Roughly three years after this web page was created, OS revised the OS32sx carburetor. The newer carbs are rumoured to have solved the rich idle / lean midrange problem entirely. However, if you have one of the earlier models (not unlikely, considering they were in production for about four yeras before OS took action to fix this problem), this following information might be useful to you.

Standard disclaimer: all tests, observations, and adjustments should be done when the engine is in its normal operating temperature range.

Setting the low-end mixture

You need the low-end mix to be very rich in order to get a decent midrange and top end. Unfortunately, the engine can be a bear to start when it's tuned 'right.' Basically, you should set it as rich as possible. This means that after then engine has warmed, if you idle for a while it wil sputter a lot when you try to run it up again. It will die if you just punch it, so you have to be careful.

OS 32 SX mixture peculiarities

The OS 32sx midrange is a bit lean as compared to the rest of the throttle range. It has been suggested that, when learning to hover, the high needle should be set a bit rich to compensate for this. For more adventurous flight, set the low needle rich; this will allow the high needle to be leaned correspondingly, for better top-end performance.

With the careful use of a very small drill bit and instructions from Doug Adams, the lean midrange / rich idle problems can be cured. See below for details.

Starting

Due to the rich low end, this is more trouble than it really should be. I find that it starts better if I kill it by pinching off the fuel line rather than simple cutting the throttle all the way off. This effectively leans the mixture the next time you turn it over.

If you're having a tough time starting it, get a friend to pinch off the fuel line right at the carb while you turn it over. Instruct the pincer to let the fuel flow again as soon as the engine fires, lest the engine starve and die again. It will want to run faster for a moment until the mixture is restored, so be ready to modulate the throttle accordingly.

If the engine is cold, it will probably die as soon as you take off the glow driver. If it doesn't die, you're probably too lean and asking for trouble (BeenThereAndDoneThat). Before you take off the glow driver, put the heli on the ground and run up to 1/3 throttle or so for a little bit - practice will tell you how long you need to do this. Some guys around here will hover up and down a couple times. No real harm in doing this too long, but if you pull the glow driver off too soon, the engine is likely to die.

glow plugs

From Doug Adams : Glow plugs are cheap, try a new one. If there's no change, then the old one is usable.

He's got a point, and it seems like something to keep in mind when looking for glow plug wear.

Drill that carb!

Check this out: Here's a quote from Doug Adams, who I understand developed the mod: I've tried a couple of diffrent air bleeds. So far the best one is described below:

=====================================
|                        ()<-.055"  | drilled square to the body.
|        OOOOOOOO     SSSSSSSS      |
|       OO      OO   SS      SS     |
|      OO        OO SS              |
|      OO        OO  SS             |
|      OO        OO   SSSSSSSS      |<-Raised Rectangular Boss on Carb
|      OO        OO          SS     |
|      OO        OO           SS    |
|       OO      OO   SS      SS     |
|        OOOOOOOO     SSSSSSSS      |
|                                   |
=====================================

The air bleed is centered above the "S" and is tangent to the top of the "S". It doesn't overlap the "S". Problem is, the S on my carb doesn't look quite like the S on his artwork. It's more italic, like:

             (x)
            SSSSS
        SSSSS   SSS
    SSSSS
  SSS
 SSS
  SSSSSSSSSSSSSSS
              SSSS
            SSSS
 SSS      SSSS
   SSSSSSSS

Well, that's kind of lop-sided... Drill in position (x), .055"

This will increase the amount of air going into the carb at low throttle settings, thus leaning the idle a great deal. Compensate by richening the low-end mixture valve. Presto: a reasonable idle mixture (so you can stat it) AND a reasonable midrange mixture (so you can hover smoothly).

Thanks Doug!