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HomeRC & PowerAircraftHelicopterEngines Plugs Mufflers Fuel › OS 91 SZ-H with pump, needle valves screws settings
05-02-2010 02:07 AM  8 years agoPost 1



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I just make my second flight today with my raptor 90 with OS 91 SZ-H with pump.

I had adjust the needle valves as per the OS manual, 2.5 turns for the high, and 1.5 for the mid and idle screw at factory setting (straight middle)

When I spooled up the engine from low to mid stick the engine seams to bug down until i passed the mid stick, then it run better, but still really really rich, lots of smoke. I should passed the mid stick carefully to avoid engine to cut off. If I spooled it too quickly from low stick to mid stick, the engine turn off (seams to be too rich or something).

once I can pass the mid stick without stopping the engine, then it run well, but really rich and seams to be underpowered, not much punch.

I read on another forum that when the fuel tank is not pressurized by the muffler via a tube pipe (my case, open to atmosphere with a long tube going down the bottom of the tank for more than 1 inches) the high speed needle valve should be near to 1.5 turn from a fully closed position, mid needle valve completly closed or 1/4 turns from fully closed, and the idle screw at factory setting (in the middle).

What you think of that,

I did not test it yet, will try it tomorrow, but I want to check first if someone as some advice or tips to give to me on that,

thanks for your help!!


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06-13-2010 12:19 AM  8 years agoPost 2


Bariloche, Rio Negro - Argentina

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same issue
I suffer the same problem, someone solved it?

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06-21-2010 08:33 PM  8 years agoPost 3


Wellsville, NY

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TTT... same question here

Mine's a 91HZ-3d ps though

My mid needle valve is almost closed off.. and it seems rich when I come off of idle as well.

04-23-2011 04:13 AM  7 years agoPost 4


Napa, CA

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My settings
Your all way to rich. My hi jet is a ~1&1/3 and the mid, two clicks past half. Runs like a top. I'm at sea level...

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04-25-2011 04:48 AM  7 years agoPost 5

rrKey Veteran

South Korea

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I had the exact same problem. After replacing the reg and pump.....twice over the course of a year, I bought the HZ-R. I even took one of the regs apart and looked to see if it was dirty. That didn't help either. From what I've been told, it's hit or miss when it comes to buying a good SZ-H Pumped engine. I have a friend running the SZ-H pumped with no problems. I've also been told, that it has something to do with the regulator and the pump making tuning almost impossible due to inconsistancy....which is why OS came out with the new regulator and dumped the mechanical pump for crank case pressure like YS.

I wish I could offer you a better solution, but after a year + of fighting with my SZ, I just bought the HZ-R and my problem went away. No more feathering the throttle to keep the engine from bogging down through the mid range during spool up.

I did manage to isolate my problem to mid range tuning, but I was unable to find the "sweet spot" before I gave up. Spool up perfomance did improve after my middle needle was almost 3 turns out which is unusual I will tell you that if your engine runs well once you get past mid range and it runs well at idle, then you should focus on your middle needle. I found that richening that needle did help more than leaning it out. Try backing it out a few clicks to see if it helps at all. I wish you all the luck in the world and I hope you get yours figured out. And if you do, please let us know what you did...then maybe I can finally get my SZ to tuned in too.


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