Don Presten

I am having an issue with air (bubbles) in the fuel line of my Hobbico NexStar. The issue is making it very difficult to to tune the engine. I performed a fuel system pressure test and that is fine. I think it could be the fuel tank itself vibrating. Looking into the very tight front comparment, I can see that the tank is installed without any foam padding. The tank seems to be wedged into a hole (cut to match the tank size in a secondary firewall and is surrounded by thin hard rubber padding.

My concern is access to this tank - it appears to have been installed during the assembly process and is so far forward of the servo access area, you don't have easy access to it.

Has anyone experienced this issue on the NexStar and if so, how did you correct it. I am about to remove the entire engine assembly in the hopes that I can access the front part of the tank from there and stuff some foam padding in there to support it. I have already tried stuffing foam arround the rear portion of the tank that is semi-accessable from the servo access area.

Campy

My Feedback: (1)

Stuffing some foam in will help.

If it is a vibration problem AND access is as difficult as you indicate there is a "trick" you can try. The trick is to add 3 DROPS of Amorall to ONE GALLON of fuel. MORE IS NOT BETTER. This will get rid of the foaming.

The downside to this - there are reports of shortened plug life. This has been reported to shorten the life of the plug from between 25% - 75%. I have used the Armorall trick and have not noticed any deteriation in plug life OR anything harmful to the engine. I used the fuel in a Saito 72 with the original plug and an OS 46FX. The original plug in the Saito lasted 2 1/2 seasons and the plug in the OS a little under 2 seasons.

If the foam and/or the Armorall do not stop the bubbles you have an air leak somewhere in the system. If the bubbles are showing up in the line between the needle valve and the carb, you can try replaceing the o-ring in the needle valve. If the bubbles are showing up before the needle valve, you will need to remove the fuel tank so you can replace ALL the fuel lines, including the line to the clunk.

If you have to remove the tank, I would hog out the secondary firewall (Make sure that is what it really is first ) so that there is enough room to wrap the tank in foam.

touch and go

My Feedback: (11)

I've never experienced the problem you described with a Nextstar but I have with some other planes. Usually there is a small hole in a fuel line. Even a hole in the line attached to the clunk can cause this problem when the level of the fuel in the tank gets lower than where the hole is. A "pressure test" may not find the hole if the line is bent so as to close off the hole. Its not very likely that foaming of the fuel is your main problem or else all the Nextstars would have the same problem and they don't. Lots of other planes have no padding on the fuel tank and work fine. Thus, going through all the work to add some padding around the tank will probably be a waste of time. Try replacing all the fuel line that you can without taking the plane apart. If that doesn't work, I'd have an experienced modeler take a look at it.

Don Presten

The bubbles appear between the tank and the needle valve. I am going to go ahead and remove the motor and pad the tank as well as replace all the lines. Thanks for the comments!

GONE2THERIVER

My Feedback: (1)

Had the same prob. mine was a small crack in the needle valve holder on the base of the barb going to the carb. Good luck!!

kdheath

It may not completely solve the problem, but an easy fix that often helps is to addd a fuel filter in the line between the tank and carb. Try it before dismantling the nose.

Raincloud48

I had the same bubble problem. I put foam rubber between the firewall and tank. I stuffed more foam between the rear of the tank and the receiver. My instructor suggested this and it solved the tuneing problems.

MikeG4936

My Feedback: (5)

I have the EXACT same problem with my NexStar.... however, for me, the bubbles are so numerous that it leans the engine out too much, almost on the verge of hurting the engine! The bubles are in the line before the fuel get to the carb, so I'm guessing its NOT the needle valve. I might try putting in a filter on first, but my guess is that in needs a small layer of rubber foam around the fuel tank. But I'm a new guy, and thats not something I'm looking foreward to... [&:]

argileh

When reading your post it felt as if you're describing my problem. I am new to rc and have about 20 flights with an instructor. Ialso have a Nexstar. Lately the engine keeps surging in the air and I see air bubbles in the line between the tank and the NV. I have twice removed the tank (of course had to reomove the engine first) and checked it for leaks (underwater pressure test) and no signs of aleak. I also put foam arounf the tank (tough job considering the tight compartment in the Nexstar). But the problem persists. Yestrday I removed the tank again and went through the same ritual, test, change tubes and put foam. Will test again this Saturday.

The thing is bubbles appear even when the tank is full. The only thing I can think of is foaming, probably cause by unba,ance prop. So the next thing I will try is to balance the prop. I will keep you updated

hurayta

I had the same trouble with my plane. I first replaced all the fuel lines thinking it was a pin hole in one of the line pulling in air. After that didnt help I removed the tank a second time, glued 1/2" of foam to the backside of the firewall and cut the hole for the neck of the tank to fit. I also removed the rubber that supports the tank and replaced it with some extra receiver padding and stuffed a pice of foam in behind the tank to hold it forward. Everything works great now. I dont think it is your needle valve leaking. If it was you would probably get a steady drip of fuel from it when you start the engine. I had cracked the needle vavle housing on my OS 46AX when tightening a screw just enough to make it drip but not enough for it to be easily noticed. I hope you get the problem resolved and have fun flying. I fly mine with non of the extras and have gone to the duel aileron servos on seperate channels to be used as flaps.

IBWingnutz

My NexStar was bubbly with my tiger .40 that was on there and i had to replace the fuel tank completely with slightly smaller one, cut out the mounting area, and wrap the tank in foam. This solved the problem!

Cisco-Kid

My Nexstart has the exact same problem. I will follow these tips and report back as well. Last night I spent an hour ballancing two props. I broke out the nail polish and sand paper. I aslo did the underwater test and put as much pressure in the tank as I could...no leaks. I added some more foam around the tank, but if that still does not work, I will end up gouging out the rubber-coated balsa supporting the tank and wrap it in foam like a freak! I'm really supprised to see how many Nexstars have this issue.

Cisco-Kid

Update. I pulled another fuel tank off of an older piper cub and installed it. I have not had a bubbling problem since! I am starting to think this is just a flaw in Nexstar's tank design.

britbrat

The "foam" that is fitted to the cut-outs in the formers is too stiff to be very usefull. That, combined with the rear needle-valve on the FXi is the source of the problem. I've seen a couple of Nexstar ARFs with TT's in them that fly without probs -- the front NV's worked without extra foam. The OS rear needle set-ups need the foam.

jollycub

My Feedback: (5)

I know this is an old post and the last reply was over 6 months ago, but I had the same problem. I rotated the tank 180 degrees and it fixed it. The tank was installed upside down. I am pretty sure this was the problem since we rotated the tank again to test it, and the fuel bubbled again. Again we rotated the tank it ran fine again.

griesel

Different plane - different engine - same problem. I have a SkyRaider Mach 1 with an Evolution trainer A engine. Engine works OK
at idle and 1/2 throttle. Have bubbles in both line to spraybar and line to muffler at full throttle - 12000 rpm 11x7 prop - just balanced.
Also surging. Fuel tank is full. I performed a leak test by pressuring the muffler line and the spraybar line closed off and no leaks anywhere. Replaced line between spraybar and carb and line from tank to carb. The surging started two flights ago
in a H9 A40 before it crashed. Did not notice bubbles on the runup before flying. Different plane - different tank - same engine.
Oh by the way here is a little tip taught me by Roy Iley. Use fiber glass reinforced tape and tape the rear of the tank leaving a 1/2 to 1
inch handle at the top rear of the tank. I used a double layer of thin packing tape. This gives a handle and makes it a lot easier to remove
the tank. Another option is cut a door in the top of the plane nose but this is not necessary if you use a tape handle and do not need to
get to any engine or nose steering nuts.

mrcoolpop

I just purchase the Nexstar and experince the same problem but, quickly discover the problem. it was the lean adjustment knob. It would only turn so far in each direction but not enough to adjust the engine. I had to take it completly off and 180 turn and the bubble and spurting was gone. It is running fine now.