I am using an old OS 120 4 stroke. My first time w/ 4 stroke and it is inverted, no pump. I made my own exhaust to fit within the cowl. Before I flew it I ran it up on the ground, the engine is used. It ran good but then I noticed the line going to the tank for pessure had melted off the nipple. I assume I put the nipple too close to the exhaust port { only 1 3/4 "}. The question is do I need to pressurize fuel tank?
Gene K

Gene,


your sure do... Try running true silicone fuel line from the pressure fitting to the tank. True silicone will not melt. It turns to ash. you shouldnt have any problem after that. Some of the lines out there are blended with certain polymers to make them cheaper to manufacture. Some may also be a natural latex. either case those melt. Silicone wont. Give it a try.

JB

"0"sink

4-stroke exhaust is hotter than 2-stroke exhaust... But if you burn the line off your muffler pressure tap, you've got something set up wrong.

The typical 4-stroke will have a 2 to 4 inch long headder pipe. You can't tap into that section, as you'll burn the silicone tube off every time. (you'll also be in a low pressure area... and could draw a vacuum on your fuel tank.)

You need to tap the muffler at a wide area (largest outer diameter is best.) The gasses traveling through the muffler "trade" kinetic energy for pressure energy as they go from the narrow headder into the main volume of the muffler. The widest portion of the muffler will have the highest pressure. (the exhaust will also have cooled a bit by the time it gets to the wider section)

The 4-stroke needs muffler pressure for the fuel system for the same reason as the 2-stroke. It helps give a more consistant fuel flow at all attitudes of the aircraft. You can run either without pressure... but the tank needs to bemounted higher in relation to the carb, and inverted (or nose-high) engine performance can suffer.

Maybe you could get away with using a pump to pressurize the tank. I know I had a Y.S. .91 and the muffler blew off and left only the header sticking out and it flew fine (loud but fine) b/c it had an onboard "supercharger" or pump on it.

My O.S. .91 is constantly throwing ts muffler off.. (partly because of the home made header experiments for the tuned pipe)

Because I know this is likely... I leave the engine tuned a bit extra rich, and it doesn't lean out far enough to be harmful. No pump. If I blow the header AND am doing a high AOA maneuver... the engine dies. the muffler pressure is more for ensuring good fuel flow at all attitudes than it is for level flight ops.

You can run ANY model engine without muffler pressure.... It just may not do well at anything other than puttering around the trafic pattern.