During the third flight today on my WM T-34 with an OS-46AX, the engine suddenly lost power about half way through the flight. I have a few hundred flights on this plane with few problems. On the way to the ground, the engine would run normally below about 1/3 throttle but ran rough above that. Temperature was in the mid 60's. I was able to fly a relatively normal landing pattern, landed and taxied back. I picked the plane up, engine still running, and put it on a run-up stand. I had to close the main needle half a turn to get back to normal operation! Tried wiggling the needle (It's remote) but there was no effect. Besides the usual O-rings on the needle, I have a short piece of fuel tubing around the needle to make sure there is no air leak. Exhaust line was still connected and none of the lines were pinched that I could see.

Any ideas?

Sometimes vibration will cause the needle to turn.....is the little "ratchet" bar that slips over the needle housing still there? Is it tight? There should be a definite "click" when you turn the needle. Also check for pin holes in your fuel line.....they will make tuning a nightmare....


CB

The ratchet is there and is tight. Also forgot to mention that the needle had not rotated. I have an L-shaped extension on the needle valve and it was still pointing where it had when I tweaked the needle this morning. (Was about 20F cooler today than the last time I was out and took less than 1/8 turn to adjust .) After I retuned it, I wiggled the fuel lines around to see if there was any effect. Nothing! Pin holes in the fuel line would tend to give a lean mixture and the problem I had was a rich mixture.

Sure sounds like an air leak. Maybe where the carb attaches to the manifold? As engine heats up it might expand.

An air leak would lean the mixture. The problem was the mixture went rich. This was the third flight of the morning so the engine was well warmed up. Also, it was quite an abrupt change while in level flight. Hard to believe that it could have been leaking for a couple of years, then suddenly tightened up.

Not trying to be difficult as I can't figure it out myself but, as a "retired" reliability consultant, I look at "If it is this, then that."

With a pressurized tank needles will not leak air in, but will leak fuel out. So that is not the problem. I suggest that a bit of crud that was in the needle was pushed out causing the engine to run rich.

Sounds like a possibility. Surprising that the bit of crud could have been in there for a couple of years as the needle setting has been relatively stable for that long.

Check to see if anything on the carburetor has
vibrated loose. If nothing is found, it probably
was crud in the needle that went through. If
it was crud, there may be more in there that
can move around, so you probably want to
blow and spray out the carb.

Jenny

Took it all apart and found no problems. Blew out the fuel lines and carb both ways. Nothing came out. Hope to give it a try later this week.

Well, it works fine! No idea what the problem was so it may occur again. As for the insensitive needle setting, the plane has a lot of flights and I suspect the remote-mounted needle orifice may be worn. I had that happen on another 46AX a while ago and replacing the needle valve got normal operation back again.

The 46 AX has a propensity of the head bolts loosening up. Runs really weird when it happens. I have tightened them with the engine running at temperatue and they will stay tight then. The safe way to tighten is to tighten head bolts run engine up to temperature, Stop the engine and retighten before it cools off.

When reducing the throttle, "saves" an engine from quitting it is almost always too lean. The carb has more fuel draw at part throttle than at full throttle so if the engine is rich, reducing the throttle exacerbates the problem and it cools the plug and dies.

I'll give that a try. However, I can't see how that causes the effects I saw.

I have had trouble with some of the stoppers on the WM fuel tanks recently. The threads in the back compression washer wear out. Does not leak noticeably, but eventually it effects engine performance.

By the way, I am on my sixth WM T-34 over a 20 year time span. I like the way the plane flies.

Brian

Hi Brian,

My planes must all be older! The clunk fell off in one of them and there was no way I could get the stopper out. Had to set up a new tank.

As part of the checkout, I both pressurized and evacuated the fuel tank and found no leaks either way.

I'm currently flying WM T-34's numbers 4 and 5. The previous three met violent ends pylon racing.

Good luck finding the source of the engine problem.

That pylon racing can be tough on planes. A couple of pics from Club 40 type racing at our field.

Brian

Not always true as learned from new Evolution 46NTs with OS 8 plugs. Not reducing throttle causes those engines to quit. You can nurse the throttle until the loaded up fuel is burned off if you are careful enough. It's a damned if you do and damned if you don't situation, but you have to stop injecting too much fuel and somehow keep the fire lit.