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Model Forum / Radio Controlled / Air Models / September 2006Extreme Air Bleed Adjustment
A couple of months ago there was a thread on this group about adjusting carbs with air bleed. Someone mentioned that their adjustments were all over the map, and suggested drilling out the hole in extreme cases. Well, I drilled out the hole on my OS MAX 25 a couple of weeks ago, and finally got to fly it today. Wow. Before the modification I had the air bleed screw all the way out, the idle was blubbery, it would stall if you idled it too slow for very long at all, and it'd really load up when you hit the throttle. Now it'll idle all day slow enough that the plane won't roll on pavement, it comes right up to full power right now, and just generally flies better. Granted, the mid range is still too rich, but I'm not complaining. The engine works much better now. So whoever posted the suggestion -- thanks. It works, and my plane is much happier now.  SignatureTim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html >A couple of months ago there was a thread on this group about adjusting >carbs with air bleed. Someone mentioned that their adjustments were all [quoted text clipped - 18 lines] > So whoever posted the suggestion -- thanks. It works, and my plane is > much happier now. Humm, I might have to try that OS LA 40 again instead of giving it away. My very first engine was an Enya .29, ran great and still does. The OS, not so good. mk > Posting from Google? See http://cfaj.freeshell.org/google/ > > "Applied Control Theory for Embedded Systems" came out in April. > See details at http://www.wescottdesign.com/actfes/actfes.html Around here we have a "Club 40" Pylon Race Circuit which requires a stock OS 40LA for power. We have over fifty guys registered and we have had 8 contests so far. That's a lot of heats, and not one problem with the LA. Cheers -- \_________Lyman Slack________/ \_______Flying Gators R/C___/ \_____AMA 6430 LM____ / \___Gainesville FL_____/ Visit my Web Site at www.LymanSlack.com > Humm, I might have to try that OS LA 40 again instead of giving it away. > My very first engine was an Enya .29, ran great and still does. The OS, > not so good. Maybe it's an altitude thing. When I lived at sea level I never had any trouble with my air bleed engines. When I moved to Kansas City, which is around 1000 feet if I'm not mistaken, I had to back all of the screws out. > Around here we have a "Club 40" Pylon Race Circuit which requires a stock OS > 40LA for power. We have over fifty guys registered and we have had 8 [quoted text clipped - 9 lines] > > My very first engine was an Enya .29, ran great and still does. The OS, > > not so good. > Maybe it's an altitude thing. When I lived at sea level I never had any > trouble with my air bleed engines. When I moved to Kansas City, which > is around 1000 feet if I'm not mistaken, I had to back all of the screws > out. 1000 feet shouldn't make that much difference. The barometric pressure drops about 1" Hg for every 1000 feet of altitude (in the lower levels), so you would see the pressure (and therefore the density) decrease from 29.92" to 28.92", a little more than 3%. The screw adjustments would have to be minor to get a 3% change in air bleed. Now, if you lived in Leadville, CA, at 9997 (IIRC) feet, you sure *would* see a difference. Dan >> Maybe it's an altitude thing. When I lived at sea level I never had any >> trouble with my air bleed engines. When I moved to Kansas City, which [quoted text clipped - 11 lines] > > Dan No, I wouldn't. I'd be dead! I was a naughty boy and smoked like a factory for 35 years before quitting. So did all of my model engines. You should hear them wheeze and cough at the field. <G> Ed Cregger >> Maybe it's an altitude thing. When I lived at sea level I never had any >> trouble with my air bleed engines. When I moved to Kansas City, which [quoted text clipped - 11 lines] > > Dan It shouldn't make much difference, but I have to agree with Robbie that it apparently does. The field I fly from is on the order of 500' above SL. Lots of newbies have showed with LA engines. Most could not get a lean enough idle mixture to run steadily below about 1500 rpm without drilling the air bleed hole out - just backing the bleed screw out to completely uncover the hole isn't enough. The mod has become pretty much SOP here. At any rate, it's a no-risk mod if you use a numbered drill bit one or two steps larger than what fits the stock bore of the bleed inlet. Too much air? Turn the screw in a bit more. Abel > It shouldn't make much difference, but I have to agree with Robbie > that it apparently does. The field I fly from is on the order of 500' [quoted text clipped - 8 lines] > > Abel Come to think of it, I might have had it exactly backwards. I haven't been flying much for the past two years, since I've gotten into the local music scene. I'll have to look at my engines and see which way the screw is set, in or out. All I remember is that I reversed the position of all of the screws when I moved here ten years ago and I haven't messed with them since. You wouldn't think there would be a difference but there is. It might even be the humidity. Where I moved from it's humid like the Amazon jungle, which makes the air less dense. >> It shouldn't make much difference, but I have to agree with Robbie >> that it apparently does. The field I fly from is on the order of 500' [quoted text clipped - 17 lines] >difference but there is. It might even be the humidity. Where I moved >from it's humid like the Amazon jungle, which makes the air less dense. Sure, it may have little to do with difference in pressure due to altitude and plenty to do with fuel head as a result of tank location, the fuel used, prop selection, ambient temp and humidity, etc., etc. Bottom line is that in some OS LA engines (and others, I've seen the same thing in a Saito 4-stroke w/air bleed carb), the range of adjustment of the idle mix falls short of what is needed to get a reliable idle. The fix is easy. Abel |
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