OS 91 PS SZ
Simon Lockington

The OS 91PS SZ less carb.

Installed in the Vigor about to have the carb installed.

A couple of weeks ago I came to the conclusion the ancient OS 91 C-Spec in the Vigor was sound like a rattly old diesel engine. I knew the old dog would need a bit of work done on it given that it has done so many flights in the last five or so years, so I tore it down to check it out.

I wasn't overly surprised to see the ring didn't have any spring left in it, the piston and liner had a few scores in it and the bearings weren't overly good. I should say that during this time I've replaced the bearings every six months or so, but the rest of the engine is essentially the same as it came out of the box.

Given that OS parts prices are so hefty, I decided just to go buy a new engine. Given the old C-Spec had the Cline regulator system (which I can't speak more highly about by the way, it's great), I had thought about just buying an engine without a carb and using the Cline, but I decided to just get a whole new engine.

While the old C-Spec had treated me impeccably, I'd been very wary of OS engines since that disaster of an engine, the OS 91 PS (Perry Pump version) came out. I had one of those in one of the Sylphides and what a big piece of crap it was. You could get it to hover smooth, or make OK power (not great, but OK) for aerobatics but you couldn't have both. After an assortment of carb spray bars from overseas I gave up and put a YS in both the Sylphides and haven't looked back since.

So when it came time to replace the old C-Spec I considered going the YS route again, but decided to give the SZ range of engines a go, specifically the pumped version, using the proven OS pump that has been in use on planks for years and years.

The SZ PS does not use crank case or muffler pressure which means no more disintegrating clunk lines and is also regulated which means no change in engine tuning throughout the flight.

The PS comes with two extra head shims as standard, one thick, one thin (I can't remember the sizes off the top of my head), I'd run an extra shim in the old engine which made it run quite a bit smoother, so decided right from the start to install the thick shim in addition to the standard one.

PLUMBING
I installed an Enya 3 plug into the engine and bolted the engine into the Vigor without a problem and bolted up the Hatori pipe, however I had to sit and think about plumbing the fuel line up. Because the tank isn't pressurised by either crank case or muffler pressure, the tank needs to vent to the open air so that fuel can be sucked from it.

On a heli that goes upside down a lot, there is obviously potential for the fuel to simply run out the vent line, all over the heli. The OS manual says you can "use a suitable one way check valve" to prevent the fuel running out the line and only letting air in, however they don't mention a suitable one, and I'm not sure the normal valves such as the Perry or the YS one are really suitable for the task.

However a quick search on Runryder found that many guys had good success by running fuel tubing from the vent nipple to a spot below the tank. Apparently this, and the little amount of negative pressure generated by the pump sucking fuel is enough to stop the majority of fuel from running out of the tank while the heli is flying around. So I did that and it seems to be working so far.

RUNNING IN
True to form, OS's recommended needle settings are way out so I didn't bother with those. I set the top end needle to 1.5 turns out and hte mid range to 1 turn out and the bottom end just above half way knowing that would still be very rich. A quick flick of the starter saw the engine burst into life and sit there idling very well without any hint of coughing and dying which was great!

The heli spooled up cleanly with a TONNE of smoke and was very rich.

I'm not one for running my engines in mega rich. I like to get them up to normal operating temperature quickly, and then flying them round like normal making sure to not load them and most importantly, never overspeeding. I always make sure the engines are running 'cleanly' (ie they're not coughing, four stroking or running lean). As the engines run in, you can lean them off a click or so at a time to really get the optimal performance out of them.

By the end of the first flight the PS was running nice producing good power, lots of smoke, running cleanly and cutting back instantly at the beginning of autorotations. I was very impressed so far with the engine. It didn't seem to produce a whole lot more power than the C-Spec it replaced, but it was running as consistently and smoothly which was great.

SUMMARY
Another 15 flights later and the engine is fully leaned out and performing great. The engine continues to run very consistently and is making very good power. Is it more powerful than the old C-Spec? Well I'd be hard pressed to say it was, it's at least as powerful, but wouldn't be massively more than the old one.

My final needle settings are 3/4 turn on top end, 0.5 turn on mid and 3/4 open on the bottom. Please don't transfer these needle settings to your engine because your engine, exhaust, fuel, and style of flying (amongst other factors!) will most likely be different and you could do your engine harm if you don't know what to listen for.

Would I buy another one of these engines? Absolutely, I'm very impressed with it so far, it's certainly leagues better than the original OS 91 PS.