55 ax: Bubbles? Help Diagnose, Please!
Engine is running in Goldberg Sr. Falcon ARF. I'm a newbie, and have only taken the controls for a few minutes once it's airborne, otherwise my experienced buddy is flying it (and loving it!). I bought the engine used from an experienced flyer who broke it in on a bench & then flew only 2-3 times on a plane; it was pristine when I got it, but not brand-new.
Starts great, runs great on ground, and sounds great in air....for about 3-5 min.
Then you start to hear a bit of sputtering, brief dropouts to the point where you don't want to push it any more & probby want to bring it in. Haven't lost power fully in flight, but of course, that's the big worry. This same 'behavior' has occured several times on several flights, not just once.
My buddy was thinking it might be air bubbles in the tank; it flies fine until the cumulative vibrations start making bubbles, in theory. We're thinking that it might be because of the way I have the tank set up without any real padding It's tight in the space behind the firewall, so my 11oz tank sits directly atop the Rx battery (which is velcroed to the fuse), it's held firmly on the sides by support/strengthening 'beams' on the fuse, and then I have maybe 1/8" or less of padding on the lid of the fuel-hatch cover. In a nutshell, the tank is defacto 'connected' to the fuselage vs. having padding on sides/bottom/top.
I could see how that could cause bubbles but I want to ask if
[ul][*] it might be something else, like a carb-gasket or carb-o-ring that needs replacing[*] could a dying plug cause this, or does the fact it happens only after a few minutes of flying negate that theory[*] would the OS 'bubbleless clunk' help out? Is there a better bubbleless klunk to be had?
I might go to a smaller tank, but I'd rather not, as the one in there now is the recommended size for this engine to get an 8 min flight or so (but I'm running 1/2 throttle most of the time...)
What do you think; should I assume it's bubbles in the tank from excessive vibration, or should I look to other causes/solutions? If so...what?