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os 55ax mid range

Old 03-30-2013, 12:15 PM
  #1  
andrewskj
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Default os 55ax mid range

I have a gp cherokee 40 with a os55ax using a bisson 4046 which is only one avail from tower and I have mid range problems good top end and seems to idle great . I understand they make a 4055 muffler do I need to try to get this the tower says 4046 is for 55ax . I amd using os8 plug and power master 15%. I also noticed i need to open the hs needle at least 3turns or more out. please advise and thank you in advance . Ken
Old 03-30-2013, 01:20 PM
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BarracudaHockey
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Default RE: os 55ax mid range

I also noticed i need to open the hs needle at least 3turns or more out
That to me says you have some sort of issue, either with fuel delivery or the carb. Sort that first and the mid range will probably take care of itself. Tower sells a Tower Brand muffler with a lot of expansion room that makes that engine run like a scaleded dog though.
Old 03-30-2013, 01:35 PM
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JohnBuckner
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Default RE: os 55ax mid range

Its a great engine the AX's are my all time favorites for a sport two stroke. Its a big time stumble when you open the throttle from a stable idle right?

You have said nothing about the mid range which is too rich from your description. Remenber the midrange is not in the same place as the common place double needle type engines. It is on the left hand side at the end of that cone and its just an itty bitty slotted screw and not in the throttle arm.

John
Old 03-30-2013, 06:29 PM
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Jetdesign
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Default RE: os 55ax mid range

How much fuel have you run through this engine? My advice is to get the factory specs from the manual for both needle valves, and return all settings to stock. Keep the top end slightly rich, and fly a bunch. Get the engine running consistently, then make one SMALL change at a time. Start by leaning out the top end slightly. When it seems to be making more power, you can adjust the low end to dial in the midrange.
Old 03-30-2013, 07:07 PM
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speedracerntrixie
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Default RE: os 55ax mid range

The low speed needle may be set too lean.
Old 03-30-2013, 07:23 PM
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Dave McDonald
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Default RE: os 55ax mid range

The root of your problem is the Bisson muffler. They don't produce enough back pressure, which is why your HS needle is open over 3 turns. With the HS needle open that far, the mid-range will end up slobbery rich, often causing the engine to flame out. The engine will also lean out too much when the nose is pointed up, and will become overly rich with the nose pointed down.

The real solution is to increase the back pressure in the Bisson muffler to about the same level as the stock muffler. Blocking off one of the exhaust tubes will help a lot. You might even need to pinch down the other tube a little with some pliers until the HS needle is open approximately the same number of turns as with the stock muffler. If you can temporarily install the stock muffler to find the correct HS needle setting, then block and/or pinch the Bisson exhaust tubes to achieve the same HS needle setting, that would be the ideal solution.
Old 04-11-2013, 06:18 AM
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rlipsett
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Default RE: os 55ax mid range

I have found that running a hotter plug such as the old a3 works best for me in the engine. Second of all if you are no where near the factory recommended presets then you will be chasing the engine adjustments around to where it looks good in most positions but not good in all throttle ranges. I have had a problem engine that needed at least 5 tanks full of fuel before it settled down. I have no idea about the muffler, but I do know about the frustration of chasing setting when they are far outside the factory called for settings
Old 04-11-2013, 06:57 AM
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racer639
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Default RE: os 55ax mid range

I have been fighting this problem also with a 55ax in a GP revolver. The engine idles great transitions to full with no hesitation and runns very strong wide open, but any thing above idle and below half throttle it runs about 20-30 seconds and die light you shut off a switch. I have tried 3 different pitts type mufflers the stock muffler and an ultrathrust all the same. The only thing that helps is power to the plug, but I hate to put an onboard driver in the plane if I can help it. I did notice that




tower shows the 55 discontinued in one part of their site and in stock in another, the one shown in stock is a different stock # and has a different carb and muffler.

the older 55 has a 40j carb the new one shows a 40k, which is the same as the 46 uses, so the problems with the engine must be common enough to warrant a change by OS
Old 04-11-2013, 07:07 PM
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fredscz
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Default RE: os 55ax mid range

Try an OS F four stroke plug. I agree with one of the other posters that the engine takes longer than the normal OS to get run in , but when you get it right it will be your favorite engine.

I tried an F plug in one of my first 55's when the mid-range had a burble and the hotter plug cured it. L ater after the engine had more run time it ran flawless on the #8 plug.
Fred
Old 04-12-2013, 06:14 AM
  #10  
mustangman177
 
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Default RE: os 55ax mid range

Also what prop are you turning ? I have 3 OS 55 AX's. Found a APC 12 x 4 works best for me. Also running a Mac's black muffler.
Old 04-12-2013, 06:17 AM
  #11  
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Default RE: os 55ax mid range

Hi!
The problem isn't the engine it itself but the silencer lack of back pressure! Use the original OS silencer or reduce the exhaust outlet on the silencer you have.
No long running in time is needed any of our modern engines, they run and throttle just perfect from the moment you start them. The only thing you have to do is set the needles right
Old 04-12-2013, 10:22 AM
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JohnBuckner
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Default RE: os 55ax mid range

I agree entirely with Jaka's post above, he is right you really need to choke of that muffler a bit more and the #8 plug is perfect with that engine. I avoid the so called pitts box mufflers like the plague prefer almost every time to cut in a simple tunnel in the firewall if neccessary. That engine is superb and I do own three that have been wonderful right out of the box in a Moskito and one in a small Ultimate. Of course I use the stock muffler.

In addition to this many of my students use this engine and they have been proven as a perfect match with Senior Kadets, none have been problem childs of course all have used the stock mufflers.

In addition they have proven (there goes that opinion thing agine) to respond exceptionally well to pinch tunning.


John

Old 04-12-2013, 11:41 AM
  #13  
airbusdrvr
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Default RE: os 55ax mid range

I have had the same problem with the 55AX. I finally "partially solved" the issue by leaning the low speed needle. Keep leaning the low speed needle to the point that you still get good idle to full throttle acceleration. It is unbelievable how much the low speed needle needs to be leaned.
Old 04-12-2013, 12:00 PM
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racer639
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Default RE: os 55ax mid range

I was just told by customer service at OS the the problem is the 40j carb, the newer 55's have a 40K carb, changed them out now no problem. I have tried 3 different pitts mufflers and the stock one and ultra thrust they all work fine now

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