OS FS-52 Throbbing
 

Several members of our club have experienced throbbing, i.e. engine speed increasing and decreasing, when flying. The usual causes such as foaming or air leakage through the fuel lines were checked out with no trouble found. The trouble was duplicated with movement of the needle valve. It seems as if air was leaking through the threads of the needle and the needle holder. The stock needle valve was replaced with one that is slightly longer from an OS-61 2 stroke carburetor that was handy. The results thus far are very good, no more throbbing.

Does this sound right to you? Comments appreciated.

Thanks,

DaveB

RE: OS FS-52 Throbbing
 

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RE: OS FS-52 Throbbing
 

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RE: OS FS-52 Throbbing
 

Wiggling needles in O.S. engines have not been a typical difficulty. Usually, we've found it's because an extension on the needle is being moved around by the engine's vibration. This is especially so if some kind of knob is on the end of the extension.

A common 'cure' is to put a piece of fuel tubing over the needle and then reinstall the needle. This puts some tension on the needle and the wiggling goes away. On some older engines, the piece of tubing was installed by the factory.

A wiggling needle can deform the needle seat in the carburetor, so changing to a needle with a slightly-different taper can 'solve' the problem for a time. If it persists long enough, the needle seat can become deformed enough that the carburetor body will have to be replaced. A polished ring round the pointy part of the needle indicates that it's either been over-tightened when it's been closed, or it's been wiggling around and striking against the needle seat.

RE: OS FS-52 Throbbing
 

Interesting I have been experiencing this same problem with my OS 52 4/C, will O.S. repair under their warrenty?

RE: OS FS-52 Throbbing
 

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RE: OS FS-52 Throbbing
 

The O.S. Max warranty covers defects in parts and workmanship of the original manufacture of the engine for a period of two years from the date of original purchase to the original owner. If you believe you have a difficulty that would come under the warranty, please send the engine to us. Please see the "sticky" thread at the top of the O.S. Max forum for details, or see the web page:

http://www.hobbyservices.com/