I have had this problem for quite some time with 3 different engines. Brillelli 50, CRRC 50, and 44cc stihl conversion. 2 different planes, one Yak and one seagull bling. 5 different carbs. One of them has the nipple soldered on the plate and a hose run into the fuse. They run fine on the ground, good transition, good idle, strong top end. When flying and cut it to idle, it will quit after a few seconds. if you try to get it to recover before it quits its like its lean, but richening the low end didnt help. I have tried tuning all of them as per two different threads found here on RCU. I'm sure i'm doing something wrong. I just cant figure it out. I have tried to make both needles richer and leaner. I posted this problem a few years ago and never did get a fix. Please you gurus like PE, Tired old man, Ralph, and Bob help me figure this one out. You guys are top of the food chain for gas engine knowledge. Any ideas would help

I would think that after
three engines,,
two planes,,,
multiple carbs,,
much engine messin',,

that I would look elsewhere...

Might start with setting proper end points for both high and low side..
Plus use the throttle cut if your radio is so equiped..

Sounds like a good idea. Since it's happening with all the engines it suggests a user induced condition. Start with the throttle end points and linkage lengths. No binding or over driving allowed. The verify the choke is fully open for every engine run after it has been started. Servo operated chokes could be poorly adjusted and causing a problem. Then go back to tuning basics, starting with both needles at the 1-1/2 turns open position to establish high and low end performance. If those don't fix it try routing that hose to the carb air inlet and see what happens.

check the carb line and tank...make sure the clunk can move around freely, and there is not a hole anywhere in the line

This seems a carb tuning AND throttle linkage setup problem.
Get someone well known with gas engines to help. I have seen a wire pull systems with rubber band return spring on the throttle work (nearly) flawlessly. Not my kind of setup, but just showing what can be made to work for you.
In my  planes I use ball links on both ends, a braided plastic covered steel wire (fishing leader line) in between with the end loops over the balls. If need be, this pull wire can be guided around curves in thin servo pushrod tubes. The throttle valve spring load takes out all backlash. Servo is a slowish high torque digital (10 Nm holding torque) metal gear. (10 usd at HK)

Agree!!
And ... while I don't advocate using it, one thing to prevent this problem could be to leave the throttle stop needle in place adjusted for the minimum idle speed. If you do that, you might consider a remote kill-switch though. Something like the 42% Opto-Kill or the Rcexl one or on a magneto engine, a micro-switch that grounds out the magneto

Idle set too low? We had a guy at the field who liked his idle set low. When he did some low speed tumbling and snaps at idle his plane would die. We suggested that he raise his idle speed. No problems since.

Glad you brought that up.

All too frequently I see people taching the idle rpm on their engines and adjusting everything downwards to fit some idle profile they came up with. The only idle rpm you need to recognize is the one where the plane does not roll of its own accord on the runway or taxiway. The numerical value means nothing. Rolling or not rolling at idle is everything.

some RC-Imac guys never have a very low idle. They kill the engine before they land. This is to keep from damaging the very expensive large carbon props.
Dead stick not necessarily means dead plane.

I appreciate all the suggestions. I dont think its throttle linkage. As is noted it works perfectly on the ground. These are not my only planes, and not my only gassers. I have tried adjusting the idle up, but then it will roll on the ground. I have to put the idle at least 2500 to maintain idle in the air. I have consulted with the guys at my club and they dont know either. I feel that it must be a mixture problem-yes user induced I''m just not sure which direction to go. I bought a carb rebuild kit so i think i will take the worst one and rebuild it. Just a note....I have been an automotive technician for 28 years. I can rebuild a Mercedes Benz V12 twin turbo with no problem, but these small carbs sometimes have me baffled.

idling at 2500 is too high....you must have a carb tuning issue ...... one of my gassers idles at 1600or so and on asphalt it still rolls.....i fly off grass so problem solved

Pump new fuel pump membranes in the carbs and go from there. Just been there my self. I had one engine that was fine last year and wasn't bad for the day's flying this year. After that, it would not stay in tune and I had a couple of deadsticks. Replaced the pump, readjusted the needles and it was cured.

You've probaly seen this page before, but here it is anyway. [link]http://tech.flygsw.org/walbro_tuneup.htm[/link]

Loose fitting spark plug caps ????

Tuning Walbro equipped two strokes is not at all like any car engine.
In the carb tune, a lean idle setting can also cause your problem, but so can a too rich setting. Find the lean setting where you have highest rpm at fast idle (do NOT adjust trim whilst finding that rpm). Then open the needle again until you see a slight rpm drop. Now adjust the high needle for best full throttle rpm, and open it up slightly, so the engine reaches best rpm after a short while.
From there, adjust the high needle as dictated by spark plug readings, but leave idle alone.
Remember that the idle needle supplies fuel ALLTHETIME, whilst the high needle only supplies extra fuel at more than half throttle (give or take a little)

Does it do the same with the cowl removed if it has one?

Does it do the same thing if you land with a mostly full fuel tank?

Have you tried a different type of carb. regulator venting system?

You might try a new carburetor as a last resort.

What prop size are you using?

Prop is xoar 22x8. cowl on or off makes no difference. fuel tank level makes no difference. I am going to try using a carb from one of my good running engines.

This may seem silly if you've been here already, but just wondering. On the bold, when you richened the low end, you also raised the idle trim to increase your idle speed, correct? What do you think might happen if you went even richer, along with another increase in throttle to compensate? That's always done it for me when I've run into what you're describing. FWIW

The other day I was tuning an engine (Zenoah btw), and I asked the guy to leave trims alone. Whilst adjusting, I noticed him adjusting trims!!! So scared he was of the engine quitting. This is something automatic and unconcious. It took me about 5 minutes to convince him I could not adjust the engine if he did not leave the trims where they were.
After primary adjustments we went out to fly the plane and add final adjustments to the High needle by judging the plug. The engine now runs reliable and idles all afternoon if needs be. (glider towing duties)