Common symptoms in a high-time servo subjected to lots of vibration. One of the small wires connected to the commutator typically breaks, giving the servo a dead spot. If the servo stops with one of the brushes on the dead commutator segment, it won't move when you ask it to. If you were to give it a shove to get past the dead commutator segment it will turn as it should. When turning, inertia will usually carry the motor past the dead spot as it turns.
as a servo get older it may work fine as it is receiving many Inputs, but once it is not receiving so many Inputs it can have a mind of it's own and wander,
One of the first successful loops I ever did was on an old Xcell 1001, when the elevator servo died with a broken motor wired. It made for a very exciting flight, but I did manage to get down in one piece. After I landed, I began looking at the swash while moving the transmitter right stick. I had NO elevator control, till I gave the swash a slight push.
Is this a recent development in a motor that has seen lots of time on it? Or is it a problem with a new motor?
If you wonder about your motor's innards, there's no time like the present to take it apart and inspect it. You can look at the piston and its skirt for damage, at the cylinder walls for damage, and if you were to put the ring by itself in the cylinder and square it up with the piston, you could measure the ring gap.
Other things to look at would be the fuel tubing inside your tank. If you're running muffler pressure, the fumes in the tank tend to take their toll on the fuel tubing, especially the inner tubing connected to the klunk. The fuel line may get real soft and collapse, it may lose its flexibility, it may develop pinholes, it can even take on the appearance of being made of small crystals about the size of salt or sugar crystals.
I re-read your post, and see that you did check the fuel line inside the tank. One more spot to check is to make sure your muffler is bolted securely to the crankcase, and not coming loose.
If you're using an in-line fuel filter, maybe it's got some goo in it. If you're NOT using an in-line fuel filter, it's time to start. I filter from the jug to the tank with my pump line, and still use an in-line filter.
Overheating inside a fuselage is a real problem you need to consider.
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