OS mixture question

[Wayne Galligan]

I helped a guy with the a similar issue this weekend on his Mintor.   Way too lean on the bottom end. His would pop on acceleration or die coming out of the outside loop.

For the OS 1.40 RX Try this:
Use an OS "F" plug.
15% nitro
I have not had to adjust the pump on any of the OS's I have set up to date.

Set the top end till it peaks in rpm(use a tach if you have one) and back off one click to the rich side.
Bring it down to an idle and try  to set the idle to about 18-1900 rpm.

Let the engine idle for at least 20-30 seconds and do not touch the throttle.
   1.  if it speeds up or surges at any time during this time the bottom end is lean.
   2.  if the engine gradually slows down and dies...its rich.

Advance the throttle with medium speed and observe:
   1.  if it burbles and picks up speed or burbles and dies bottom end is too rich.
   2.  if it falls flat or hesitates then accelerates or dies it is lean

This may take several tries to get but once its set then its just minor top end adjustments depending on weather conditions.

Changing pipe lengths will also effect the low end transition.   A longer pipe setting will allow you to set the bottom end a little richer and give better downline braking.
I find that running the bottom end as rich as possible with out it loading up gives the best downline deceleration.

Setting your pipe is a whole other story.   My best method is on a test stand.   Set the pipe long and run engine, tach the top end at peak rpm.
Shorten the coupler or pipe about 1/4" at a time and observe the peak rpm.   It will increase with each shortening of the pipe.  
Once it peaks in rpm or no longer increases or drops in rpm this is the top tuning spot for your engine and pipe for max rpm(not really the desired tuning).
At this point back the pipe off about 3/8 to 1/2".   This will give you a good starting point.  I usually lengthen the pipe if I go to a higher loaded prop or
shorten it with a lighter loaded prop.

The OS 1.40 is a very predictable engine.   If it starts to load up and stall in the midrange for no apparent reason or drops rpms
after the glow plug is removed first check the plug . If it continues to do the same thing then the bearing is probably on its way out.
Replace the bearing with a sealed bearing or a stainless steel bearing that goes in the EFI version.   I have about 200 flights on one
of my OS's that has the sealed std steel bearing from Bearings Limited in it and it is still going strong.   Another way to check the bearing is move the prop by the tips
perpendicular to the engine and if you can feel end play the bearing is getting loose.   

Wayne Galligan
  ----- Original Message ----- 
  From: Kurt Bozarth 
  To: discussion at nsrca.org 
  Sent: Monday, April 18, 2005 7:22 PM
  Subject: OS mixture question


  OS 140 RX (non-EFI) question:



  When I advance the throttle, the rpms remain at idle for approx. one second, "pop" and then advance to full throttle rpms  I am not quite sure if this "pop" and hesitation is a result of having too lean of an idle mixture or too rich.  This is an older motor released back when Hobby Services offered a free needle valve assembly replacement to correct for mid-range richness issues.  I do not know if this was done or not - something I will check for tonight.  I have never adjusted the pump setting.  Let me know your thoughts.  Thank you.



  Kurt Bozarth 
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