I've worked in the trades, and like them all, except for plumbing; it takes a special breed, and I have a lot of respect and appreciation for plumbers. I went back to school to get out of the trades. I got into this hobby because I like planes, and I hope to work in the aerospace industry when all is said and done. Well guess what I got stuck doing today? Yep, plumbing.

I've been noticing a bunch of fuel all over the fire wall, and have been chalking it up as spilled fuel from filling. But it's been getting worse. Finally yesterday I heard some roughness in my engine when flying, and upon further investigation came to the conclusion I had an air leak somewhere. But where? I drained fuel from the tank, pressurized the system, and could find nothing. On a hunch, I tightened the bung screw and replaced the line from the fuel tank to the carb. Today, there were little bubbles in the line going to the carb, and had the same problem when flying. So I spent my whole day at the field today, plumbing[:@] For me, plumbing is fixing one thing, then finding a problem in the next thing. Every can is a can of worms, and it's been that way for nearly all of my 30 years of life. So today, I removed the engine, fuel lines, fuel tank, etc. Pressurized just the fuel tank; pressurized just the lines around the remote needle valve. Heard nothing, saw nothing, but replaced some lines anyway since I had the whole thing apart. Put it all back together, and couldn't prime my engine. Thumb over the carb (wide open) like I do 4 days a week, two turns of the prop, and no fuel. A few more turns of the prop, and the little air bubbles reached near the carb, but would not get sucked into the carb. So I tried starting anyway, and did get it started, but not as easily as normal. Now the engine runs spectacular (as it should, it's an AX) both on the ground and in the air.

Why can't I get it primed right? It sounds like an air leak, but I checked and changed everything! Which brings me to a simple question: when the engine is hot and the cylinder sleeve has expanded, does priming actually get more difficult due to the looser fitting piston/cylinder seal? But even if it did, I had the engine out and fuel lines apart, so the engine had plenty of time to cool down.

I wish we had a field plumber. I can do all of the other trades very well; I can assemble a plane, repair a plane, fly a plane, tune an engine, pretty much anything, except for plumbing. Boy do I hate plumbing. I guess most people do, 'cause it seems to be the last thing anyone wants to help with

No sport engine should ever be run to the point where it gets too hot to be refueled and restarted as soon as it lands. At the first sign of any sagging, it's time to land and investigate.
Are you sure that you don't have the lines crossed?
If you can pump fuel out of the tank with your fueler or a syringe, then the engine should have no trouble drawing fuel.........unless the carb is clogged. A syringe is handy to suck out clogs.

You know what's screwie..Trying to fine air leaks from drains traps...we had to re dig the darn entire block.lmao
Of course Mr. Murphy was around...becuase out of darn 20 houses, it was that last one.
I don't mine new plumbing...it's the used ones.[:'(]
Thank god it was new houses..becuse my buddy hit the gas line...and I counldn't get out of the hole fast enough.
I ran like hell He was stuck in the excuvator.lol
That was a good AIR LEAK..lmao...They only had air in it incase we hit it.lol

Well..I heard B1s take off for 4 years of my life. Those 4 after burners are narley.lol

Anyway, I was have a similar problem with the GP cap 232. I couldn't get that darn thing to prime
no matter what with a starter. But I was thinking the tank was mounted too low and too far back,
plus all the crazy bends I did for the rear needle valves.
I used three different pitt style type muffuler...and still no dice.
So I put another engine with a front needle..and she fired right up.

On a trianer i usually just blow into the return line.
Humm...maybe I'll try a syringe with sometype of fuel tubbing adaptor..so I don't have to blow.lol

Maybe some debris got stuck in the carb's orifis and worked it's way through.
Or maybe the seal to the tank it too dried or cracked.

You probably have an air leak in the klunk line inside the tank. The only way to find this is to take the tank apart and inspect the lines VERY carefully. Bend them over your finger, roll them around, and stretch them. Small holes will become obvious.

Another way to prime the engine is to put your finger over the exhaust outlet and then crank the engine over by hand. This will pressurize the fuel tank and drive fuel to the carb. You can also use this method to see if fuel is leaking out of any air holes in the lines between the fuel tank and the carb.

Finally, don't discount the possiblity of a piece of crud caught in the needle valve or the spray bar assembly. If you have a remote needle, back flush the needle valve with clean fuel. Most carbs are easy to pull the throttle barrel and inspect the spray bar. It doesn't take much crud to keep an engine from running right. We had one yesterday at the field, and when we took the barrel out, there was a tiny spec, smaller than the head of a sewing pin. When we cleared it out, the engine ran great.

Brad

That's just way too funny!!!! I don't mind plumbing when it's new either but the one house built in 63 was A thrill, everything from the main to the bath tub wouldn't seal so it was always dripping when you were working on something.
Tank plumbing is another story though, I would do all your plumbing and covering if you would do all my sanding. I have been known to mention before how much I don't like sanding.
Brad got right to the point and someone else mentioned crossing the fuel lines, I have done that A number of times with my three lines, either not marking them when I assemble the tanks or forgetting what the markings meant, even done that with color coading too.
Most the time when I have A bad line inside the tank it is broken or has A hole in it right at the spot where the brass tube ends and the line begins. The last few years I have been soldering barbs on the inside brass tubes and smoothing them out with A rubber wheel and this seems to have cured that little problem.
I have had poopyugler in my carb before but most the time I have found A broken line inside the tank.
Do the easy thing that Brad said first and if it isn't crud then go straight into the tank. I keep extra caps and plugs in my building box just for those nasty little problems.

You think it's bad now... Just wait until you get into high performance engines with pumps, check valves, fuel filters, header tanks, fuel injection, and three billion moving parts.

Dang Nate, don't try to scare the kid, they will cover all that in about his third year of engeneering school!!

I love sanding!!! You can get a piece of wood to be any shape you want by magic of sand paper. I will sand things all day and get them to look perfect. I will happily trade with you. I should have mentioned that new plumbing is enjoyable; it's fixing plumbing problems that I don't enjoy, or adding new things to an existing old system.

I have different colored fuel lines, so there's no chance, well a tiny, tiny chance, that things are hooked up wrong. I don't doubt there's a problem inside the tank, or with the cheap plastic bung plug provided. I'll also flush the carb, sprayer, and needle valve.

I think the engine was hot from tuning at full throttle, and probably because I wasn't taking breaks between flights, although they were short and not with lots of throttle. I am running 10% nitro and a medium #8 glow plug. I used the tach this day while tuning; got 13,800 rpm's peak, tuned it down to about 13,200 with the needle valve.

Nate, by the time I'm ready for those kinds of engines, I imagine I might enjoy trying to fix them. At least I want to keep this idea in my head as I progress towards some pattern equipment.

Yeah, just kidding..... Well, not really.

I'm sure you'll figure it out.

What the heck are you guys saying?????

Just having fun.
My youngest student left for vacation during the middle of his first build right after we assembled his fuel tank. The lines are color coded and I have it written down as to what color goes where.
Problem: I have been writing notes using my paper pad????? Things could have gotten real ugly with just A few squiggles of the pencil and the rip of A sheet. [&o]
Oh well, not my plane!!!
GA, I took some photos for you today showing A Y set up. Old plane but worth A look.

Remove the fuel tank and lines. Fill your sink with water. Place the tank underwater and blow down the line. Any bubbles? If so, you have a leak. Now remove the tank so you can see the clunk line. Blow again. Any bubbles not coming from the tip? If so, you have a leak.

I'm not understanding this part - I assume you mean remove the stopper, and I should blow into what? And look for bubbles where?

Thanks for the help.

Put the whole tank under water with the ends of the tubing coming out. Plug all of the tube openings except one....Blow. If you get any bubbles you have a leak there.

Now, take off the stopper and pull it out of the tank. Blow into the carb line while plugging the clunk. This tests for holes in your clunk line (behind the stopper). If you get any bubbles you have a leak.

Sorry, I should have explained better. [sm=red_smile.gif]

No apologies necessary. Thanks Nate, I'll check it out.

No leaks.

So just a recap, I was having trouble priming the engine-fuel was visibly not going from the tank to the carb when I covered the carb with my thumb and spun the prop two or three times- the way I prime my engine every day. I was able to hold my thumb over the carb and spin the engine a lot, maybe 12 or so times, and got fuel close to the carb, then trying to start the engine (with a stick) I got it to start with lots of tries.

Once the engine started, it ran very well, so it was able to draw it's own fuel, maybe because of the help from the pressure line?

Again, no leaks from the clunk line all the way to the carb. Will check carb for cleanliness.

Hi gaRCfield,

I have two lines sticking out of the firewall of an almost completed Ultra Sport 60. Since I ran those lines last December, I have no idea which is the fuel pick up line and which is the pressure line.

I'll try to figure it out before the first flight!

I think I'll fill the tank half way, then stand the model on it's tail and try to de-fuel through one line. If I can get fuel out, then that is the pick up line as the clunk is at the bottom of the tank and will have fuel to pump.

If I get nothing but air, then that line must be the the vent/pressure line and I'll proceed accordingly.

Clear as mud, eh?

Oly

Sounds like it would work. I just checked mine; the lines are color coded, but never hurts to check. Pressure line's on top and clunk line's on the bottom just like it should be.

Carb is all flushed with good clean fuel.

you dont happen to have an idle stop screw on your carb do you? i had the exact same thing happen to me a few weeks ago. my idle stop screw fell out and there was a huge air leak from it. if you normally have one check if it is there... if not that may be your problem.

I don't think I've ever had one.

i have an old engine so....

With a 46AX you don't have one.
http://manuals.hobbico.com/osm/46ax-manual.pdf page 28 is your carb
Item #8 are the carb mounting screws. If one is loose or missing air will leak in.
Item #5 is your rotor guide. In some carbs this also can function as a throttle stop. If missing-big trouble.
Item #7 is the o-ring. If the carb is not seated on this fully you can leak air. It can also be cut and therefore bad.

All that stuff is in good working order. Thanks for the suggestion.