I own an OS 46AX. Purchased new. It was broken in exactly like the manual says. It has about 12 tanks through it. 6 flights, mostly at about 1/2 to 3/4 throttle. Seemed to run okay during the flights, but because I am a newbie, just very gentle flights. Very short flights, only used about 1/2 tank each flight (10 ounze tank).
Now I am trying to fine tune the engine. First I set the high speed needle. Run it full throttle, lean the high speed until the max RPM is reached and then richen until the RPM's drop about 5-600. At that point it runs really well for the first 1/3 tank of fuel after I made a very small adjustment to the low end. I thought I had it running really well. Transitioned great. The pinch test on the low end sounded great. Everything going great, then after like I said after about a third of a tank, it starts running real eratically. Speeds up without any throttle change, then drops back down in RPM. It almost sound like it's leaning out, then starts to run right, then leans out again. Moved the low end needle back to where it was originally, same erratic performance.
At that point, I emptied the tank, and put it away for the day. Next day fill her up, starts right up, first or second flip, runs great for about a third of a tank, then starts acting up again. This happened three times now, all three on the ground, not in flight.
Anybody have an idea what could be wrong? Brand new number 8 glow plug is being used.

Sounds like a fuel system issue. May be a hole in the clunk line or an air leak in the stopper, such that when fuel reaches the stopper you get inconsistent fuel mixture to the engine.

One thing you can do is run an auxiliary tank, just for ground testing purposes, and see if you still have problems. Then you will know if it's the engine or the tank setup.

It sounds like your fuel tank might be a little too high in the fuselage. Try setting the needles as you described, but at 1/2 tank,. when its running good, then shut down, fill the tank & see how a higher fuel level effects it. if tanks too high, it will be too rich on the high end & a little bit rich on the low end. The center of the carburators spray bar should be about 1/4-1/3 of the total height below the top of the tank. When its lower than that, the engine is not really pulling the fuel as much as it is when the level drops. At full tank, the fuel free flows to the engine & the needle setting is restricting that free flow, but as level goes down, then the engine has to pull the fuel up to the engine & through that needle setting, it is too lean to do so. Bill

The centerline of the tank is exactly on level with the spray bar. I'd have to lower the tank floor to lower the tank. Before I do that, I will do what garcfield says first. Maybe I made a bad connection in the lines, or a pin hole. I have another tank, and I can play with the level too.
I read somewhere that the fuel can bubble up and make the engine run lean. You think that could be the problem also? How could I remedy that. The tank is shoehorned in pretty tight. In the engine manual they show a "non bubble clunk". Think that would help or is it a waste of money?
Thanks for your help!

garcfeilds aux. tank will tell if its the engine or the tank set up, but at 1/2 the tank above the spray bar, gravity is pushing the fuel to the engine when you fill it up. The needle setting will be different than if the engine is pulling the fuel from the tank. I have an Ultra-sport with a .68 in it with a 10 oz tank, & I get 15 minutes at full throttle. You might consider a 6-8 oz tank which will bring the center lower from the spray bar which should help. if your tank lines are at fault, then trying to set the engine at 1/2 tank level will not produce a good setting. It will pull air & cause eratic running, but if the lines are fine, then it will set good, & go rich after you fill the tank & restart it. The fuel in the tank shakes around alot & if theres no foam around it then that too can cause the same simptom by letting air into the clunk, but you will see bubbles in the line if thats the case. You will also see bubbles if the lines got a hole in it.

Can you see the fuel line where it connects to the carb? Look for bubbles in that fuel line; if there are bubbles, try to trace it back to where they start - at a T, filter, tank, or hole in fuel line.

All the above stuff makes sense to me, except that I've got a 46AX and a 55AX, and a 120AX for that matter, in planes with the tank well above the carb and haven't had any issues related to tank position.

I have found that when replying here to help some one, is that with out the engine, plane, tank or what ever the issue is, right here in front of you, All that can be done is just give some fixes that are common happening in the areas the problem lie. I too have planes that the tank set up is higher than what would be best, but run fine through out the whole tank. An engine that has a good venturie size will pull fuel better than one that is sized differently. If set to restrict the push of gravity on the fuel entering the carb, then also allow good vacuum to pull the fuel as the level goes down, then it will not be a problem. I also have engines that will pull fuel fine & on another engine of the exact same , that wont. Mid range on one, perfect, on the other, rotten. At some point, the remedys given by the replys should be enough to get the problem solved. It also helps the newbies gain valuable info & insite as to how these things work, & how to self diagnose there problems with the new info they learn.

I can see the fuel line where it attaches to the carb. The engine is in a sig midstar. I will check all the lines like you said. BUT, I mis spoke earlier. The TOP of the tank is exactly level with the spray bar. I said earlier that the CENTERLINE of the tank is level with the spraybar. So therefore when the tank is full, the fuel flows to the engine easily, but when the level drops, then the fuel has to travel uphill to get to the carb and maybe therefore leaning out the fuel mixture? Maybe that's the problem?
Anyway, I will check all the lines for holes etc. like you said.
Thanks!

Bigal, your set up looks good. Tank height is not your problem. The pic helps alot, I would run it with that hatch off & check for bubbles in the lines. if there are between the tank & needle valve, then the air leak is in the tank or at there attachment to the tank nipples. I see in the pic that you have yellow line to the needle, but pink line at both tank nipples. Any connectors through the firewall could also be at fault. If the bubble are only between the needle valve & the carb, then it is in the needle valve assembly. I have had a few O.S. nipples leak where the line hooks up from the tank. A shot of thin C.A seals them right up. Also the threads of the needle valve can suck air too. thick grease, or some teflon pipe sealant goop, will seal them up. Hope all this helps. Bill

I have a 46AX which is inverted and a high tank and runs well!!!!!!!!! Lucky me..

The only thing that I see is the yellow feed line would be too high for me in such a set up and I would try and have it with as little as an upward loop as I could. Hope this helps..Clean your tank out well and check all lines for a split especially right on the very end where the tube meets the nipples.

Make sure the clunk has a free movement through the tank..and make sure it is not reversed in any way.... or stuck somewhere. My guess is that since you say that after 1/3 of the tank...it may be sucking air inside the tank and not fuel..or a mixture of both... when fuel level goes lower than where maybe the clunk is stuck....if it's so, then cut a couple of mm the clunk fuel tubing inside the tank.

Saludos,

Jorge

Hi

I t is little doubt that this engine has been running lean at some point, from the color of the engine it looks like it has been very hot at some point.
I would also have shortened the yellow fuel line so it doesnt go so high out from the firewall, it looks ok the fuel line install but...
Also if the engine has been very hot, i would also check the head bolts to make sure they are not loose.

Update: Over the weekend, I changed out all the fuel tubing. Started the engine right up, and same thing. At first the engine runs great, but when fuel in the tank is about 1/2 empty, the engine starts to run erratically and speeds up as if it is leaning out.
Next, I raised the tank 3/4", so that the centerline of the tank is just about 1/4" below the carb. Before the centerline of the tank was over an inch below the carb. Now the engine runs great as before, then starts the erractic performance, BUT there is less than a 1/4 tank of fuel left in the tank at that point. A 10 to 14 minute flight probably will not draw 3/4 tank of fuel, therefore I should be alright.

Now all I have to do is modify the hatch so that it will accomodate the tank 3/4" higher. That should be no problem.

This may sound stupid but check the muffler nipple to see if it's clogged. This would cause the symptoms you describe.
Max