Pumped carb vs. muffler pressure

Model Airplane News - RC Airplane News | Pumped carb vs. muffler pressure

Hi :  I read your “engine clinic ” articles all the time and enjoy them.  My question is this.   I was given an O.S. .70  4 stroke “Ultimate:” as a gift.  I would never have bought a pumped engine but I’m stuck with this one.  I’ve been running 4 strokes for the past 20 years a love em’ but have no experience with a pumper.  I have a new model almost ready to go &  all the necessary “plumbing” installed to give this “Pumper” what it requires including the “T” necessary to fill
the tank.   My question to you is this:  Can I ignore all those nipples on the backplate (4)  and simply plumb the fuel tank directly into the carb and direct muffler back pressure into the tank as I do with my Saito .80 and O.S. 1.20.??   In other words,  can I run this “jewel” as though it were not a pumped engine..??  Will appreciate your thoughts on this and keep those great articles coming.   –Ed

Yes, you can bypass the pump and use muffler pressure. However, the pumped engine usually have a larger carburetor throat. If you experience any leaning problems during maneuvers, then it would be best to connect up the pump.

Clarence Lee

Updated: July 16, 2015 — 10:26 AM


  1. Hi Clarence,
    With regards to the os pumper engines, I have the original series OS 120 surpass with pump and c12 carb. this engine was recently involved in a dispute with the ground about 40 yards from the runway…and well, the plane lost the argument. But the engine survived, kind of. The needle broke off along with the threaded portion where it goes into the carb body, Believe it or not, this engine, pump included, ran like a dream with loads of power. I was wondering if you can point me in the right direction for a replacement carb that works with the 120 pumper? Or maybe a fix? I absolutely love the way this ran and this is the only issue after the crash, so any ideas would be much appreciated!

    1. I’ve forwarded your question to Clarence; please check later this week for his reply in a new post!

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