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HomeAircraftHelicopterEngines Plugs Mufflers Fuel › OS 91 SZ H non Pumped
07-13-2008 09:56 PM  9 years agoPost 1


Covina, CA

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My setup is a Hatori SB18FH
running Cool power 30%

Anyone know what the mid and high needle valve settings should be on a 91 sz h non-pumped to achieve optimal performance both in hot and cold weather?

07-14-2008 03:46 AM  9 years agoPost 2


South Jordan, UT - United States of America

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You realize there's no such thing as a universal needle valve setting, right? The needle valves need to be tuned according to the following, at a minimum:
* Altitude
* Temperature
* Humidity
* Brand of Fuel
* The exhaust system (muffler, pipe, etc)
* Diameter of fuel tubing
* Length of fuel tubing
* Clunk used
* Whether or not you use a header tank
* Resistence caused by fuel filters
* Amount of muffler pressure
* Age of engine (ie. new engines need to be richer until they're broken in)

Unless you live in a very mild climate, you will have to change the engine's tune as the season changes from summer to winter.

What works for me will probably be too lean for you. What works for one specific engine won't be the same as an identical engine at the same field, burning the same fuel.

The best advice I've seen for those that aren't familiar with tuning an engine is to simply keep track of the head's temperature.

You want to start at the manufacturer's recommended settings - IIRC, that's 2 1/2 turns on the high speed needle, and 1 1/2 turns on the midrange (but CHECK THE MANUAL). From there, look at the head temperature. If it's below 100C (boiling point of water), you can lean it out. If it's above 100C, you need to richen it a bit.

But that's just a guideline, and is NOT very accurate way to tune the engine; it relies only on the fact that as the mixture becomes lean, the temperature of the engine rises rapidly. But if you run at a higher RPM, then the engine heat is going to be producing more heat, and will be hotter - even though it is properly tuned. 100C is somewhat arbitrary - it's around the right range, but it generally means the mixture will be on the rich side.

More accurate ways of telling if you're running rich or lean involves pulling the glow plug and looking at it. I can't recall what means you're rich or lean, though.

The throttle response also tells you if you're running rich or lean. Lean engines don't spool down to idle smoothly when you hit throttle hold. Rich engines hesitate when spooling the engine up, and may quit in the process.

Always err on the side of being too rich. If you run too lean, you'll end up damaging the engine - maybe even enough to make you rebuild the engine. If you're too rich, you just waste fuel and don't get as much power. The fuel is easier to replace than the engine.

The last thing I want in a heli is a few loose screws.

07-14-2008 03:51 AM  9 years agoPost 3
Funky Trex

rrElite Veteran

Westerville, OH - USA

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I have a few 91SZs and they range from about 1 turn to 1 1/4 for the high needle and around 1/2 +- a few clicks on the mid needle. Like Pariah said, there are so many things that can effect these settings so you'll have to find what your particular motor runs the best with.

07-15-2008 07:32 AM  9 years agoPost 4


Swalmen, the Netherlands

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All true, but to give you a head start on my settings:
S91SZ (non pumped) Wildcat 30%
- Low: 50% (centered)
- Mid: 1/4
- High: 1,4

I added a Carbsmart to compensate for the cold and warm temps.

Walter Robijns, 3D rules !!!

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