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OS .52 FS Unusual Problem

Old 12-04-2015, 08:48 PM
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This is a strange one that I've been playing with for about six months, so thought I'd post it on RCU to see what comments pop up. Before getting into it, just a bit of background about me - I've been doing this since the late 1960's, currently have over 100 engines in my collection, both two and four stroke. Some years ago I co-authored a book on running and modifying Fox engines, which Clarence Lee plugged in one of the issues of RCM magazine. So, a long way from being a newbie with glow engines. Just wanted to mention that to avoid all the usual "did you try a new plug, or different fuel" type of responses.

So here it is. During the Summer I acquired two OS .52 FS engines from the estate of a local modeler that had passed on. One had been run, but was low time, the other was NIB and never run. Other FS engines I own in that size range include an OS .48, a Saito .50 and .56, Magnum .52 (X2) and ASP .61 (X2). The ASP .61 is just a bored out .52, otherwise same as the Magnum .52. Like the other OS FS engines I own (ranging from .40 - 1.20) I expected the .52's to perform well. However, that turned out not to be the case at all. In fact, both of the OS .52's turned out to be nasty little beggers. Well below the performance and reliability of my Magnum .52's. Off a good 1,000 RPM, or more, for any given prop. and numerous deadsticks. After experimenting with a range of prop makes and sizes, I eventually settled on the Scimitar 12 x 6 S2 series, which also happened to be the one which worked the best for my Magnum .52's and .61's. While the Magnum .52's could easily turn in the upper 10,000rpm range (I fly them at about 10,500), the OS could barely manage upper 9's, but had to fly it in the low 9.000's. (A bit more with an 11 x 7 but the models flew better with the 12 x 6) It acted, and adjusted, much like an engine with poor fuel draw. Note that both OS engines had been swapped into 3 different, existing models that had been running Magnum engines (same mounts, so an easy swap). Same result each time, so not related to tank, installation, etc. But, such as it was, the lower power was not the most nasty aggravation. That was the poor reliability, mainly a sudden "hard".miss and/or occasional deadstick, that presented somewhat in the way you'd see with water contaminated fuel. I did eventually find that the engines (both) ran the best when set to absolute maximum lean setting possible. Basically, run it like you stole it. Sometimes I could get several flights in a row without an "incident" but not fun to fly. Pondering a "possible" fuel draw problem, I then installed a Perry oscillating pump on one model and, zing, the rpm came up to 11,000! But I soon discovered that, like before, it had to be set close to max rpm, to minimize the hard miss problem, although the deadstick landings came to an end. The "miss" is odd too. It's instant, hard, and prolonged to the point that I'm already thinking it's deadstick. Then, it will come back to life, just as quick, and the flight continues normally. Heart stopping, though, for that instant, but it has not actually quit now in almost 200 flights with the pump installed. And, with the pump, it outperforms the Magnum .61 so, I've just kept using it. However, I'd really like to solve the problem for real, especially since I now have "three" OS .52FS's. A club member recently just "gave" me one that he also could not get to run properly, and claimed it acted much like mine. Tired of deadsticks, he had put it on the shelf some time ago, and bought a Magnum to replace it, which ran just fine.

Were it not for the fact that it's an OS, I'd be on the verge of calling this a design problem of some type. But it "is" an OS and I'd never heard of this type of problem before with an OS FS, so there's something odd going on here, but what? I have investigated this a bit further, in that I have disassembled one of the OS's and one of the Magnum's for comparison. The Magnum is supposed to be virtual clone, after all, so why does the cheap clone work so much better? Aside from the fact that the internals of the OS are plated, and the Magnum not, the only notable difference I saw was the head design. While somewhat similar, the OS head is smooth and sculptured, if you will, while the Magnum has hard defined lines/edges. Also, the OS has notably more clearance (space) around the exhaust valve (relative to the side of the head). The head gasket on the OS "overhangs" the extra clearance around the exhaust valve, which has me wondering if that's creating some kind of a flow problem? Glow plug position, and depth appears the same in both OS and Magnum. Camshaft lobes and timing appears to be the same. As for fuel - I'm running what's available in my area, that being Cool Power (5, 10, and 15%). Most people here are running 15% in their four strokes. I typically run 10% although I have tried 15% and also tried some 10% Omega, but in all cases the problem remains. I've experimented with various plugs, including some two stroke plugs. The OS type F always works best, of course, but I had to try all the same. Interesting, though, that the Fox miracle plug worked perfectly well in the Magnum engines, as well as my other OS FS engines, but the OS .52 just hated it, ran awful. I should also mention that the elevation here (West coast of Canada) is basically sea level. And remember that running the engine a bit "richer" makes the problem worse. I tach it before every flight to make sure it's turning "at least" 10,700rpm. It's even happier at 10,900 - 11,000rpm. The pump keep the fuel coming no matter what, and it does not overheat. In a normal situation, I'd prefer to run it more like 10,500. Most of my flying is in the 1/2 - 3/4 throttle range, in any case, however, the miss usually happens at full throttle.

Any comments or ideas would be welcome. I'm about at the point of trying some "franken" engines - OS with Magnum head?

Best regards,
George R.
Old 12-07-2015, 07:49 AM
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Without seeing the engine, we'd really not know what it might be, exactly. It does sound like a fuel draw problem. We'd start out by resetting the carb. Make sure the idle mixture is set with the screw flush with the housing or turned out a bit from there. If you start out with the idle too rich, then you know it will need to be leaned. Remember, if the idle mixture's set too lean, it can interfere with high-speed fuel flow.

We normally prop for 10K or a little higher when leaned to just rich of peak RPM on the ground. Once we can get the engine to sustain full-throttle running when leaned a bit rich of peak, then we work on the idle. Any idle 2,500 RPM or lower is "good".
Old 12-07-2015, 09:20 AM
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Hi Bill, thanks for the response,

I should have mentioned that I did also try the carb from one of my magnums installed on the OS. The problem remained. Also, two OS engines exhibit the same problem. With the pump, I can sometimes go a half dozen flights with perfect operation, but not so without the pump. Without the pump, if the engine/s are adjusted for a good setting with the nose level, once the nose of the model is raised, it sags and must be made even richer. We're talking less than 10K here, and well under that to be able to fly the model. Idle and acceleration are excellent with or without the pump. Fuel tank installation, and height are good, and (in three different models) the tank is close to the firewall with fuel lines as short as possible. Still, it acts like a fuel draw problem, but not with with equivalent Magnum engines installed, which will easily run in the upper 10K rpm range, without a pump, including nose up. I've had both the OS carbs apart to check all the O rings, which appear to be in excellent condition. Wiggling the HS need while running does not affect the mixture. I've also tried additional sealing where the carb meets the intake stack (RTV) as well as adding a gasket where it bolts to the head. Still, no change.

The miss occurs predominantly at the full throttle position, but not immediately. For example, it is most likely to occur near the end of an extended knife edge at full throttle. Or, near the top of a large, full throttle loop (either inside or outside loops). Normally, this may suggest a lean mixture, however, making it richer will worsen the problem significantly. Interestingly, the problem does not present during a long, full throttle, vertical pull, but may happen after the (low idle) downward leg. For example, if I perform a large square loop (with 1/2 rolls) there is never a miss on the vertical leg, however, after the long downward leg, and inverted pull out (all at idle), I roll out, and as the throttle is increased (not to full) there can be a hard miss at that point. After the miss, which is hard on the nerves since the altitude is low at that point, the engine will catch again and run fine. Without the pump, the miss can occur at almost any point in the flight, and much more often. Plus the engine power is poor since it can't turn up as it should, With the pump, holy ***p does it ever perform! People don't believe it's only a .52. The model I'm currently using for these engines is a Phoenix Model SeaBee that weighs in at almost 6 lbs. I fly mostly old-style pattern with it. As I said, it has never quit with the pump, and with the incredible performance, I'm less inclined to swap the engine back to a Magnum. If not for the occasional, heart stopping, miss I'd likely not have even posted, and just used a pump with my OS 52FS's. I'll continue to pursue this though, just need to do a little more brainstorming.

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