So I have a new OS 55 AX for my Revolver. I followed the manual and opened the needle-valve 2.5 turns to start with. So the engine idles nice...if I slowly open the throttle, I have it so it will run decently up until 3/4 throttle for an extended period but go further than that and it'll quit. I thought it was too lean....but no matter what I try I can't get it to run at full throttle. I should mention that I'm running a Bisson Pitts Muffler. I'm assuming I need to enrich the idle mixture with the mixture control valve? I haven't touched that yet... Thanks!

You have the engine too fat, bring it up to that point just before it wants to quit and lean in the NV until you can get it to run at full throttle then stop, that way it is rich enough to break in and lean enough to run. Then follow the instructions for the rest of the break in.

Have you tried to open the throttle full open with the glow driver still connected? This would help keep it running if it were to rich. My guess is that your running a little lean and the engine will die even with the glow driver connected. If it does die then open the needle at least another half to full turn. The recommendation settings are a ball park type setting assuming that your fuel supply is good.

Open the throttle to the point where it may quit then pinch the fuel line. If it stops almost instantly then the high speed needle is too lean. If it gains revs before stopping the high speed is too rich. My betting is the high speed is too lean because an engine will run extremely rich at full throttle even without the glow starter on. Pitts type mufflers tend to have too little back pressure feeding to the tank which means the high speed needle has to be opened a little more than usual.

The recommended initial needle setting is generally on the rich side. Try closing it 1/4 turn then advance to full. If that doesn't do it, try another 1/4. Once it runs at full throttle, adjust the needle so you get maximum RPM then richen it slightly so you get a 200-500 RPM drop. since this engine is new you may want to run it even slightly richer than that for the first flight or two. Next adjust the low speed mixture screw. These also generally arrive a little rich from the factory. Adjust it so you have a smooth, reliable idle and a good transition to full throttle with no sagging or coughing & spitting. Be sure to re-check the high speed after resetting the low. You may also have to re-adjust your idle speed after setting the low speed mixture.

Once it is running good, don't mess with it until there is a fairly decent change in outside temps. Even then, you'll just need a few clicks to compensate.

I say that the factory settings are usually rich. I've found this to be a general rule but there could always be an exception out there too.

Isn't it possible that he's not getting enough tank pressure with the pitts muffler? I've heard that can happen, and other than his needle position I don't see a good indicator of the engine set rich or lean.

I would try the stock muffler just to see if anything changes. If the muffler is the cause, try plugging one of the exhaust ports on the pitts muffler to increase exhaust pressure to the fuel tank.

Just my thoughts...

I agree, sufficient back pressure is what creates sufficient fuel pressure for the engine to be able to operate through the extremes that we take for granted.
I doubt that the Pitts muffler has the same amount of R&D that OS puts into their product line.
The Pitts muffler reminds me of the old "Cal-Custom" line of automotive performance parts that didn't work as well as stock.

I have several OS AX engines. All of them, ALL OF THEM, ran just fine blubbery rich for the first two tank-fulls. I would run them up to full at a somewhat lean setting so I could get it there, and got it there with no problem, by the way, then richen it up so that it sounds like a four stroke and pumped out the blue smoke like crazy.. spitting oil and fuel, but still runs.

I would run it full at that blubbery setting, for a full minute then lean it out a bit to allow it to get up in RPMs for about 20 seconds, then back to blubbery rich again. I would repeat this for the full first tank, then refuel, and repeat, but a tad leaner on the blubbery side and increase the leaned out time to about 30 seconds.

After the second tank was finished, I just fly the thing, and run the engine a few clicks rich. Yeah, I get lower RPM's but it will run cooler and break in just fine.

The more you fly that engine, the stronger it gets.

CGr.

A lot of the suggestions here are valid but it always helps to review the basics. Is the engine upright, inverted or sideways? Is the tank centerline at or below the height of the engine spraybar? Are you using a recommended prop ..... OS recommends up to a 13x7 but personally I think that's too much for this engine. 12x6/7 should be enough. Is your fuel fresh and have at least 10% nitro and 18% (or more) lube?

OK! Now go back to that 2 1/2 turns, start her up, leave the glow driver on and go to full throttle. She should run but be sloppy rich. Adjust the main needle to lean her out, pull the glow driver and then follow the break in instructions high speed for 10 seconds, back to low speed for ten seconds for a full tank (or two). These adjustments are done with the main needle, NOT the throttle. By the third tank you can open her up with the needle (but not at max rpm yet) and run as long as you want but it's then you adjust the idle with the low speed needle. Don't pinch the fuel lines or point the plane up and down. Just bring her to idle for a few seconds and then quickly advance the throttle. Coughing and slow acceleration mean the low speed needle is too rich, and abrupt stop means the low needle is too lean. Adjust it for a smooth transition to full throttle, readjust the high end needle and go fly! High end performance will improve through the first gallon of fuel and then you can optimize your prop and high end needle settings for best performance. Keep track of your rpm readings and you'll know when she's fully run in.

I've never had this happen using a quality Pitts muffler on a .46 or .55 AX. In fact I have only seen it one time on a .91. Of course anything is possible and it would be easy to check and verify or rule out.

Also things like the tank position will not be an issue here. There are tons of these Revolvers out there (mine included) with side mounted .55-AX's and tanks in the stock position and they perform excellent. Tank plumbing could be a factor though if it is incorrect or has a leak.

The point is that you will almost never fly at the recommended needle settings. Those are given as general guidelines so that the engine will start in most places and conditions. Yes, run it rich for the first couple flights but the manual settings will often be too rich.

Now all of this is just an educated guess based on experience with a lot of my own and other peoples engines. There is no way to be 100% certain without seeing it.

Hi, have the same engine, muffler and airplane. This engine with the stock muffler was working perfectly on an other airplane. Had to plug one of the exhaust ports on the pitts muffler to increase exhaust pressure to the fuel tank so it could work ok. If the carb was adjusted to work ok when the plane was level, the engine would died when it was hold in a nose up position, if it was adjusted to work ok nose up, there was nothing to do to have the mid-range work properly when level. A tight piece of wood with a screw through the exhaust pipe works ok. Suggest plugging the left one for better gas evacuation. Also check if the lower front right side of the fuel tank is rubbing against the triangular piece of wood glued for re-inforcing the fuel tank compartment. Mine was and it cut my tank, still waiting for it. Had to call them, send e-mail with pictures for a replacement. They dont sell this tank separately.

So first of all, thank you guys for all of your replies...MUCH appreciated. These forums are great.

Anyway, I'd love to reply with the exact steps on how I got my engine running right but I can't. What I did was I put the idle screw and mixture back to default and put the factory muffler on...that didn't help. I tried everything and then it randomly started running right. I put the Pitts muffler on and it continued to run just fine without any plugging of the muffler. I can't figure it out but it seems to be running just fine now. It's a little sluggish in the mid-range but I assume it's a bit rich (which I want) and that can be corrected by dialing in the mixture a bit. Anyway, it was a very frustrating few days with this engine.

On a bright note, I flew the .55 in my Revolver today...it was the maiden for both and it went pretty good. The Revolver is a sweet plane. I'm running a 12*8 prop...is that a decent match? I'm assuming after 3 or so flights I can lean things up right? Thanks again.

Joe, the first clue to the OPs problem was 2 1/2 turns out. Just for grins turn in the needle on one of your running two strokes and I will bet money you are close to 1 1/2 turns out on yours, that is about the normal place on an engine. 2 1/2 turns is just where the factory says to start the engine to make sure it never goes lean on you, not even close to where they run at.
I have never had a Pitts muffler that didn't give enough pressure but I hear of it happening once in a while. I use Bison Pitts myself and they have a baffel system and produce pressure just like a stock muffler. Some of the older mufflers were just cans with two pipes and the pipes were too big, I don't hear of the problem much any more but I'm sure it still happens.
New bike?? What did you get??
Gene