OS 40 engine misfires and fuel bubbles from carb

by Brian
(Cincinnati, Ohio, USA)

I'm relatively new to flying and have a question.

While flying on a hot day (90 deg) my trainer with an OS 40 engine would misfire every minute or so.

Upon landing, fuel would be bubbling from the carb.

Can somebody tell me what's going on before I ruin the engine?

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OS 40 engine misfires and fuel bubbles from carb

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Jun 30, 2009
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Duarte nailed it
by: brifro

Well, Duarte nailed it. I damaged the airplane last weekend and had to disassemble it during a fun fly. A guy at our club noticed a couple of problems with my fuel tank: one was that the the exhaust tube went straight into the tank, not at an upward angle. The clunk was also not able to move as freely as it should. Great forum. Thanks.

Jun 29, 2009
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OS 40 engine misfires and fuel bubbles from carb
by: Duarte

Brian,

Just guessing, since you say you are new to this.
In the tank you have a pressure outlet, that goes to the muffler.

Check if, inside the tank, the metal (or plastic) tubing its bent up. If so (and it should be), check if it goes all the way to the top of the tank.

If not, add some fuel tubbing in order to achieve that, when the engine is running, you do not have fuel being pumped into the muffler.

Remmember that the pressure from the muffler is also blown back into the cylinder, via the exaust port, and if you have fuel there it will accomulate in the engine and flood it.

Hope I've helped. Sorry for the not so good english but I'm Portuguese.

Happy flying.

Jun 21, 2009
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Good deal
by: Matt

I'm glad to hear that you got'er lined out.

Jun 21, 2009
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thanks for the replies
by: Brian

Oh no. The center of the tank should be level with the carb or higher? The center of my tank is probably 1.5 to 2 inches lower than the carb. It ran OK today when a club member adjusted it. They thought it might have been running too hot that day, in part due to the high heat. I was out there alone that day with no help from the experienced guys.

I put the plane together correctly and the engine, I was told, was a good match with the plane (an Aero-Star 40). To clarify for the other commentator, it was sizzling, I guess. It wasn't "filled" with fuel, I just noticed bubbling out of the carb. The problem was apparently temporary. Thanks for the input.

Brian

Jun 21, 2009
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Fuel problem
by: Bill

Brian:

What is the relationship of the fuel tank to the engine? In other words the center of the tank should be in line with carb or slightly higher but no more than that. If the tank is too high it's possible that gravity is flooding your engine. As to the misfiring it could be related to the flooding of the carb and/or your plug might be too cold. Your information with the engine will indicate the proper plug but my guess is that the flooding of the carb is the culprit.

Jun 21, 2009
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bubble or sizzling?
by: Matt

Hi Brian,

We're practically neighbors. I live in Carrollton Ky.

I'm not quite clear what you mean when you say fuel is bubbling from the carb. Do you mean it so hot it is sizzling? Or is the carb completely full of fuel and bubbling out?

There really should be that much fuel just sitting in the carb. Are you sure it's not running too rich? Have you tried the pinch test to make sure the engine isn't too rich?