took plane to field, checked plane on stand, throttle servo worked great, very smooth servo function. fueled plane and started it, and the engine surges wide open, low, mid range, wide, and the throttle arm actually moves back and forth like jello sometimes. with throttle at idle it may suddenly go wide open or vice-versa. range checked radio and it's ok, is the throttle servo bad, receiver bad? all other controls work fine with no jumping. any suggestions? thanks

Any metal on metal contact around the push rod?

Steven

don't think there is any metal to metal, unless you count connectors from split ailerons, got a long lead on there and it's kinda cramed into the fuse. linkage does move freely though

Was the range check with the engine running ?

I was more think about like a metal clevis on a metal carb arm

Steven

Need more info to answer your query. Is this a glow engine or an ignition (spark plug) engine? If glow, does it have an onboard battery for the glow driver? Any other info on equipment brand, type, model, size of plane etc.

the engine is my reliable tt .46. no on board glow. and no metal to metal contact. i'm thinking servo because the other controls, a, e, r work fine with no stuttering. guess i need to pull it and replace the throttle servo and see what happens. seems like receiver is fine, too, as i did range check it and again all other servos work fine.

I think your diagnosis is right on, most probably a bad servo that acts up under vibration.

when you say it range checked good what exactly do you mean? was the engine running and did the throttle servo respond correctly?
when the throttle started surging was the transmitter close to the receiver? could be you were having some over drive issues..this is where with the transmitter close to the receiver your actually over driving another channel and it appears on the throttle channel...the fix is to walk away a few feet....

well, the throttle clevis is plastic, so i guess that's out. when i range checked it, i walked away about 50' and could not see the throttle, just a,e,r working fine. it works fine when engine not running, so i'm guessing bad servo under vibration. going to royals white sox game tonight so will be later this week before i can check it. i'll try to post solution when i find one to thank all for their comments and advice.