Hi All,

I have a Honda GX31 31cc four stroke ingnition engine that I'm having problems with and hope someone out there might have a clue what's going on. The GX31 pressurizes the tank through the carb and was doing so on the first flight of the plane. On the second flight the engine quit and I ended up doing an off field landing. Brought the plane home and did some minor repairs to the wing. The next time that I went to the field I couldn't get full RPM from the engine so I brought it home again and check all of the plumbing on the tank as well as cleaned the carb.

I'm now getting 5700 rpm with an 18x10 prop but the fuel system is drawing a vacuum, not pressurizing. Checked the carb again and everything seems in order. Does anyone know what I'm doing wrong?

Any help would be greatly appreciated.

Happy flying,

Jim

Jim,

Are you sure the engine is supposed to pressurize the tank? All of the gas engine carburetors I am aware of pump the fuel from the tank, and the tank is vented to the atmosphere.

Hi Pauly,

Yes, the tank is supposed to be pressurized. The tank that comes with the engine has two lines on it: one for fuel and the other for pressure. When I originally ran the engine the fuel would spit out of the line after a run when I took the filler cap off(just like a YS engine). Now I can see that it has a vacuum on it because it draws the fuel back into the tank when I unplug the cap. I spoke with the engine supplier and he also said it is supposed to pressurize the tank. Tomorrow I'm going to fuel up the plane, tie it down and let it run until it stops to see if it can get through a tank without stopping due to fuel starvation.

Happy flying,

Jim

You sure the other line is for pressure? - maybe it's just a vent, or return for excess fuel from the carb pump.

I've never heard of a pressurised petrol ( gas) tank. - what does the "pressure" line connect to?

Is it not more likely that the vacuum in the tank is the result of the fuel being drawn from an unvented tank?

I can't see how it is pressurised through the carb, perhaps I'm a bit dumb, so If anyone wants to correct me - that's ok!

Most carbs I have worked on with the primer bulb and return line,are set up so when you prime the carb, it fills up the fuel cavities in the carb, then the excess is sent back to the tank via the return line. Many fuel caps have a one way rubber valve called a duckbill valve that lets air into the tank, but not back out again, so you can tilt it on it's side and the fuel won't run out of the tank vent.

Jim, sorry but I think someone told you wrong. The Original tank did and does indeed have two lines, as flypaper stated, one is the fuel feed line and the other is the fuel/vapor return line mandated by the EPA on small gas engines. ( to help keep vapor and fuel out of the air and off the ground) If you check the original tank you will note the return line went into it at or very near the top of the tank to allow unrestricted flow back into the tank. AND, as you stated, the return line has a vacuum on it after the engine starts, that is to draw fumes from the tank into the carb. More EPA S---.

Just try, this, disconnect the pressure line and use it as a vent line for the tank. Run it on the ground, pick it up, turn it up side down and any way you can, see what happems.

AND --- what kind of tank do you have in an airplane that has a "cap" on it?

Good luck.

Hi Guys,
Which props do use on this engine. I wanted just to try my new Honda GX31 in the air and put it into old Extra 300 (15lbs with this engine). Was turning MAS 16X8 Classic prop around 6300 revs.
Model barely could fly horizontally, although seemed to keep revs in the air. Was extremely nose heavy and abviously stalled in the first turn and crashed.
Thanks for any advice.