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Engine basics => Engine set up tips => Topic started by: Matt Colan on September 05, 2011, 12:15:03 PM
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My grandfather has a P-51 that he built it about 15 years ago. Last year he decided to finish it and we started flying it. The plane has a Brodak 40 in it with a not so good motor run.
When the engine is running on the ground, and it gets up to operating temperature, it will start to detonate, surge, go slobbering rich, and almost shuts off. We have tried a different Brodak 40, and got similar results. We've changed props, spinners, fuel lines, fuel tanks and it still does it. We beefed up the nose of the Mustang thinking that it may not have been built correctly all those years ago, and it hasn't helped the motor run.
When we started flying it last year, there wasn't any issue with the motor run and the plane flew right off the board. When we were running it in a 2 cycle last year, it was fine. But now, even at a 2 cycle, the motor will still surge but not as dramatic as when it goes into a 4 cycle.
Does anybody have any ideas what might be causing this issue with the motor run?
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Did you make any revisions in the cooling system?
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Matt,
its hard as you know to diagnose these kind of things, but judging from the fact that you have replaced all the significant fuel system parts, and the engine, I would suspect that perhaps there is a structural reason or a prop balance issue. Did you try a different spinner? Fuel? plug heat range?
aside from that I would look at the area next to the wing, and the motor mounts to see if perhaps there is something loosing its grip on the wood structure.
It could be a heating issue I suppose, did you fly it in the same weather conditions last year? if so then I kind of doubt its heat related unless you are using different fuel.
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Matt,
its hard as you know to diagnose these kind of things, but judging from the fact that you have replaced all the significant fuel system parts, and the engine, I would suspect that perhaps there is a structural reason or a prop balance issue. Did you try a different spinner? Fuel? plug heat range?
aside from that I would look at the area next to the wing, and the motor mounts to see if perhaps there is something loosing its grip on the wood structure.
It could be a heating issue I suppose, did you fly it in the same weather conditions last year? if so then I kind of doubt its heat related unless you are using different fuel.
We flew it last year around late August until we couldn't fly anymore. We changed props that we knew were balanced (just checked them on the balancer). We haven't changed the fuel from what we were using last year. We changed the spinner because the spinner that was on it, wasn't balanced. We changed plugs multiple times; ones with idle bars, without idle bars, different lengths, and it still does it.
I keep thinking it's some type of structure issue because this plane when it's running is vibrating a lot!
We ran it today in some drizzling rain and 65 degree temperatures with a different spinner on it, and 3 different props, and it still did the same thing.
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HI Matt,
Not sure what to say except a "vibrating a lot" nose usually leads to lean runs due to fuel foaming.
You say it goes slobbering rich........ not an indication of a flimsy nose in my experience.
Best of luck sorting it out.
Big Bear
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You may have a resonance issue where the frequency of the engine excites the natural frequency of the airplane. You did not say what prop size you are running, but you might try a very flat pitch prop and run the engine very fast. Something like an APC 10X4 and launch at around 11000 rpm. This is probably not ideal for stunt, but if you have a low speed resonance this will tell you and help understand the problem. Besides, it is easy and cheap to do.
Steve
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Matt, you can bench test for fuel foaming with a separate tank, off the airplane. Helper #1 holds the airplane, helper #2 holds the tank and you start and needle the engine. If this stabilizes the run, then start by adding two drops of armor all to a gallon of fuel. If that doesn't resolve matters, than you have to disassemble the front end and inspect carefully for cracks, loose fasteners or bad joints/broken structural parts. Flex and twist the fuse to try and get the cracks to open up. Use good light and take your time. If you can find the source of the vibration, dry it with K2R or talc and then epoxy/fiberglass it back together. Unless something else came loose, I don't think resonance in the controls is likely the culprit, because the airplane worked right when it was first put in service. You might have to look it over any number of times to find the problem. Best, Tom
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When you flew it last year with good engine runs was it with a B' .40? The same or differant B' .40(?), or a differant engine(?). I'm looking for the Brodaks setting up a harmonic resonance...although that should cause lean runs...but
W.