I guess there's a lot that could be wrong here, especially since my plane crashed and spent the night in a corn field; the engine did look very clean however - nothing in the carb.

So the 55AX is running poorly at mid throttle- anything over idle and under full throttle. Sounds bogged down or something. I've tried changing plugs and fuel and nothing changed.

I tuned her up with a 13x5 prop and got about 9000rpm, though the engine was even sputtering a bit when I was tuning. After a flight I switched to a 12x8 prop and found 12000rpm. I went to fly and it wouldn't rev up, so I pulled it into the pits and on a hunch leaned the high speed 2 clicks and the rpms shot up, and was back up to 12000rpm.

I flew a few circuits and landed, the rpms never seemed high enough in the air. I re-checked on the ground and was only getting 10000.

The biggest problem is at mid throttle though; it's not enough to move the plane on the ground, and sometimes sounds like it wants to choke out.

I'm going to check for leaks and clean out the carb. Any other ideas/suggestions?

The LHS said he had a 75AX which did the same - idled great, top end was great, but middle throttle it gurgled. He said some engines are just like that. I refuse to believe the 55AX is going to run that way with nothing wrong with it.

lean the low en and 8th of a turn, fly around a bit, then land turn off engine turn in another 8th of a turn an repeat process till you find it to be on the sweet spot, and also try not to change fuels to keep it running tuned....

heres one test to check to see if your low end is too rich..... have yur engine running at idle for about 30 seconds, then give it full throttle, and if it hesitates too much to hit high rpms its too rich on the low end.

Of the few 55AX engines I have delt with they all seem to be sluggish around mid throttle until well broken in. Seems I read this here on RCUniverse also. Mine come around after a gallon or so of fuel. I don't understand why it was only sluggish at mid throttle but it was. It acted as though it was rich except at WOT or idling and no amount of tuning the low or high side altogether fixed the problem until after a gallon or so of fuel

Run another tank or two through it then lean out the low speed needle 1/8 of a turn at a time until it is corrected. It sounds a little fat to me. Most engines are like that from the factory.

I disagree with your LHS. If an engine runs like that, especially an AX series, it is either out of tune or has something else wrong. My .46's, .55's and my .75 all run excellent throughout their RPM range.

Ok ill keep running it. I did lean out the low end and tgeres no hesitation from idle, just what appears to be richness at half throttle; pinch test at half throttle proves its rich, but again low ends been leaned out some

The low end needle has a big effect on mid-range. I wouldn;t be surprised if it tolerated a little more leaning out on bottom. Just be careful not to go too much.

How do you know when its too much? I just stopped leaning it when the hesitation from idle stopped. Maybe its that simple?!

It should transition well and have smooth idle & mid-range. If it starts sagging bad or dies when you accelerate then you may have gone too far. Just lean it in very small increments, clear it out then see what the adjustment did. If you lean it too far, just back it out 1/8 or so until it comes back.

When you are getting close to the right low end setting an 1/8th turn can be way to much. A 1/32 turn OR EVEN LESS may get better results. Take your time keep it a little rich for the first 1/2 gallon or so of fuel. to lean will cause A LOT OF WEAR. [8D]

Have you tried a smaler prop, say 11x5, 11x6? My ST51 loves an 11x5.
Good luck, MikeB

I have two of the OS 5AX motors and have quickly decided they are my favorite two stroke ever. My first one I bought for a 3D plane and I noticed the same thing about the mid range. It was a bit sluggish in the mid range with the bigger props. I had a 13x4W APC on it when it was brand new. Stepped it down to the 12x4 APC sport and it came to life. Now that both of mine are broken in I have no problems with hesitation. By the way a 11x6 is also a great sport prop for this engine. I would very much consider taking the Carb apart and making sure there is no debri inside any of the jets. Then go back and follow the advice of some of the other posters here. Very small increments on the low end needle should help you solve your problem. I also don't agree with the LHS owner. Your engine probably just needs some more fuel through and some TLC in the tuning.

Seems to me a few questions unasked here. You say this is post-crash, how did it run pre-crash? Is it in the same plane now? Is the clunk line doubled back and now in the front of the tank which could effect fuel flow? How much impact force got to the engine, could it have damaged the carb?

You had previously stated that the engine was running well before the crash, so something obviously changed as a result of the crash. We recently maidened a new GP Extra 330 with an OS 55 AX and it was exhibiting exactly the behavior you are describing (idles well, ran well at full throttle, but transition was poor, and it would deadstick at mid range). This engine had a Pitts muffler. After doing some more research, I found out that the OS 55 AX is very sensitive to low exhaust pressure on the tank. We had to plug one of the outlets of the Pitts muffler to raise the exhaust pressure, and that cleared up the problem. Now the engine runs beautifully.

In your situation, its possible that the crash resulted in damage to either the exhaust pressure nipple, the exhaust pressure line, or there is a small leak in the fuel tank itself that is releasing pressure. Before you mess with the tuning, make sure that all your exhaust pressure connections are not leaking.

Brad

I don't recall stating that the engine was running fine before the crash, and if I did I could have been mistaken as I'm not really sure - it was getting dark and I was in a hurry to get the plane in the air (ok probably not the best decision, but that's in the past). The muffler has little fins that stick off at one end - they look like they could be for heat dissipation, though more than likely they are for looks. Two of these were chipped as a result of the crash, and there was damage to the plane under the fuse near the muffler.

I have disassembled and reassembled the fuel tank, there definitely are no leaks now. I will clean the carb and reinstall the fuel tank and try again. There IS a possiblity something happened to the carb - at one point I could not get the plane to idle as the (injector?) was not lining up with the hole on the opposite side. This happened after running the plane after the crash, and after trying to tune the low end mixture valve. I took the carb apart, adjusted the idle mix screw, and everything seemed to line up fine.

Thanks for the info; please keep the questions and/or advice coming as I am SOOOOO eager to see this 55AX run the way it was designed to run.

The fins are definitely for cooling and are meant to give additional surface area to transfer heat to the airflow.

Can you use the manual http://manuals.hobbico.com/osm/55ax-manual.pdf and the carb part list http://www3.towerhobbies.com/cgi-bin...I=OSMG2518&P=Z to explain what YOU call an injector?

Closely inspect the casing of the engine for hair line cracks, especially where the carb attaches to the case and where the muffler goes.

These are all too small for the .55-AX. OS recommends a 12x7-8 or a 13x6-7. I feel that the 13" prop is probably a bit of a stretch but likely doable. The 12x7 is near ideal. If you use an 11 inch prop on this engine it is probably best to keep the pitch fairly high (i.e. 11x8 or more). Despite the advertised "practical RPM", this engine leans toward low end "grunt" rather than a high RPM screamer. It has the torque for the 12 inch props and thrives in that RPM range.

I ordered one from OMP. The person that took my order said most of the 3-D fliers are using 11.5x4 and 12.25x3.75 APC's. The manual recommends the bigger props, but the emphasis is more on keeping prop noise down rather than performance the way I read it. When I get it, I'll do a short breakin on the bench then mount it to my 4*. I'll try the 12x7 after it gets some run in time, tht would be impressive!

That's interesting. I really don't see the point though. A typical .46 will pull those props just fine so I'd think the extra torque of the .55 wouldn't be put to much use. I say take advantage of that torque and run a bigger prop with more thrust. Maybe a 13x4 for a fun fly/3D plane. Maybe there is some advantage I'm missing though, who knows. Maybe I will experiment one of these days. Since this is a beginners forum though, the recommended prop range is probably a safe place to start.

Ok, so at one point the "carburetor rotor" was getting hung up on the "mixture control valve assembly" and was not able to close all the way. Unscrewing and rescrewing the low needle helped and now it has full range of motion.

Today I got to the field and my old instructor was there, and he said a guy was coming who knew all about engines. He helped me tune her up, which included richening the high needle and leaning the low needle; it was sounding pretty good, blowing plenty of smoke (it's still new) and idling great at 2100rpm and topping out at 12000rpm. I had 2 good flights on it and it deadsticked on my 3rd takeoff. My first dead stick! I kept the wings level and put it down as gently as I could - it landed in a giant puddle and somehow missed the corn, so no real damage.

We cleaned it up and started it up before leaving, and it started up, advanced fine, got to full rpm and cut out. Tried it again and the same thing happened, so now my engine is cutting out at full throttle, though sounds just fine getting to that point.

Keep working with the guy that knows about engines. Get him to help you figure it out. Small pieces of debris can cause havoc with the clearances on the spray bar assembly.

Don't pull the low speed needle to clean the carb. It just messes up the settings. Its better to just remove the entire barrel. There should be a stop screw on the crankcase that prevents the barrel from coming out. Just loosen that screw and the barrel should come right out. Make sure you don't loose the spring. This will allow full access to the fuel passages. Flush them all out with clean fuel. Also disassemble and clean the high speed needle valve assembly. You may have had a stray particle that came loose and got stuck in the spray bar. When this happens, the engine goes lean and overheats, loses compression, and deadsticks.

Brad

Ok, will clean high speed needle. The guy who knows his stuff was visiting for a day and is gone. He lost his home, cottage, and $8000 of rc gear in Katrina. Im gonna try to get him on here and get a thread going to get him flying again, he has nothing. I have a radio and batteries to give him, not much but a start.