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Problem os 50 sx

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harris08/04/2012 23:34:26
29 forum posts


I'm getting problems with my engine where i keep getting air bubbles running through the fuel line. I tried using a new fuel tank but this has not fixed the problem. I think it maybe the O ring on the needle valve possibly. Does anybody agree this maybe the problem or or possibly something else?


Stephen Grigg09/04/2012 06:44:57
8691 forum posts
1128 photos

replace all the fuel piping including the clunk first.

Stephen Grigg09/04/2012 06:45:36
8691 forum posts
1128 photos

and is the fuel tank bung nice and tight

Dave Bran09/04/2012 06:58:52
1898 forum posts
5 photos

If you have air bubbles in the fuel line thats before the needle unless its a remote needle engine (my SX's were all needle on inlet).

What is this on??

Split Line

Poor Joint

Vibration (more a Heli thing)

Steve Hargreaves - Moderator09/04/2012 07:42:16
6742 forum posts
191 photos

Is it a remote needle? Where are the bubbles in the feed to the needle r between the needle & the carb? As the others have said a pin hole in the fuel line is a possible....if its between the carb then the 'O' ring might be to blame.....try slipping a short length of fuel tuve over the needle tube before screwing the needle itself into the tube....the idea is that this short length of tube seals the joint between the threads. A spot of vaseline on the needle tube thread can achieve a similar result...

Others have suggested dropping the needle into some petrol for a time...the idea is the petrol swells the 'O' ring slightly.......not one I've tried myself however....

harris09/04/2012 10:40:18
29 forum posts

Thanks for the replys. The needle valve is a remote needle valve. The engine has not been used for a few years and i gave it an overhaul including a new piston ring and back bearing. I can get the engine to run but i can't seem to get it running sweet when i adjust the needle valve and the mixture control in the carb.

I tried a new tank and checked under water to see if there are any leaks, new pipes too and also the fuel pipe on the needle valve to seal it. I tend to get the air bubbles when throttling up and down when it settles and the bubbles dissappear. Could air bubbles cause a massive problem as the engines does not seem to run t full chat ?


Alan Cantwell09/04/2012 10:58:20
3039 forum posts

after a run, check the tank, is it full of froth? if it is, its a vibration issue, check the prop and spinner balance, a drop or 2 of Amoral in the fuel will stop the frothing, but you need to find the soarce

Josip Vrandecic -Mes09/04/2012 11:29:45
2993 forum posts
260 photos

Hello Harris,I had the same problem few years ago,when I forgot to remove needle valve,from my os fx 46,during the winter(over storage).....I have replaced the "O" ring and the problem was resolved...and as Steve says, control fuel line beetwen needle valve and carb.

All the best


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