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OS 120 Surpass II Pump spitting fuel

Old 07-11-2007, 11:47 PM
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Default OS 120 Surpass II Pump spitting fuel

I have an OS 120 Surpass II with pump that was NIB when I started running it towards the end of last season. Although last year, it was running well, I've been having problems this year getting consistant runs. While doing a ground run-up the other day, I had the cowl off and noticed fuel spitting out the hole in the pump screw and think this may be related to the problem I'm having. I have not touched anything having to do with adjusting the pump, and have not disassembled it. What would cause the fuel to spit out the hole under all throttle settings?

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Old 07-12-2007, 09:32 AM
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Default RE: OS 120 Surpass II Pump spitting fuel

When fuel comes out of the hole in the center of the brass screw of the original O.S. FS-120 Surpass pump unit, it means that the regulator diaphragm is leaking. That hole is the side of the regulator that's supposed to be exposed to atmospheric pressure. The other side is the fuel side. When it starts to leak, the diaphragm's shot. Unfortunately, there is no repair. O.S. would just replace the entire pump unit. However, they have not been available from us or O.S., in Japan, for a number of years. They've been out of production for too long. You may be able to locate a new pump unit through one of the online swap-shops, the RCU Marketplace, or through one of the online auction/selling sites. Also check the ads in the back of the model magazines, including the classifieds. Many dealers who sell to engine collectors advertise there.
Old 07-12-2007, 11:14 AM
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Default RE: OS 120 Surpass II Pump spitting fuel

Ok, Bax, thanks. Kind of what I expected. When I first got the engine a few years ago, I was wary of pumps and long term parts availability so at that time, I purchased the parts to convert it to a regular, non-pumped OS (crank, backplate, carb, intake manifold) so that's where I'm at now. At least now since the pump is shot, I can take it apart with a clear conscience and see what it's like inside.

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