The usual suspects -- check the tank, tubes, and filter for leaks.  If you find yourself saying "oh, that couldn't be the problem" -- check that.  Then check to see that the venturi is in there good (all the smaller OS engines I have seal the venturi with an O-ring; I assume the 46LA does too).  Ditto the needle valve.

If it were going lean I'd say check the backplate, but it wouldn't hurt.  Check other oddball things, like whether the head is tight and the glow plug seals -- most of those problems would either not act like fuel feed at all or would act like the engine going lean, but hey -- you did say you didn't know what's going on.

Beyond that -- well, if it were me I'd be asking you.

I had the same problem years ago.I changed everything and I still had the problem. I found that the uniflow tube in the body of the plane was restricted. A bug was blocking it. I could fuel the plane through the tube but it ran like yours every flight. I changed the tube and problem went away. This was on a Cobra that had  three years of flights on it before the problem started.
Ed

I wonder if the uniflow tube is not in the right place; has come unsoldered or something.  The engine will richen up when the uniflow comes out of the fuel.  I'd pop the back of the tank off and take a look.  I wouldn't put a commercial tank on an airplane without having opened it up to see that all was well.

Floyd,  A simpler way to check if the uniflo is causing a problem is to fly a flight with the uniflo vent capped and the overflow vent open...just like an old veco tank.  If the engine runs as you'd expect on straight suction (slightly leaner at the end of the flight) you can be pretty sure the uniflo vent might have a problem.

What prop are you using?  Could the engine simply be unloading once it gets airborne?  Is it piped?

Ted

You just need to revisit your definition of "good flying day", and then you'll be right back on track.

Try changing to a hotter plug.  We had this problem with a Brodak 40 and would go so rich, it would almost quit, or the plane would slow down so much you could almost taxi it on the ground.

Matt makes a good suggestion.  What plug is in it now?  Have you tried others?

Trimming the prop sounds like a step in the right direction as well.  That APC was made at the request of David and I when we were first starting to run piped engines.  Worked well on VF .40s and .46s on pipes but might well be too big a load for the LA.  You might want to simply stick an 11 X 4 wooden two blade on it, tack it up to about 10K on the ground and see what happens.  There's a 50/50 chance it'll hang right in there.

If an engine is too "loaded on the ground" it will "unload" in the air as the angle of attack of the blades is reduced due to forward motion.  If that prop is on the ragged edge of too much cutting the diameter would be enough to bring it back from the abyss.

FWIW, when I got my .46VFs running their best I was running a slightly undercambered Eather three blade that was in the vicinity of 11.5 X 3.6-7.  That was in airplanes as big and heavy as the original Trivial Pursuit and provided plenty of thrust to fly it.  Don't feel compelled to go BIG on diameter just because it sounds like bigger has to be better.

Ted

  Marc is correct I have a Tiger .51 that goes so rich after I launch it. All my othe Tigers are fine. I havent tried it for a bit  maybe this Winter I'll get ot it  after my other stuff is working right.   Matt , do you have a suggestion for hot plugs? I have used Thunderbolt 4-cycle, RC long ,Enya 3, Hanger 9 four cycle , Ys 4 cycle, OS F . OS 8     to name a few   .
  John

We always try to stick with Thunderbolts when we can. We have a box of "good" plugs and it can be hard to identify what plug is what.

Ted,

The prop we currently have on our LA 46 is the exact prop Randy Smith recommends for the PA 40UL.  For some reason, my motor didn't like it at all, and we tried it one day on the LA and it was a major improvement over what we were using.  I think it's an 11.5 diameter with 4.25 pitch, with a little undercamber (It's a majic prop) turning around 9600 RPM.

I put up one flight yesterday (Sun.) with the APC 12.25  X3.75 cut down to 11.6".  No other changes.  I got a near-perfect engine run! I'll have to make more flights to see if that really solved the problem.

You learn something new every day!

Floyd

I tried a new LA 46 in my over-weight P-40 winged Shoestring—the design in January 2011 CL World—at the weekend.

The setup is Brodak wide profile tank, standard large venturi, std OS NVA and Bolly Clubman 11.5" x 4.5" prop. Fuel was 22% oil 50/50 Coolpower and castor, no nitro. It was new out of the box so I ran about 10 oz of fuel trough it on the bench and flew it.

I was happy with the first flights and former Oz champ Mark Batty put it through its paces. His comment was, 'change nothing, leave it like it is.'

I'm told the Bolly prop is flatter than a true 4.5" pitch but all I've done is balance it. Don't have a pitch gauge.

Yes.  I suppose other "fixes" might have worked.  Since everything depends on everything else...(sort of like a differential equation) the shortened prop put everything in balance, along with the existing fuel, glo plug, etc.
"Could be that changing glo plug, fuel, might have worked also.  Who knows?

Floyd