bob27s
Posts: 4819
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
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Hi,
Thanks for writing.
There is likely nothing truely wrong with the engine.
What you described is not uncommon.... we get it every so often from the whiplash and dust guys....and you sorta have it figured out. You are getting bubbles in the fuel.
More accurately, the fuel is foaming. Tell me what aircraft, what type of fuel tank you have, and how it is installed.
Our first rule here, is to put the engine in a test stand. Put a tach prop on it (same prop, clip 1/4" off of each tip, balance it... dont fly with it). Run the engine. If it runs, behaves, needles well, and gets up to RPM properly, there is nothing wrong with the engine. Although you have it broken in, and have run it in a stand before, please do this (it will be the first thing Dub will ask if you send the engine for service). Letting the engine run three or four times for about 3 minutes each will help recover your seal. Start running it rich, slowly approach peak rpm. When you find it, back off 1/2 a turn. Let it run there.
Of note, the Jett remote needle valve should not let air into the system. The remote needle has no rubber seals. Even with the packing nut loose, to the point that the needle will turn by itself due to vibration, the needle has sufficient internal threads that it will not leak. The nut is primarily to adjust tension on the needle.
Following the test run....
Let me assume for a moment that you checked and replaced all of the fuel lines, and the clunk line inside of the fuel tank, and there are no kinks or restrictions.
You said you didn't see bubbles in the intake. That was on the ground. You are probably correct there... or the bubbles were tiny where you might not easily see them at that point. What is likely happening, is that after you launch the plane, the engine is unloading (it hits a higher RPM than you will see on the ground), and that is causing an airframe vibration which is creating fuel foam.
There are two ways to prevent fuel foaming
1) ensure that the tank is wrapped in foam, and ensure that no part of the tank is touching the airframe.
2) use a bubble-free fuel tank. One of our bubble-jett tanks, or a Tetra tank, or which ever fits best in your application.
Fuel foaming caused by high RPM and high vibration frequency is something that is unique to high performance engines.... most sport pilots will never experience it. It can drive the average modeler nuts, but it is just part of the hobby that is 'learned' by experience.
What you described was one of our biggest problems with Q-500 and QM40 racing. I've had my share of zeros and blown plugs over the years, because the fuel tank somehow slipped out of the foam and touched the fuselage side or firewall. Engine needled great on the ground. As soon as the plane made it to turn 1 and the engine unloaded, the engine leaned out. Required immediate shut down, or the engine would be ruined. To solve this, EVERYONE in 428 and 422 racing uses bubble-free tanks. No foam problems, because there is no air that can mix with the fuel  It also provided a consistant needle setting to the very last drop.
That inconsistant needle is another way to toast a good engine. With a regular tank, when you are down to about 1/3 fuel, many engines begin to lean out. It has a tiny bit to do with where the fuel level is in relation to the carb.......but it has a LOT to do with the clunk bouncing around out of the fuel, and if there was any small amount of foam in the fuel.....that is what the engine tries to run on.
Let me know how you make out.
Bob Brassell
bob27s@hotmail.com
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