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Bad Fuel Pump on a OS 91 FS four-stroke

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Old 07-20-2014, 04:40 PM
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GRandolph
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Default Bad Fuel Pump on a OS 91 FS four-stroke

I have a pumped OS 91 FS II and for the second time the fuel pump has stopped working. The common denominator for both pump failures was that the engine sat for a year or two without being run. My question is this - do other people experience a high failure rate for these pumps or is it just me? Why don't these diaphrams in these pumps last longer? I use after run oil during long storage (transmission fluid) but I don't put it in the fuel pump.
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Old 07-21-2014, 12:02 AM
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I have the 120 with a similar pump.

The pump was faulty when I received the engine new, would not pump enough fuel.
On advice I flushed clean fuel through it with a squeeze bottle & some blue jelly like goop came out.
This made it run ok but still not perfect. The carb & pump were both replaced under warranty.

Like you I use after run oil but don't put any in the pump. I have had no problems since the pump was
replaced. What oil are you using? I'm using Coolpower at 18%. - John.
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Old 07-21-2014, 07:46 AM
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Most engines, pumped or not, will need some servicing after sitting for a year. If the fuel you use has any natural castor in it, it will harden over time. When engines are run regularly, castor varnish is flushed out before it has a chance to get really sticky. I haven't had a pumped engine before, but if you are able to open the pump to service it, cleaning out all of the fuel from inside and storing it dry for long term storage would probably help. You'd have to rewet everything later to get it to work right I'm sure, but I think you'd be able to avoid having to replace any parts that way.
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Old 07-21-2014, 09:51 AM
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The OS fuel pump is one of the reasons I went to YS engines. Yes, if they sit too long then the pump will clog so it's normal. Pulling the pump and back flushing got it going again the first time it happened to one of mine, I flushed with straight alcohol.
Reason I went to YS is they sell everything you ever need to do a complete rebuild including diaphragms. Most YS users always have extras on hand because it's about the only part that can get a clog or wear out, it can be replaced in a few minutes and it only costs a couple bucks.
OS had a better idea, they made there pumps one piece so when you have a problem the complete pump needs replacing, about $60. or $70 bucks. If you can get your hands on the material that the diaphragm is made of they can be rebuilt. A friend from RCU once rebuilt one for me.
If you can't flush it out then a new pump is in order. The diaphragm in the regulator can also go bad. OS required me to send in the complete regulator assembly to install a new diaphragm, those two screws and spring must be way too complicated for the run of the mill user to replace. With shipping that cost me almost $50.00. I still have a couple of OS .91 but no longer have any pumped OS engines.
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Old 07-21-2014, 11:57 AM
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I will take the pump apart and see what I can find. I haven't tried flushing it yet - maybe that will help. Unfortunately, it would not suck up a single drop of fuel. The previous time it failed, it would still pump but would give an unreliable mixture causing the RPM's to change. I 've learned my lesson with this pump - never by an OS pumped engine unless you fly the snot out of it. I don't have much time to fly, so this is not the engine for me.
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Old 07-21-2014, 01:39 PM
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I don't mean to bad mouth OS but they could have changed the design of the pump years ago, they did it once before when they had the old metal type of pump. I have one of those old engines under the bench too that you can't get pump parts for. They could make it more user friendly and sell the needed parts so we could change the diaphragms ourselves instead of having to buy a complete replacement or having to send in main parts just to replace a 50 cent part.
As long as I kept using my OS pumpers I had no problems. Now that I know how to take there plastic pumps apart and get them back together again I'm sure I wouldn't have had the plug up problems. The engines aren't cheap and having to spend that kind of cash to do a simple repair is beyond my thinking.
I haven't bought a new OS product in years but I really like pumpers so the YS has worked out well for me. I just clean out the regulator and leave it loose when I store a YS, when it is going to be used again I just tighten two screws and fire it up.
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