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OS MAX .25 carb/throttle action

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Old 05-01-2012, 04:53 PM
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ahickman
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Default OS MAX .25 carb/throttle action

Good evening. New to the forum and new to RC motors. I have what I believe is an OSMAX .25 and am pulling it down owing to having sat for 5 years in storage with fuel in it. Not familiar with the engines at all. Last I had was a Cox .029 with a simple needle vavle. This one looks to have a full throttle plate of sorts. The carb is missing the throttle lever and I believe the barrel inside the throttle is stuck/froze. Can anyone tell me if the brass colored cylinder/bushing i the picture is supposed to rotate?

Thanks,

Aaron
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Old 05-01-2012, 05:12 PM
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Default RE: OS MAX .25 carb/throttle action

Hi,
buy one new, I've already bought from this seller and I recomend.
http://www.ebay.com/itm/O-S-MAX-No-3...item46035e90ee

regards
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Old 05-02-2012, 06:18 AM
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Default RE: OS MAX .25 carb/throttle action

That is an RC carburetor. The "brass colored" object is the end of the throttle barrel, and it is supposed to be able to rotate. The black screw holds the throttle arm onto the carb. Here is the part number:

OSMG9000 22081408 THROTTLE LEVER #1A-3A US$2.29

These parts can be ordered through any hobby retailer who sells O.S. Max engines and accessories.

If the throttle barrel is frozen in the carburetor, it can be freed by using a heat gun to warm the carburetor body. The carb body is aluminum, and the throttle barrel is steel. The aluminum will expand more than the steel, and so it should become easier to move and remove. You can also soak it in rubbing alcohol for several days to see if that will loosen the congealed fuel residue, which is the likely "glue" holding it all together.

If all else fails, a new carburetor would have to be purchased. Depending upon exactly which OS engine it goes to, one of the modern carbs may work, or you may be able to locate one through one of the internet hobby sellers or auction sites.
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Old 05-02-2012, 09:18 AM
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Default RE: OS MAX .25 carb/throttle action

Many thanks for the part number.  I was able to soak the entire engine and carb in some solvent.  Everything freed up and seems OK.  Only issue I suspect will be the "air bleeder" screw which you can see in the second picture.  It was broken off.  I was able to get the stud out, but have no clue as to the original length.
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Old 05-03-2012, 06:02 AM
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Default RE: OS MAX .25 carb/throttle action

The original length really is irrelevant. As long as you have a screw that has the proper threads, it just has to be long-enough to close the air-bleed hole when turned in. A spring is used to apply tension that keeps it from vibrating out.
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Old 05-09-2012, 12:00 PM
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Default RE: OS MAX .25 carb/throttle action

OK, I reassembled the engine and came up with a test jig of sorts.  Preadjusted the carb as best I could and fired it up.  Very suprised at the response to the throttle plate.  A couple of problems though:  1.) difficult to idle, 2.) spits out massive amounts of fuel from the exhast and 3.)went through 8 oz of fuel rather quick (3 min. run time). 

I looked over OS's website for how to tune this engine, but still can't get it right.  Where does the air bleeder come in to play?  Why the poor fuel efficiency?  Anyone give me the short on how to properly tune this bugger up?

Thanks,

Aaron
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Old 05-11-2012, 05:09 AM
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Default RE: OS MAX .25 carb/throttle action

Anyone?
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Old 05-11-2012, 05:56 AM
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Default RE: OS MAX .25 carb/throttle action

At full throttle, the engine should be leaned with the high-speed needle to peak RPM and then richened a bit. If you do a quick pinch of the fuel line you should hear the engine pick up a bit in RPM. If you hear no change, then you need to richen the needle a bit. If you burned 8 ounces in 3 minutes at full throttle, you have the high-speed needle much, much too rich. When properly leaned, it will be somewhere between 1-1/2 turns to 1-3/4 turns out from closed. It all depends upon where it needs to be set for your propeller, fuel, and location.

The air-bleed screw should be set so that it blocks the air-bleed hole 1/2-way across. Adjust from there. Turning the air-bleed screw counter clockwise will lean the idle mixture. Clockwise richens. This is opposite to the main needle valve.

Your engine will discharge quite a bit of oil because the oil will not be burned, and the fuel will contain about 18% oil by volume.
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Old 05-11-2012, 08:13 AM
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Default RE: OS MAX .25 carb/throttle action



Do I adjust the air-bleed screw after the highspeed adjustment?  BTW, when first starting the engine (cold), should I leave the carb wide open or closed?

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