have one part of the cone go in.... don't worry about the dreaded water lock as your not putting enough in there to do that....
the water will make the tranny fluid crystalize and break up the carbon... then you just wait for it to clean it's self up before shutting the engine off (normal exhaust, no smoke)... then you might want to add a fuel injection system cleaner... Zmax does a great job of breaking up carbon... all the way down to the O2 sensor, but it's $20... you can go with lucas' fuel additive for a few tanks and you'll notice a difference.