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
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.
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
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.