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50SX & Hatori pipe

Old 10-03-2006, 08:38 AM
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shinbones
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Default 50SX & Hatori pipe

Hi,
I've just fitted an old (.45 size) Hatori tuned silencer to an OS50SX, and before I fly it, am wondering about pipe/header length. I have no access to a tacho, but the engine runs sweet... with fairly even throttle response, smooth idle & healthy top-end. The only change I've noticed since fitting the pipe (other than more grunt & less noise) is that adjusting the main needle has little or no effect on rpm between about 2 & 3 turns open - and this concerns me. Any more than 3 turns open and it dies suddenly... less than 2 and it starts to lose RPM, but in between 2 & 3 turns I can hear no difference when it's on the ground.

The engine is only about a gallon of fuel old, spinning an APC 11x7, on 15% nitro & 20% Coolpower. The header is about 220mm long from exhaust port to the pipe (including a 65mm-long spacer). I've tried shorter spacers, and no spacer at all but coudn't get it to run as sweet as with the 65mm spacer. (In fact, with no spacer - i.e. a header length of 155mm - I couldn't even get the engine to run for more than a few seconds!)

I've heard that too short a header will make an engine very sensitive to needle settings, so I'm wondering if the opposite might be true? That is, does my engine's relative insensitivity to needle setting suggest that my header is too long? Or should I look for another problem? Everything else setup-wise seems fine, and with the stock OS muffler on, the engine was much more responsive to needle adjustments. Any advice or wisdom welcome.
Thanks,
Scott

PS- the pipe is a Hatori #40T (I think) - it's about 15 years old!
Old 10-03-2006, 09:27 AM
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Bax
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Default RE: 50SX & Hatori pipe

Proper pipe tuning requires a tachometer. You run the engine, lean it to peak RPM and note it. Shorten the pipe assembly at the header by 1/4". Restart the engine and lean it to peak and tach it. As long as the RPM comes up, you keep shortening the assembly. When the RPM doesn't change, you leave it there. Then you fly it and listen carefully to how the engine's running. You'll have to make some final adjustments based upon how the engine runs in the air. Please realize that the pipe will only be good for a fairly-narrow RPM range when the prop can unload...or for whatever RPM range you tune it for.

Lower RPM requires a longer pipe assembly, higher RPM takes a shorter assemmbly.

To get a proper setting, you must stop the engine between adjustments of the pipe length. When you re-start it, you have to let it run long enough to let the pipe assembly warm up. Pipe temp affects the tuning, and no, we don't have a temp number to give you.

A too-short pipe assembly will cause the engine to trend lean. Too long and it will trend rich. Usually, the high-speed needle will become more sensitive when you're using a pipe and it's close. If the needle has little effect, then either the pipe's too long or the needle seat in the carb's damaged.

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