Conference vmszoo::rc

        I'm not a real expert on carbs, but I can relate my experiences
    with mine.  Just last week I went to the field and for some strange
    reason, my .15 was not cranking out the rpm's.  So what I ended up
    doing was closing (completely) the low idle screw.  I'm still not
    sure what the problem was, but it solved the problem.  The engine
    picked right up.
        As far as the throttle linkage, my engine runs at idle when the
    throat opening is about 1/32 to 1/16 of an inch open.  Any more then
    that and it runs away on me.  It's all a mater of playing with all
    the adjustments.  For example, this is what I did;
    
        - turn on radio, turn on receiver, set your throttle trim all
          the way up, set your throttle stick all the way down
    
        - look at your throat opening, if it is open to wide loosen up
          the throttle linkage and move the stick up a bit, tighten the
          linkage and set your stick back down, (you only want a little
          opening showing at those settings), if the throat opening is
          closed, than reverse the procedure, with the stick all the way
          down start moving your throttle trim down, the throat opening
          should finally disappear, (this will kill the engine)
    
        - at this point I adjust my fuel and air flow to get the lowest
          idle  without killing the engine
    
    I'm no expert as I said, but this methos works for me.  Hope it helps!
    
    Ken

Carbs are very simple things once you have adjusted them a couple of times. 
With the risk of creating a few flames I will say that the OS carb is probably 
the best and easiest to use and adjust.

It is a good idea to have the throttle set up so as throttle stick back and
throttle trim forward is a fast idle (normal flight idle); you pull the trim
back to half way and have a slow but reliable idle (landing idle); finally you
pull the trim fully back and the engine stops. If you don't have endpoint
adjustments on your radio it is a bit of work to achieve this result as it
necessities changing angles on arms and radial positions but it is worth the
effort. Throttle stick full forward is obviously going to give no obstruction to
the carb throat. 

OS has two basic types of idle mixture adjustments; air bleed (which you
probably have); and fuel metering carburetor which is used on the larger
engines. The two types are readily distinguishable, the air bleed carb has a
adjustment screw on the body of the carb, the fuel metering carburetor has the
adjustment in the center of the rotating barrel reachable via a small screw in
the center of the throttle arm. With the air bleed type you wind the screw out
to lean the mixture as you are actually adding extra air to the mixture. With
the fuel metering carburetor the adjustment screw is wound in to lean the
mixture. 

The method I find most effective is to first set the high speed needle; to do
this open the throttle to full, adjust the needle to gain maximum RPM, then
wind the needle out (richer setting) until the engine note changes, now wind
the needle back in again the smallest amount to clear the note. Do this
adjustment slowly as it takes a few moments for the engine to stabilize after
adjustment. With the high speed needle set pull the throttle back to an idle,
wait for a few seconds, if the engine idles then speeds up a little and stops
the mid range is too lean. If however the engine gargles and then slows and
stop it is too rich. Adjust the mid range setting and retry the above test.

After you have adjusted the mid-range setting to get the engine to idle the
next step is to finely adjust the mid-range setting. Allow the engine to idle
for 5-10 seconds and then quickly open the throttle and listen to the engine
pickup speed,  if it gargles and eventually picks up it is too rich; if the
engine stops it is probably too lean; if it does it quickly and cleanly you
have the setting just right.  

Hope this is of some help.

John.

    The OS engines typically have two screws on the carbs.  As in .3,
    one is for the air bleed (this one is parallel to the ground), the
    other effects rotation of the throttle barrel in the carb.  Loosening
    this one too much will permit the throttle barrel to slide out of
    the carb, something you'd need to do if a nose first landing in
    mud required cleaning!  If the engine appears to 'load up', that
    is at idle the engine appears to run richer, and doesn't respond
    to forward movement of the stick well, then open this screw about
    an 1/8th turn and retry stick movement.  The opening is a very small
    hole in the front of the carb closest to the prop.  Unless you have
    the stable hands of a surgeon, its safest to stop the engine when
    performing the adjustment due to having a screwdriver blade fractions
    of an inch from the rotating propellor...
    
    			-Mike
    

    Dan, I just thought I'd write what we had talked about...
    
    If your engine spit off the muffler, lost pressure and still
    ran a low throttle setting chances are that your low speed needle
    setting is too rich. Losing a muffler should increase RPM's. Losing
    pressure should decrease the amount of fuel seen at the carb. Yet
    your engine still continued to run. A rich setting would also 
    contribute to the amount of fuel/oil seen on the plane. Also if
    I'm not mistaken Red Max is known for a high oil content in their
    fuel. 
    In retrospect I would have opted for a dead stick landing and run
    out the fuel. Being your first flight you had plenty of battery.
    Call FOX today. They can Blue Label the part to you and you'll have
    it by wednesday.
    
    						Tom

    Charlie
    
    It's quite simple.... You have not fuel system problem. Your high
    speed needle setting is to lean and the low speed needle is to rich.
    I suggest that you set the high speed needle a little rich. Then
    start the engine and bring it up to full throttle. Let the engine
    warm up. Now reduce throttle to idle. Wait app 20-20 sec. Now advance
    the throttle. If responce is sluggish the bring the engine back
    to low throttle and lean out the low end. I'd try 1/4 turn to start
    and then two clicks leaned every adjustment after that. Repeat the
    from low to high throttle movements seeing how the engine responds.
    If the engine goes to the high end and then dies the richen the
    high end. Also remember to wait 20-3- sec. every time you reduce
    the throttle to a low setting. this will give the engine sufficient
    time to load up if that's what it's inclined to do. Continue this
    until the transition from low to high is a smooth action. 
    	Now with the low setting adjusted set the high for max rpm's.
    Gently apply pressur to the carb intake line and see if a slight
    increase in rpm's is realized. If yes then the high is set ok. If
    no increase is seen the the high is still to rich. If the engine
    races higher and then dies the high is set to high.
    
    
    					Tom

    Tom,
    	I believe that my loading-up problem is due to having to set
    the high speed needle too rich.  If I lean it out with the nose
    level, the throttle response is good and It doesn't load up at idle.
    However, it will lean out too much with the nose up.  I think that
    I may have a tank problem, either too high as Al suggested or too
    far from the engine.  THis causes the engine to have to draw up
    hill to the carb when I pull the nose up.  I can try your setup
    suggestion though,
    
    Thanks,
    
    Charlie
    

    
    Well, Al and I tried to get my OS40SF running better the other day...
    Thanks to the suggestions in here.  It seems to be doing better.
    
    However, I still have a couple of concerns...
    
    1 - High end - a couple of clicks rich from max rpm is only about 1/2
        turn open from fully closed.  Most engines that I have seem to be
        set more like 1.5 turns from fully closed.  Should I be concerned?
    
    2 - The engine RPMs INCREASE when I pull the pressure line off the
    	muffler?!?!  Is this because of reduced backpressure in the
    	muffler?  Or is the pressue creating havoc in the carb when the 
    	line is attached??
    
    Thanks!
    jeff
    

Jeff, when I had the problem of RPMs increasing after I pulled
the pressure line it was because the vent tube in the tank was
semi-clogged.  I replaced the line because it clogged from a
too-sharp bend I'd put in.

    Jeff,
    
    Another thing that _could_ be wrong and would fit both symptoms is that
    the high-speed needle valve/and or seat could be worn.  This would
    allow more fuel to pass than normal and would require thurning the
    needle further in (leaner) than expected.  Muffler pressure would tend
    to aggravate this by delivering fuel to the carb under pressure.
    
    If you have one, try a spare/new needle valve and see if it has any
    effect.  As far as cause for concern, yer' probably OK but, like you,
    I'd be a bit uncomfortable at the loss/lack of tuning latitude.
						 __
				|      |        / |\	   	       
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      | |        00	       <|  ~~~  ____ 04 ---- | --------------------
    |_|_|        (O>o		|\)____/___|\_____|_/	   Adios amigos, Al
      |     \__(O_\_	        |	  |___/	 o	   (The Desert Rat)

    Jeff,
    	It's normal for RPM's to pick up when you pull the pressure line if
    it's running rich under pressure.  The mixture will lean out when you
    pull the line which will cause an RPM increase unless it goes way lean.
    It will continue to go leaner as the fuel level gets lower.  That's the
    reason for muffler pressure in the first place.  It reduces the effect
    (doesn't eliminate it) of the change in fuel 'head' pressure due to
    attitude changes and fuel level changes.  Many planes still lean out
    some as the level drops and when the nose is pulled up.  The advantage
    of a pump or pressure system is that it further reduces the fuel flow
    fluctiations due to changing attitude and tank level.  
    
    Charlie
    
    P. S. The major disadvantage of a pump is the additional complexity -
    one more thing that can be misadjusted or can fail on you.