OS.91-SZ (non pumped) Needle Settings

[CitationX]

I did a search for "needle valve settings" on the 91-SZ just to get a feel for what guys are running out there. One thing I know for sure is that I am way, way too rich. I am breaking in 2 new non-pumped SZ engines and, not knowing any better and using the practice I have used on previous engines, I have the high and mid set at 2 full turns out. The low end is still at the factory setting, which is about 1 turn out. It is no wonder why both engines hydraulic lock at every start attempt! Furthermore, I am wondering if the rich setting of the carb is causing excessive vibration in the tail, particularly on my Vibe.

Anyway, I am curious about what setting others are running on their machines.

Thanks

[JohnSSN700]

Whoa, you are wayyyy to rich. That little pump really puts out the fuel. Forget the manual, try these settings, they should get you in the ball park:

L-Factory or 1 o'clock
M-Closed, you could go 1-4 clicks out but probably will used completely closed.
H- 1 1/4 turn out

I hope this helps you.

John

[DavidH]

Whoa, you are wayyyy to rich. That little pump really puts out the fuel.


What Pump? It is a regular SZ not the Hyper.

David

[JohnSSN700]

oops, he did say "non-pumped" didn't he!! :arggg: Sorry about that, just ignore me........

[RossoOz]

High 1.5
Mid 1
Low is 12 o'clock (standard setting mid point of range)

Fuel is 18% MV oil, 20% nitro (home brew)
OS 8 plug
MP2 muffler
Standard head shim.

Running the motor around 14500 rpm (1850 headspeed on my heli)

I'm still rich on those settings, but Ive just put a new set of bearings in this motor that I bought second hand. I expect to lean out a little more to get the motor dead smooth. Also I will start using a colder plug than the os8, its a bit to hot for this motor IMO. I want to use enya 4, but I can't get any local at the moment.

A rich motor will make the tail vib and kick a bit, just start leaning off the main a few clicks until the vib goes away, but keep checking the backplate temp to make sure you don't go to far.

I would be guessing your back plate temp is stone cold at the moment.
My backplate is only slightly warm to touch, not hot at all.
I don't tune for power, I tune for a smooth running engine.

Also the idle needle isn't one turn out, it can only move 180 degrees. Its a press fit into the carby, not a threaded needle like the mid and high.

Great engine, about the same power than my YS91, at the moment, not as smooth. I expect it will make better power once the needles are a bit leaner.

Ps, Im operating at around 2300 feet, summer temps of around 30-35C, so I have to run a bit richer for the heat.

[Rada70]

So the hoter the temp the richer you go, and vise versa. I read the oposite by Curtis in rotary magazine.

[DavidH]

The warmer the ambient air temperature the leaner the mixture the will. This is due to the air molecules being less dense so they don't mix as much with the fuel.

When the temps are cooler the richer the mixture will be. This is due to the air being dense and mixing well with the fuel.

That is a basic explanation not a scienitific one.
The reason he has to richen his mixture is the altitude he operating the engine . Less oxygen.

David

[RossoOz]

I guess what I was trying to indicate is the fact that its the fuel running thr the motor that does most of the cooling, and that why I need to run the needles just a touch richer.

If you had a perfect tune on your motor at 10 degrees C, then raise the outside temp to over 35 degree C with no other changes to air density, mositure content etc, the greatest effect to enable you to cool the motor back down to normal operating temps is to richen the mix. I'm not talking about more than a click or 2 here. By doing this, you are going to lose some power, but the motor will still run smooth and cool.

Cold dry air will give you the best power, verses a hot humid day. The mositure lowers the oxygen density.

I am always quite amazed how well such a simple carby can handle such a wide range of different conditions and still work so well.

[ferincr]

Hi, I'm running this engine in my R90 and I'm trying to find the needle setting too, at the moment I have the mid at one turn (haven't worked it yet, but I think read that someone was using it totally closed to have a fast transition from idle to full speed (not really worried about that yet).
My high neddle is at one turn and 7 clicks (less than one turn and 1/4) and it seems like the engine can be leaned even more (temp wise) the back plate seems to just warm, I can keep my finger there forever. I'm not sure if I should keep leaning it, it seems like everyone is running this engine around 1 1/2 turns.
I'm flying at around 4500 ft ASL weather temp is about upper 80's and quite high humidity.
Any suggestions?