I have a Nextstar Select ARF. The last ltime I ran the engine to make sure that everying was running okay. I notice air bubbles in the engine to carborator fuel line. Is this bad? Can I still fly it like this? I did the pinch test and everything seemed okay.

Air bubbles usually mean that you have an air leak some where in your system. As for flying the plane, it would not be recommended as the engine will more than likely lean out and die. Fuel line is cheap, might want to consider changing them. Good Luck, Dave

If the bubbles are very few this can be normal. But if the bubbles look like they are taking up 10 or 20 percent of the volume passing through the line, then this indicates a leak. And often it is inside the tank. So check the fuel lines both inside the tank as well as from the tank to the carb.

ive had some experience with this. make sure to check and see if air bubbles air comming in when you have a full tank. When fuel levels are low fuel can bounce around and cause air bubbles to travel to the carb in some cases. last time i had problems with air bubbles i checked my remote needle valve and found fuel escaping from a crack. when you spin up your engine check to see if any fuel is being blown back from your remote needle valve. if so be sure to replace the assembly

Vibration can also cause bubbles as the tank gets down past 1/2 way. If your prop isn't balanced it can give you fits.

RCken wheres the pic?

if the bubbles stay in one spot in the line, its fine.
if their is one steady stream from the tank to the carb then its bad

Extra air can cause a lean condtition.

I haven't given him this one yet

This is an excerp from the article I wrote on tanks. [link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=1068]Click here for the rest of the article)[/link]:

At any time you may experience problems with your fuel system. Here are some of the more common problems and things to look for.

One of the first things to look for in any situation is any sign of bubbles. Not all bubbles are bad, but you should know the bad ones from the acceptable ones. The first type of bubble is just a little foam that may accumulate in the fuel line (Pictured on top of illustration at left). It is usually just sits in a high spot in the fuel line and it may or may not have much movement. Pictured at the bottom of the illustration is a line of bubbles that are moving from the tank to the carb. This is a definite sign that you have an air leak somewhere, and it needs to be corrected.

An air leak is usually caused by a bad connection, a pinhole in a line, or a malfunction in some apparatus (Like a fuel filler or remote needle valve). Now it's time to do a little detective work!

Start by filling the tank. If the bubbles appear while the engine is running with a full tank, you can pretty much figure that the leak is outside of the tank because all of the tank's internal components are surrounded by fuel (That's a good thing - you don't have to take the tank apart!).

The next thing to check is the line from tank to carb. Is there anything between the two, like a fuel filler or remote needle valve? If so, do the bubbles start before or after it? These are the types of questions you'll need to find the answers to. For instance, if there are no bubbles from the tank to the filler, but the bubbles are seen between the filler and the carb, replace that piece of fuel tubing. If the bubbles persist, your fuel filler valve is leaking air.

If the bubbles only appear at a time when the tank is less than half full, then the leak is most likely inside the tank. The easiest remedy is to replace all of the internal tank lines.

Thank you for the advice. I concluded in the 3-way system that the carb line needed to be replaced. I replaced the old line with the same diameter tubing but thicker. I should be able to get the tank back in without taking off the engine. Right?

i just did it w/ my gp rapture. it wasnt easy but it want hard either.
my problem was the plane dieing on takeoff. if i could get it in the air it would be fine. but it would die/struggle on takeoff
so i raised the tank as high as i could (center line of the tank was a about1in lower than center line of the carb, now its about 1/4 of a inch lower)
took out my fuel filter and tried to make the line more direct.

what i had to do was take a long piece of fuel line and run it through the muffler and engine feed holes, out to the middle of the fues where i could plug the tank in. then i very carefully pulled the lines w/ the tank back into its origional position (from the muffler/engine ports).

Sounds like you fixxed the problem. But for future reference, If the air bubbles are in the line going from the needle to the carb ONLY, then there is an air leak in the fuel line itself, the o-rings that are on the needle are leaking or one of the fitings are loose.

This is a pressurized system, and the only place that will have negative pressure is from the needle valve forward.

If your fuel line was leaking between the tank and the needle valve it would show up as fuel leaking instead of bubbles. If you have bubbles in the line from the tank to the needle valve you have vibration problems with a full tank because the pickup tube would be fully submersed.

Something I haven't seen mentioned often is today most of our fuel has an antifoaming agent in it and we get very little foaming from vibration these days. In the past foaming was a real problem if your plane vibrated very much and we really had to soft mount the tanks with a lot of foam to keep it from happening. Not so bad these days.