You said the first time you flew it you had experienced pilots tune it, so you know its tuned properly. This is an incorrect statement. The high speed needle should be checked every day at the beginning of the day. Atmospheric conditions change on a daily basis that affect the high speed needle. If you tune it on a hot, humid day, the air is thin and full of water, requiring a leaner mixture. Then when you go to fly on a nice cool, dry day, the needle will be set too lean, and the engine will overheat and deadstick.

High speed tuning is simple, takes a few seconds before the first flight of the day. Its easiest with a tachometer, but can be done by ear. After starting the engine, go to full throttle. Now turn the high speed needle a few clicks in the richer direction. You should hear a slight drop in rpm. If you hear an increase in rpm, you are lean. Continue richening until you hear a slight drop in RPM. Now your ready to go. If you hear a slight drop in RPM when you initially richen the mixture, then start leaning out the mixture until the rpms no longer increase. Now go back in the richening direction until you hear a slight drop in RPM. Go fly.

Bubbles in the fuel line shouldn't be a problem unless your seeing them in the line between the needle valve and the carb.

You also may need to adjust the low speed mixture as the engine breaks in. If the engine is deadsticking when you try to go from idle to full throttle, you may need to adjust the low speed needle. This takes more time and patience, and should be reserved until you've run several tanks of fuel through the engine. The good news is that the low speed needle setting should never have to be readjusted once you get it properly set.

Brad