White plugs are indicative of running lean. Great way to need a replacement engine. Try turning your mixture screws a little rich (out)

When you spray carb cleaner or WD-40 around, while the engine is running, if you have a vacuum leak it will get sucked into the engine and change the RPM's. It will cause your RPM's to rise but any change you get is worth respraying and double checking.

Remedy for leaky carbs is a rebuild kit. You might want to try the screws on top of the carb but they are usualy very tight. Don't crack your carb by putting too much preasure and work in a star pattern.

Not sure what soaking the throats in laquer thinner will do. All the mechanisms that need cleaning are internal.