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  1. #1

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    OS 120 AX Problem ?

    Hi,
    I've had trouble in the past with an OS 120 AX . I bought a Slimline inverted Pitts stylemuffler and could not get it to work properly with this engine. It had numerous problems including little response to the needle valve position to overheating , poor starting etc.

    I abandoned trying to use this muffler and got excellent performance from the stock muffler.

    Have any others experienced this situation ?

    I now want to try to use a Pitts Style again and wonder if there are any that have had success with this set up ??

    Cheers,
    M,M,

  2. #2
    ThumbSkull's Avatar
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    RE: OS 120 AX Problem ?

    Often the Pitts style mufflers do not create enough back-pressure.
    The usual solution is to plug up to 1/2 of the port area. Completely block one or 1/2 block both.

  3. #3

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    RE: OS 120 AX Problem ?

    Yes the pitts mufflers do not supply enough pressure to feed the carb. three things to do if want to run the pitts muffler

    1.Block off one exhaust tube or pinch both of them closed 1/2 way with pliers

    2. run a 30 dollar perry pump

    3. run a smaller carb


    all the AX engines not like to run hot.

    I am very well versed in the OS 120 AX engine. If you have any questions let me know. The 120 is the strongest one out there and it is also the lightest. It is even stronger than a Jett 120.
    AMA # 126183
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  4. #4

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    RE: OS 120 AX Problem ?

    Hi,
    Thank you for your replies.

    I wonder why the muffler manufactures don't include these considerations in their product(s) ? Is it just the 120 AX that presents these problems?

    Thanks
    M.M.

  5. #5

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    RE: OS 120 AX Problem ?

    no because most of these mufflers at the time they were produced worked just fine. IE: lower power engines with smaller carbs.

    It is the same with these mufflers on most of the engine sizes. These mufflers are usually less power than the stock muffler so they made them have 2-3 exits to try to get some power back that the stock muffler has. doing this they lost some back pressure. Some of these also have the muffler pressure tap in the wrong spot. near the engine exhaust can have a positve and negative pressure pulse leading to the low tank pressure. You can take the muffler pressure and try a 3 dollar check valve to see if it helps.
    AMA # 126183
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  6. #6
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    RE: OS 120 AX Problem ?

    the Bisson pitts mufler works great.... the exhaust tips are pinched off which give better back pressure
    \"Propellers are notorious for inflicting serious bodily harm while vigorously defending their space\" George Aldrich

  7. #7
    Rocketman_'s Avatar
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    RE: OS 120 AX Problem ?

    ORIGINAL: Merlin Man
    ... I now want to try to use a Pitts Style again and wonder if there are any that have had success with this set up ??
    Yes, I am having success with two OS 1.20AX engines side mounted on a scale twin war bird using Slimline large volume Pitts style mufflers. The engines are being run with no pumps, no regulators, no check valves and no plugged exhaust pipes. They are very easy to start by hand (chicken stick).

    One of the engines quit during the second flight of the model while transitioning between high and mid throttle, which is a common complaint about this engine. I found that I was being too gentle with them during break in by running them too rich. I said "enough of this rich crap" and I leaned them out and they've run very well ever since.
    The pictures show the wedge shape of the muffler which allows it to fit entirely inside the cowls.
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  8. #8

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    RE: OS 120 AX Problem ?


    ORIGINAL: Rocketman_

    ORIGINAL: Merlin Man
    ... I now want to try to use a Pitts Style again and wonder if there are any that have had success with this set up ??
    Yes, I am having success with two OS 1.20AX engines side mounted on a scale twin war bird using Slimline large volume Pitts style mufflers. The engines are being run with no pumps, no regulators, no check valves and no plugged exhaust pipes. They are very easy to start by hand (chicken stick).

    One of the engines quit during the second flight of the model while transitioning between high and mid throttle, which is a common complaint about this engine. I found that I was being too gentle with them during break in by running them too rich. I said ''enough of this rich crap'' and I leaned them out and they've run very well ever since.
    The pictures show the wedge shape of the muffler which allows it to fit entirely inside the cowls.
    why did you buy extra mufflers when the 120 comes with a pits muffler?????
    AMA # 126183
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  9. #9

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    RE: OS 120 AX Problem ?

    HI,

    The 120 comes with a pitts muffler ? Mine didn't . The standard (stock) muffler isn't a pitts muffler ...............Is it ?

    Rockerman, I notice your set up is not inverted. ( I think that the sidewinder set up is one of the best going, but unfortunately I cannot employ this .)

    I can't think of any reason why the side mount muffler works better that the inverted one , but apparently it does. ?! I know that the side mount motor has less problem with flooding at the start but otherwise ??

    I am starting to think that the option of a petrol ( gas ) motor might be the way to gowith their pumped carbys.

    Thanks forall your suggestions,
    M.M.




  10. #10

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    RE: OS 120 AX Problem ?

    THe 120 AX comes with a Short cylindrical muffler for which a 90 degree adaptor can be purchased to make it fit like a pits. it still has the exit at its end.

  11. #11
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    RE: OS 120 AX Problem ?


    ORIGINAL: airraptor
    why did you buy extra mufflers when the 120 comes with a pits muffler?????
    I bought the wedge shaped Slimline mufflers because they fit entirely inside the cowls and made a neater installation for my application. The exhaust pipes protrude, of course.

    The OS 1.20AX is sold with or without a muffler. The square cylinder shaped OS Power Box Muffler with the 90 degree adapter would not fit inside the cowl so I purchased the engines without the OS mufflers.
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  12. #12
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    RE: OS 120 AX Problem ?

    I am using the stock OS muffler with the 90 degree adapter. Performance wise, it is great with no issues. Fitting it in a cowl is another story. It will hang down more than an aftermarket Pitts style muffler. The other two issues, the pressure nipple now points straight at the webbing of the motor mount with almost no clearance, and to tighten the thing I have to go through the cooling holes in the front of the cowl. The muffler and cowl have to be removed as a unit. Some planes will no doubt have more clearance. This is on a GB ULT and the lower cowl cuts back in a hurry.

    david
    I never want to see a crash. But I don't want to miss one either.

  13. #13

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    RE: OS 120 AX Problem ?


  14. #14

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    RE: OS 120 AX Problem ?

    Sounds more like a fuel meter problem. You have an air leak some place. Check;cyclinder head, back plate, needle valve "O" ring and carb seat. I have used both SlimLine and Bisson mufflers and never had a problem with my OS 160FX.

  15. #15
    Moderator w8ye's Avatar
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    RE: OS 120 AX Problem ?

    A 160 is much easier to tune and more forgiving than a 120AX.
    Attended the CutFinger Institute of DirtNap University for years but never did graduate....
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  16. #16
    Rocketman_'s Avatar
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    RE: OS 120 AX Problem ?


    ORIGINAL: w8ye

    A 160 is much easier to tune and more forgiving than a 120AX.
    Very true.
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  17. #17

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    RE: OS 120 AX Problem ?

    ORIGINAL: airraptor

    Yes the pitts mufflers do not supply enough pressure to feed the carb. three things to do if want to run the pitts muffler

    1.Block off one exhaust tube or pinch both of them closed 1/2 way with pliers

    2. run a 30 dollar perry pump

    3. run a smaller carb


    all the AX engines not like to run hot.

    I am very well versed in the OS 120 AX engine. If you have any questions let me know. The 120 is the strongest one out there and it is also the lightest. It is even stronger than a Jett 120.

    The correct answer is #2 I've been running a 120AX on a perry pump for 6 years.
    I had the exact same problems and this solved them all. Most people on here will curse the Perry pump but they don't know the proper tuning procedure. I'll find it and post it.
    You will be amazed at the performance when you have it done, but you have to drill and tap a pressure port in your backplate.

  18. #18

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    RE: OS 120 AX Problem ?

    Here's the foolproof tuning method for the perry pump, if you try to tune with the adjustment screw on the pump as the instructions state, you will lose all your hair, and throw the pump onto the ground and stomp it to death, save yourself the heartache.

    Open up the muffler if you've had it blocked off, peak the high end, don't back off rich. Go to idle, if engine idles well for 15 or 20 seconds, lean the low speed 1/4 turn, Go back to the top and re-peak, go back to idle, same routine, if it idles for 15-20 seconds, lean another 1/4 turn, continue this process until engine will not sustain a idle, it should speed up and die within 20 seconds. Now richen the low speed 1/16th turn at a time until engine will again sustain a idle,after each adjustment on the low speed repeak the top end, as soon as the engine will sustain a idle leave the low speed alone. Each flying day just repeak the top end and back off 150 rpm for flying.
    You have to go lean on the low speed needle to the point of no idle then come back up to get it right. Transition from idle to midrange up to full should be crisp without sputtering. If you run the high speed to rich you will have the same problem with sputtering. My O.S. 120, 160 and 1.08 performs flawlessly with this setup using a Bisson pitt's muffler, I had to lean the low speed 1-1/4 turns to get it to run properly, just make sure you peak the high end and only back off 150 rpm, be sure to use a tach. On my 1.08 it took about 7/8 of a turn on the low speed to get it right.

    When your done your low speed needle will about half open to what it was with the stock muffler, I think mine is about 7/8th to 1 turn open when stock calls for 2-2.5 turns out I think, it's been awhile since I played with it.

  19. #19

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    RE: OS 120 AX Problem ?

    Just for reference, I have a new ASP 1.20 2c that I've been tweaking and was having problems loading up in the midrange with the standard muffler pressure. I wanted to move my tank to the CG so I installed a perry pump and attempted to use your method of setting it up. With the perry, the midrange loading was much worse no matter how close to max rpm on the HS or how lean the LS needles were set. Transition from idle to WOT was great but forget trying to run it in the midrange for any length of time.

    Long story short, I tubed up the bypass system method recommended by Perry and the pressure was just enough to get a rich 2c no matter how much past about 2.5 turns on the HS I would go. Now I can idle it down to 2K rpms and go fix a sandwich, come back and get a clean transition to WOT and the mid range sings with a smooth 2c. The HS needle is very insensitive until you get about 500 rpms from max and is easy to dial in with a tach.

    Conclusions - with a two needle carb, the higher the fuel pressure, the richer the midrange with HS and LS needles being optimally set.

    I'm at about 1 gallon of fuel thru the engine and it tachs 9800 max with a 16X6 breakin prop. With it set to 9600 rpm on the ground, my 9 lb Phoenix 540 120 pulls out from a hover easily.

    ..Joe

  20. #20

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    RE: OS 120 AX Problem ?

    i have a question for you, I have 2 120 ax that sat around in oil for about 2 years, one overheated on the first test flight and the other one I tried out in the yard with a slimline muffler (not sealed just bolted on) and a 17x6 prop. The reason, the other 120ax that overheated caused my new airplane to crash on it's maiden, so I am weary of the engine since it has been sitting around. So, I can start the engine, but when I bring the throttle back it dies, it will start and I can get full throttle, then it dies when I bring it back to idle.

    I am thinking I can save this engine, any suggestions for tweaking it?

  21. #21

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    RE: OS 120 AX Problem ?

    I had the exact same problem with the 1.20AX inverted and using a JTec wrap-around Pitts muffler. I had no high-speed needle valve response and couldn't get it rich enough. The motor ran hot and would quit after a few seconds in flight.

    I tried a stock muffler on it last weekend for the first time and it ran great. The problem ended up being where I installed the muffler pressure port on the Pitts muffler. I didn't really know where to put it when I got the muffler and ended up putting it in the 'can'. At that location, there wasn't enough pressure to supply to the tank. I moved it as close to the exhaust outlet of the motor as possible and the problem was solved. The high-speed needle valve responds just as it does with the stock muffler.

  22. #22

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    RE: OS 120 AX Problem ?

    What temp dose the 120ax run at? I was checking mine this past weekend its run around 300F its enclosed w/ a Pitts style muffler on 20% with a 17-6 vess prop but the motor seam to run strong at wot im only getting about 8000 rpm

  23. #23

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    RE: OS 120 AX Problem ?

    To me that seems high. I would look to better cool it.
    AMA # 126183
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  24. #24

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    RE: OS 120 AX Problem ?

    For what it's worth, I think a 17x6 is just a bit too much prop for a cowled 1.20. You will get more hp at >8500 and it will run cooler. Also, 10% or less nitro.will run cooler with only a minor hit in output with a large 2c nitro engine.

    On my Yak 54 I use a 16x6 when I want speed and an APC 17x4W when I need to slow it down. Both turn around 9400 on the ground and hover the 9 lb Yak at around 3/4 max rpm. The APC is a very heavy prop tho, and spool up is terrible on glow. Gas engines sure have much more lower end torque.

    Anybody know why?


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