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I think there is a lot of misunderstanding about how the OS carbs work. This is probably why everyone gets worried about where that mid-range needle is running.
The 60 series carbs all share the same operating principles. In the engine manuals, OS goes to great lengths to discuss how to tune these motors. I've found that virtually nobody bothers to even read the manual, much less take the time to understand it. OS puts a graph in the manual to describe the hi speed and mid-range needle tuning and how they are related. They also have a great flow chart which describes the tuning process. If you understand the following, you can envision how that graph was developed. You can also understand the mechanical design of the carb, and get a better idea of how to tune these engines.
The Hi speed needle on these carbs is similar to the front door to your house. EVERYTHING that goes into the carb has to go through this needle valve first. Here's a diagram I put together quickly a while back to better understand the internals. I planned on making something nicer based on this, but I just ran out of time.
The fuel flows through the hi speed needle, which controls the flow through the entire carb, then into the Mixture Control Valve.
THis is an overall photo/desciption of the carb body functions.
Here is a photo of the Mixture Control Valve (MCV)
This mixture control valve has a window in it's center. Fuel flows from the hi speed needle through this window. The cateye rides inside the MCV, and fuel flows through the window, into the cateye's opening. The exposed opening depends on the throttle position. The more throttle you give, the more opening is exposed. One point to note...only one half of the cateye opening has a metering effect. THis is because only one half will ever become exposed in the window.
Here's a shot of the cateye.
Some fuel also flows around the MCV, and is available to the mid-range chamber. I say 'available' because it doesnt necessarily have to be used. THe primary metering channel of these carbs is from the hi speed needle, through the window in the MCV, through the cateye, and into the spraybar.
The throttle barrel, or rotor as it is described by OS, has it's own cateye. THis is the midrange cateye. Fuel flows, when demanded and available, through this cateye, and down into the spraybar via a 'backdoor' inside the rotor.
There is a port drilled from the mid-range needle chamber to a small hole inside the carb's case.
This port is exposed to the rotor's midrange cateye at about 20% throttle opening. THis means that the midrange channel of the engine is not effective before 20% barrel opening. It is completely covered again at approximately 90% barrel opening.
What does this mean? THis means that at full throttle, the midrange needle has zero effect on tuning. All fuel is flowing through the primary metering channel - the cateye in the center of MCV that is attached to rotor assembly.
OS gives us a very good description of every control on the carburetor in the manual. Pages 18 and 19 in the SZH manual has all this info. On pages 23-27 we have the tuning info.
So now that we've looked at some mechanics, and reviewed our manual, what does this tell us?
Since all fuel must flow through the Hi-speed needle FIRST, this is the most critical needle on the carb.
When tuning, we should first make a preliminary adjustment of the MCV screw to set an acceptable idle and transition to higher throttle settings. If she stumbles and chokes on spool-up, you are too rich. If she cackles and races to higher throttle settings, the MCV is too lean.
We then hover the model at an arbitrarily 'safe' needle setting. I typically use 2 turns on the hi and 1 turn on the mid if the motor has been broken in.
In a hover, we can tell if she's insanely rich or lean, so we make our first adjustment. The manual tells us to move the hi needle in 3-4 click increments initially. Then we do 1 or 2 click increments later for fine tuning.
So we adjust the hi needle so the heli hovers relatively smoothly. Notice we havent touched the mid-range yet.
Now its time for the most important part of OS 60 series carb tuning - running at full throttle and setting that hi needle once and for all. Please make sure you have your collective pitch set correctly before attempting to tune. Start with 10/-10. I cant tell you how many helis I've tried to tune, where the owner insisted he had 10/10 and it was really +14/-8. You will NEVER be able to tune your engine and make it run well if the heli's mechanical setup is not correct. I start with 10/10, get the machine tuned close, then slowly increase pitch (while fine tuning hi end) Don't start with 12/12. Its a 90, but only a select few can manage pitches greater than 11/11.
Now, back to the hi end tuning. Whats important here - and hopefully now that we understand the carb's internals a little better- is to make sure the mid-range needle is entirely out of the picture. This means we need to be at pretty much full throttle and loaded.
This can't effectively be done doing tic tocs, or chaos, or anything silly. You'll drive yourself nuts doing it like this. What this means is.......get that heli doing a maneuver which demands full throttle for an extended period of time. Giant loops, where you are loading the heli hard on the initial climb, and barely having to lift off the throttle over the top. I find that hurricanes are a great tuning maneuver for the high end, because you never have to reduce the throttle setting.
You will tune the hi needle like this until the engine sounds smooth, and pulls really well without cackling, tail wagging, sagging, etc. I'll typically come out of the hurricane and roll level inverted - transitioning to a loaded half outside loop. At the very top of this loop I hit throttle hold.
If the engine settles to a nice idle within a second or two, and you are happy with how she pulled through the big maneuver - you have completed the hi speed needle tuning. If the engine 'hangs' or cackles for more than 2 seconds before settling to and idle (or if she never settles to and idle) then there are two possibilities...
1.) your throttle trim or throttle hold setting are too high
2.) you are too lean, and the engine is hot and angry (richen and repeat process)
Through all of this - the midrange needle still hasnt been touched. Why should it be? It has been pretty much non-functional, as all fuel was going through primary cateye into spraybar.
Now it's time to set the mid-range. The mid is there to supplement fuel flow in that 20-90% throttle range. Here is where we can do our tic tocs, sudden stops, chaos - any maneuver that will transition from hi needle range (90% and above) down through the midrange and back. We are looking for a smooth transition.
This is so hard to describe in text. It's a 'feel' thing. You are looking for a nice transition. You want a powerful and smooth midrange and you want her to immediately transition to that powerful and smooth hi end that you set earlier. If she's too rich in the mid, you are gonna make the transition from mid-range to top end and notice that it takes a second or two before top end really kicks in. Thats because she was loaded up in the mid, and it took a second or two to clean that excess rich mid fuel out of engine and transition to your perfectly tuned top end. If she cackles in the mid, well, you are too lean. Like I said - its a 'feel' thing that must be learned.
The 60M-C carb in it's stock form has a larger cateye than the older 60K (its actually the 60M's cateye). OS did this to eliminate the lean mid-range issue that the 60K had on the gold headed Cspec. As a result - more fuel flows through the primary channel of carb, and less midrange is theoretically necessary.
Once the hi end and mid are tuned....the relationship between the two is established. When the temp or climate changes, you should only have to adjust the hi speed needle. You should not have to mess with your midrange again. Of course, if you change fuels, helis, mufflers, etc - its back to square one.
The pumped 60M-P utilizes the smaller 60K cateye because the pump regulator system increased flow to the hi speed needle 'front door.' I flew the 60M-P with the 60M-C's large cateye for some time, and it was almost impossible to tune correctly.
The 60M-P fuel delivery system pushes enough flow through the primary metering channel that the midrange needle does not need to be open very much (under typical conditions)
As for the tuning info above - there are other variables. Shims and plugs also enter into the equation. That's for another post, cuz I've rambled too long.
Hopefully someone will find this info somewhat helpful, and clear up some misconceptions about tuning these carbs.
Oh, and did you ever wonder what the balls were on your carb? |
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发表于 2008-12-18 14:27
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