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OS FS-120-SP Supercharged Help Needed

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OS FS-120-SP Supercharged Help Needed

Old 10-18-2004, 02:12 AM
  #1  
Thoughtful_Flyer
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Default OS FS-120-SP Supercharged Help Needed

I am having serious problems attempting to run a FS-120S-SP supercharged engine and I hope somebody can help. I've carefully followed the advice in the manual and on the FAQ page on the OS website without success so I hope you may be able to tell me I'm missing the obvious!

I bought this engine from a modeller who was emigrating. It appears to be in immaculate condition and has been stored (wrapped in a poly bag) in its box for about a year. I have installed it, inverted, in a model and connected up the fuel tank exactly as shown on page 5 of the manual. It is on a solid (i.e.. not anti-vibration) mount. I've fitted a new OS Type F plug and am using 20% nitro, fully synthetic, fuel.

From cold, after priming the engine as suggested in the book and setting the needle 2.5 turns open, it starts quite easily and initially runs well - but rather rich. After a perhaps 20 seconds at full throttle (or sooner if I reduce the throttle the increase it again) the engine starts to surge and knock and soon cuts. It seems to run a little better and for slightly longer if the glow is left connected. Each attempt at re-starting then seems to get worse with (what seems to me) excessive amounts of fuel coming out of the silencer. Closing the needle valve a half, one or even more turns seems to make little if any difference. I have also tried turning the mixture control about 30 degrees clockwise from its mid point with no effect.

I had wondered about a sticking valve and therefore removed the rocker cover and very slightly increased the clearance between the valves and rockers. This made no difference.

Can anybody help – please!

Thanks

Chris
Old 10-18-2004, 06:40 AM
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BlackB12
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Default RE: OS FS-120-SP Supercharged Help Needed

Sounds like it's just way too rich, most likely the low speed adjustment needs leaned out some since it's mounted inverted. A stuck valve would make it have no compression so you'd know that immediately.

Mike
Old 10-18-2004, 07:51 AM
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jaka
 
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Default RE: OS FS-120-SP Supercharged Help Needed

Hi!
Helped a friend with one 15 years ago and it was very hard to set....if not impossible.
I think this is the only OS engine that is not working properly whatever you do to it.
Just a collector item to gaze at ...not really to have in a flying airplane...sorry!

Regards!
Jan K
Sweden
Old 10-18-2004, 11:17 AM
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Bax
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Default RE: OS FS-120-SP Supercharged Help Needed

2-1/2 turns out from closed is super, super, super rich. We start out with the needle about 1-1/4 to 1-1/2 turns out from closed. The engine gets leaned from there. When the engine's broken-in enough to be leaned to peak RPM, the needle will be about 3/4-1 turn out from closed.

We found that the engine ran well with high-oil fuels (22%-24% oil), and tolerated 20%+nitro quite well. The engine will continue to break in over the first five to six gallons of running. It won't really perform well until it's had that much running time.
Old 10-19-2004, 03:17 PM
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Thoughtful_Flyer
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Default RE: OS FS-120-SP Supercharged Help Needed

Ummm….

Many thanks for everybody’s help so far.

I agree about it being far too rich, just makes you wonder why OS tell you to set it 2.5 turns open!

I tried reducing it and it seemed to run best fully closed! (Yes honestly). This made me suspect the needle so I tried one with a finer taper from another engine. Adjusting the mixture control (leaner) improved things somewhat and it now seem to run half reasonably with the main (new finer) needle about ¾ of a turn open. With a 14 x 14 APC prop I’m getting about 7,400 revs max and have a nice idle a bit below 3,000.

I would be grateful for comments and further suggestions.

Thanks, Chris
Old 12-08-2016, 03:28 PM
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tejasnumu
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There are some significant concepts but I have a 1.20 FS SP. The engine will probably always run a little rich. I run a 383 ci small block Chevrolet with a small bore GMC 6-71 converted to pump gas. Supercharged engines of roots design (back of your 1.20 FS SP is a roots supercharger.) My 36 runs rich and retarded in order to keep intake valve shut at the proper time. If intake valve opens with fire in cylinder; the plenum turns into an explosive device. I have never broken down mine as it used to invalidate the warranty. I have not run mine for awhile but there are folks in U.S. that freshen them up and will give you crib notes on the care and feeding. I change lots of stuff on 2 strokes, a little on 4 strokes but I have no intention of cracking my SP. I'm just using what I know about blown auto engines. Check around. Some FS wizard could probably make it like new. Mine used to run a 10-300 about as good as my YS 1.40. Terrible strong engines when running right.
Old 12-09-2016, 08:07 AM
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12 years. That engine is sitting on a collectors shelf now.
Old 03-07-2017, 09:04 AM
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tejasnumu
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Default 1.20 fs s

Unless you nailed it to the shelf, mix a batch of nitro methane; methanol; synthetic oil and a little castor. Take it out and use it! A low wing "Basic .60" tail dragger is good. They are so much more fun when you run them rather than just look at um. There is a whole new generation of folks that will come over and admire it. I've got an Ohlsson and

Rice that pulls a little free flight like a new Estes .049. Unlike most blower motors it likes lean.
Old 04-29-2017, 10:49 AM
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moparbob498
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Hi ya, well I'm new to this, so my thought may be fubar, but here gos... Is there a pressure line to help pump the fuel with the supercharger set up? Not familiar with it yet, just from what you described sounds like it does & it's flooding ,too much pressure under Supercharger, & above it is vacum?? Anyhow, is this making sense, or is there no air line going to tank for pressure assist?

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