OS 91 SZ, Fuel wont go into the carb.

[montarok]

My Rap 90 SE had been sitting for a couple of weeks, I put some after-run oil in the engine after the last flight. When I tried to start it today, fuel won't go into the carb. I removed the fuel tubing that goes into the carb and turn the engine with the starter and it pumps out fuel, but when I try to blow air into the carb, I can't. What do you guys think is wrong and how can I fix it.

[ChrisLaFollette]

Does it have a cline regulator on it or the OS pump and regulator or are you running muffler pressure? Make sure all the needles are open so fuel can flow through the engine.

[montarok]

It has the OS pump and regulator and I'm not running muffler pressure. The high needle is 1 1/2 open and the mid is 2 clicks open.

[R-4-L]

and the mid is 2 clicks open.

you mean 2 clicks open from fully closed?

[ChrisLaFollette]

I dont know whether or not the OS regulator is like the cline, a fuel demand regulator. Meaning in order for it to open and flow fuel it requires a bunch of pressure, according to an old post by David Harkey the cline systempressurizes the tank to about 7psi so im assumbing it takes 7psi to open and let fuel flow. I do know that there is no way in heck you could stick a piece of fuel line in your mouth and the other end on the nipple and blow and make it open... I tried. :lol:

Im thinking maybe the pump isnt pumping enough fuel to make the regulator open and flow fuel, if it is a fuel demand regulator. Now it could flow fuel without pressure and just regulate the final amount going to the carburetor seens I bet the pump pumps quite a bit.

If it were mine I would take apart and clean the carburetor really well with fuel or rubbing alchohol and then stick it back together and then I would check to make sure the pump was doing its job... I think there is a short piece of fuel tubing between the pump and the crank case, not 100% sure about that but check and make sure that piece of tubing is fine and doesnt have any splits/holes in it. Personally I would replace it for peace of mind.

Let me know what you discover.


PS: To anybody who knows, how DOES the OS pump/regulator system operate? :dontknow

[WayneBrown]

I put some after-run oil in the engine
Unfortunately this may be the problem. Silicone regulator diaphrams do not like petroleum products. Your diaphragm may have dissolved...
Another issue I have had with my YS pumps is at the conclusion of the last flight, I shut it off from the tx instead of pinching the line off resulting in the fuel evaporating and leaving a slug of oil at the carb.
Oil being denser than fuel, and harder to burn creats a problem. I use a syringe filled with fresh fuel, a short piece of tubing attached to the syringe and inject some fuel into the nipple at the carb. Suck the fuel back out, and the passages should be clear. Fresh fuel will cause the oil to return to suspension.
FWIW: OS SZ motors are similar to the YS's, Pressure tap at the backplate, checkvalve between motor and tank, regulator to meter fuel flow.