Posts Tagged ‘os91’

OS91HZ-R (Regulated) Almost Ready

February 8th, 2010 2 comments

First production is scheduled to complete early March.

It looks like O.S. is getting very close to releasing the new 91 HZ-R engine, which is basically a stock 91 HZ that will ship with the new regulator system that I have been testing since June of last year. This regulator works with crankcase pressure and delivers a very consistent run from beginning to end, it also improves performance as well as mid range response. It is unknown exactly when it will be released in the US, but I am guessing sometime in the Spring. It will of course be available in Asia first and I’m sure a lot of people will order it directly from an Asian shop

It is also unknown at this time, but apparently O.S. will sell the carb by itself, so that it can be installed on any O.S. 91 HZ motor or even on an Align H motor. More info about this system here:



  • This is a main fuel tank pressurized regulator system integrated version of the 91HZ.
  • Newly designed 61E Carburetor integrated with a regulator surely regulates pressurized fuel. As with other 91HZ engines, it features twin needles which can adjust hovering and high speed flight independently.
  • Regulator is located at a place where heat does not suffer and access is easier.
  • The system ensures stable fuel supply irrespective of rapid model attitude and G changes.
  • The system does not use muffler pressurized fuel system which requires needle adjustment according to the change of
    muffler pressure.


  • Displacement : 14.95cc (
  • Bore : 27.7mm (1.091in)
  • Stroke : 24.8mm (0.976in)
  • Weight : 625g (22.05oz)
  • Output : 3.4ps/15,000r.p.m.

OS Regulated Carb

OS91HZ Manual

July 13th, 2009 No comments

…because I couldn’t find one anywhere on the net, here’s a scanned copy 😛
OS 91HZ Manual

Categories: General Discussion Tags: , , , ,

Review: OS91HZ with Cline Carb Modification

June 23rd, 2009 No comments

Anyone that’s been in nitro helis for a while knows of the OMI cline regulator mod for the OS91SZ, which works pretty well. For those that are concerned about engine performance this seems to be the more robust solution, as pumped versions of the engine have been known to break down.

Normally an OS91SZ has some of the pressure redirected from the exhaust back into the main fuel tank. In turn when the carb opens and the pressure from the tank forces the fuel through the carb. The more the carb opens the more fuel flows, simple. This system works okay but wont guarantee the same fuel flow for all orientations and through out an entire tank, particularly during aggressive 3D flight.

By comparison the cline regulated fuel delivery system uses pressure from the back plate to pressurize the tank (separated by a oneway valve), which is substaintially more pressurized then a normal setup. When the carb opens it creates a “suck” which pulls a diaphragm (like a scuba regulator) which proportionately opens the cline valve to allow pressurized fuel to enter the carb. The end result is a smoother running engine at any attitude which doesn’t lean towards the end of a tank. Also you don’t have exhaust fumes in the fuel tank which can eat cluck lines and fuel magnets.

What about the OS91HZ?

The OS91HZ in general is a far superior engine to begin with:

  • Redesigned performance head
  • Carb inlet hole on the crank is bigger
  • Crank is rebalanced
  • Port timing has been changed
  • Piston is lighter with a cut on the skirt for better fuel flow
  • Piston is ported on the top
  • Carb barrel is sealed with an o-ring
  • Carb mount clamps are sealed with o-rings
  • Carb has a thermal insulator

Needless to say the NON-pumped version runs great out of the box but wont give you the 3D edge. The pumped version however is designed for 3D flight and when the pump is functional it seems to perform really well. Unfortunately there still seems to be durability issues with new OS fuel pumps, already one person at my field has had one die. A cline regulated system by contrast, is simple, effective and has less parts that could fail.

Cline HZ Flight Performance

OS Check Valve (OS 72403061)Amazingly smooth and powerful, while not being too hot. The main needle is set quite low too, just over 1 turn and the mid at about 1.5 turns. Obviously those settings are not gospel but are working for me in the current UK weather (low 20c’s). I’ve been running this setup in conjunction with an OS check valve (OS 72403061) which works great. It is note worthy that the cline needs as much of the engine pressure as possible in order for the regulator to work correctly (not lean). To give you an idea, a typical YS has a check valve with about a 6PSI thresh hold (using a spring) by comparison the cline needs a thresh hold of no more than 2PSI. I have tried a YS check valve (the smalller 2 stroke one) and the engine definately runs hotter. I have heard removing the spring works well in the smaller YS check valve or using the larger 4 stroke YS check valve.

SZ Carb

My setup uses a stock new carb from an SZ which was cline modified already and had an insulator sleeve added to the carb in order for it to fit. I have another HZ which I’m about to break in which has the stock HZ carb cline modified, which I assume will be very similar in performance, I’ll report back on that soon.

OS91HZ with OMI Cline carb modification attached

OS91HZ with cline carb mod

OS91HZ - Cline modification


This is a great upgrade for your new OS91HZ if you are a die hard 3D pilot. You get the performance of a pumped engine with out the hassle.

Rating: 9/10

3 Needle Engine Tuning Guide (OS91hz)

June 16th, 2009 No comments

OS91 HZSomething I would have appreciated while I was on the steep learning curve of “rc nitro heli engines”, is a simple to follow engine tuning guide. This assumes you want your engine to run at optimal performance suitable for 3D flying. The reality here is optimal performance is very close to breaking point with these engines.

One analogy used to describe it would be model rocketry.

For this I’m going to base the tuning on the 3 needle OS91HZ engine but generally I’ll try to keep it generic.

Set your needles to factory setting and start the engine up. Bring the heli into a hover for a minute for so in order heat the engine (it wont be full operating temperature but warmer), then land and spool down.

  1. Idle needle (governor OFF) – if when you now spool up the heli splutters or hesitates (while it’s  initially spooling up) you are rich and need to lean the idle needle. In order to ensure you are not too lean find the “splutter point” and then wind back until it’s smooth on spool up, it should relatively close.
  2. Main needle (governor OFF) – is active when the carb is fully open so you need to test the main needles effectiveness during a climb out with full collective/throttle deflection. There is a bit of an art to testing this needle, what you are looking for is a good climb out rate which also produces plenty of smoke but not dripping from the exhaust. Also the engine temp needs to be warm to hot but not melting. You should be able place your finger on the back-plate for about 5 secs easily.
  3. Mid needle (governor ON) the mid need is really all about recovery and transition and in many ways it’s what makes the engine feel like it has power. My technique for tuning the mid needle is with a governor enabled, I’m not saying that’s 100% correct but it works for me.  After the head has loaded the governor will open the throttle in order increase the head speed and once that is achieved  it will drop back into the mid range. Now if the mid is reloaded again quickly (in a high power move) and is badly tuned the recovery of the engine/head speed will be slow and the heli will appear to bog. I use an aileron “tic toc” to load the head and then make decisions on whether to increase richness or lean based on the amount of smoke, temperature and power.
  4. General flight test – once I think I have tuning close I’ll fly normally (3D) for a few minutes to bring the engine up to operating temp under normal flight conditions. And as long as the engine is not too hot (test via the back plate) and it’s producing good power then generally I say it’s tuned.

These days I tune my engine once before every session (excluding the idle). Mainly because I’ve toasted a few engines, for the practice but more importanly for the power. I’m taking between 1-4 tanks to get it me it’s worth it.