I have a os155 pumped motor in my large scale space walker. when i do some maneuvers where i roll the plane inverted the engine blows smoke and falters . If I do same maneuver later in flight when tank is about 1/3 full . the engine runs good ? i thought with a pumped engine there would not be a problem like this ! The tank is in normal position behind bulkhead and the center sits just above carby {the motor is inverted} Any clues Help would certainly be appreciated Thank You

Most pump/regulator devices will lose regulation if exhaust pressure is applied to the fuel tank vent. Disconnect muffler pressure, if it is being utilized, and see what happens.


Ed Cregger

When I look at the instructions for the FS 155a engine, on page 19, they depict the fuel tank vent line as being open, not connected to the muffler.
http://www.osengines.com/manuals/index.html
So that may be your problem.
You also might want to review the OS instructions on adjusting the carb etc. too.

Thank You for your replies .No the there is no pressure line to the tank from muffler {there is no nipple on muffler anyway} The lines from tank are set up as per the book.I have made adjustments to the idel needle to improve throttle responce. There must be something in the fact that she runs well once volumeof fuel in tank is less than half?
Berge

For whatever reason, your engine's fuel system is not regulating the fuel flow if it is displaying the symptoms that you have described. Something is amiss. A return to the service center might be in order.


Ed Cregger

If it is running OK inverted with the fuel tank level low, it may be the vent system is not working as expected. If you flip the plane over inverted with a full tank, does fuel flow out of the vent? I normally run a loop in the fuel line so it goes up and back down under the plane for this. That might prevent a slight vacuum effect if the fuel is flowing out the vent line, which would affect the fuel pump.  Of course the simple thing may be the the tank or tubing is moving and getting pinched on you when you go inverted too. There can be a lot of Gee forces involved.

The other thought is to use dual vent lines like they do for control line planes. The vent on top has a tube exit out the bottom and the vent on the bottom has a tube exiting out the top. Thus if the plane is inverted the bottom vent is now the top vent, and vice versa.

You could use a "Uniflow" fuel tank setup too. This keeps the fuel flow more constant as the tank empties.

I had forgotten which engine the OS 1.55 actually was.

If the carburetor is a diaphragm-type, be sure to run a static line from the diaphragm vent port so that it opens to the atmosphere inside of the fuselage. Just the immediate pressure change that occurs around the engine when changing flying attitudes (such as upright to inverted flight) can negatively affect the fuel regulation of a diaphragm type of regulating system.


Ed Cregger

Thanks for all your suggestions, Problem rectified by leaning out bottom end. Thanks again Berge