1) Those cute little yellow caps are not trustworthy to be leak free. Don't use them. A short piece of fuel hose with a bb stuck inside is a sure thing. A uniflow system has to be sealed. If it leaks someplace, it won't work correctly.

2) The uniflow inlet is normally routed to the inside of the circle enough so that all the fuel is outboard of the air inlet. I'm not sure that your setup will or won't work, just sayin' that it's not the usual setup. Maybe somebody will chime in on this. It could easily be a tank problem; even internal leaks in the uniflow tube are not good.

3) What sort of prop pitch are you using? And are your props trustworthy to be the advertised pitch, or are they way off, like Master Airscreweds (sic)? A genuine 5" pitch should be good. Not sure what you can get over there, but I'd try a 10-5 Graupner or APC for a first flight.

4) Are you launching rich, or peaked? I'd set it so it was 4-cycling on the wheels and breaking to a 2-cycle when you raise the nose to 45 deg.

5) What is that muffler, and how big is the outlet? Randy Smith says a bigger muffler outlet acts like a larger venturi. Makes sense. Less backpressure makes it pump more effectively, so suck more fuel in.

6) How long are your lines, and what diameter?  

7) Looks like your venturi is in contact with the muffler clamp. This might just cause the venturi/O-ring to leak. I would not suggest shortening the venturi, but it looks like you could carve a relief on the outside to get a little clearance. No touching allowed! I don't like OS NV Assy., but others do. They can look fine and not work right.

These are some of the things that came to mind about diagnosing your setup problems. Then, there is the vibration of a profile nose, tank mounting, props are balanced, etc.?   Steve

Edit: There's another thread on OS .35S Runaway (that's what you've got) below. The link might work, not sure!
http://stunthanger.com/smf/index.php?topic=23080.0

Phil, the nose of your model looks like it's kind of cracked, adjacent to the lugs.  Just wondering if vibration might be part of the problem?

I've got a couple of 35Ss - no idea what size venturi is in them, but probably whatever they originally came with.  They'll both spin a RAM 10x6 at close to 12000, so I'm guessing it's not an overly small venturi.  Regardless of that, I have no problem getting a good run out of them using the stock muffler.  On the ground I set them so they're just 'beeping' between a 4 and 2 stroke; once airborne they settle into a steady 4, with a nice break in manoeuvres.  Never had a problem with runaway (and I even use the little yellow caps!).  If you can launch at only 6800 and still get a runaway, then something pretty weird is happening.

Steve

As far as your particular setup: The "makeshift" muffler might be causing a heating issue. Can you run open exhaust just to experiment? Cutting the venturi won't help anything but it probably won't hurt anything either. I have never had any luck at all with those wedge type tanks. I use uniflow square type tanks. RSM makes some nice ones for profiles that work well. Plastic tanks can also be made to work and take up less space. Vibration could an issue, AND finally, this engine may just not work well on this airplane, I have seen this happen before.  

Added:    Air leak in tank could be causing this...seen that too. Give it a pressure test (in water) before the next flight.   

No cracks in the nose; what you're seeing is the solartex covering lifting a little. I don't actually launch at the revs I mentioned, these figures were used to illustrate the peak rpm with the different venturis whilst static. I release at about 8500. I'm going to cut the strap some more to clear the venturi. This damned wind is still blowing at 25+ mph so no chance of testing today. BTW latest prop is the RAM 10x5 made in the UK. Have only heard good things about these props, so happy using them. If I can add a couple of 1/10s of a sec with the smaller venturi and use 63ft .12s lines I reckon I'll get close to 4.6+ sec laps which I'll be happy with.

Thanks to all for the help  Phil

Ward, I hadn't thought of the vibrations affecting the fuel in the tank, will add some thick foam behind the tank before the next flight. I confess, I don't use fuel filters and I know I should. BTW I forgot to mention that Jim Lee's service is excellent and would recommend him without hesitation.
Thanks Phil 

I see this on every profile plane that has an outboard mounted tank - the fuel pickup point waaay outside of the spray bar and the 'centre of pumping action' of the engine.'

So its no surprise that it leans out ferociously when in flight. My bet is that you are not so much a victim of runaway but more an observer of the natural law of physics and centrifugal force.

Solution? Try and align the fuel pickup point that is inside of the tank more to the centre of the engines mass by either/or -

1. Using a dedicated profile tank that is higher and not as wide,
2. Mount the engine on pads (Lord knows you have the bolt length to accommodate at least 20mm here!)
3. Or most drastically mount the tank on the inboard side of the fuselage.

The 'pad' method would be a quick and dirty method to find out if you are heading in the right direction mate and would have to at least counter the pad that you already have under that existing tank .

Good luck.

Thought it only right to give an update after all the advice I was offered! I tried smaller venturi, different tanks (with foam padding) a variety of props, severel amounts of castor in fuel (up to 25%) even tried a homemade muffler which was hopeless. The only time I could get a 5 sec lap time was when the muffler snapped off in flight! Coz there was no one about I fired up the motor again (had to adjust the nva a little) without muffler and 4.9-5 sec laps every time!! Perfect, but Very loud!! Perhaps the standard muffler is causing overheating issues, I'm not sure but I do know I can't get away with flying without one.  So I replaced the engine with a newly run in Enya ss25s (same weight as the OS35s) + 9x6 Graupner and well, well, 4.8 sec laps every time.
Using a square homemade metal tank with muffler pressure.. I'm a happy bunny again!!
Phil 

Hi Bill, Yeah thanks for that, its Great! I think the bottom line is the OS35s is too powerful for the S1 Ringmaster, well for mine anyway.. I plan to use this on an extended version 'Mercury Cobra'  46" w/s winter build. Was going to use the Enya 25 on a CG Shoestring yet to be built. Suppose I'll have to get another one for the Shoestring, It seems to be a really nice well built engine, According to instructions can use 20% castor or   synthetic even tho its iron piston 

 Phil

Phil, if you're looking at another SS25, you might want to consider the SS30 instead.  As well as being more powerful, it's half an ounce lighter, and not much more expensive. 

Hi Phil,

I recommend you trying using a 4 to 5 pitch prop with the Enya SS25 and launching around 10k rpm and see how you like. With 6 pitch prop you are probably not running at best rpm for torque. Although the speed could appear to be in control in level flight at 5.0s a lap you may not have the best speed control when maneuvering. With a 6 pitch prop and lower rpms, you may loose speed or gain too much speed when maneuvering.  If you feel the speed control is not ideal I suggest you try one of these props with the Enya SS25: 10x4, 10x5 and the  APC 10.5x4.5. I use the APC 10.5x4.5 with my Ringmaster/Enya SS30 and the larger 7.0mm venturi.  If the APC 10.5x4.5 loads to much the SS25,  try the bigger 7.0mm venturi. If still loading the engine too much cut the prop in 1/8 increments. The 10.5x4.5 works amazingly well on my Ringmaster. I like lots and lots of power and I have flown my  Ringmaster/Enya SS30/APC 10.5x.45 in some extreme and turbulent wind conditions with not too much trouble.

In the beginning I also had several problems with my RingMaster/Enya SS30 because of fuel delivery. I started using the same tank shown on the picture in the first post, which BTW seems to be mounted inverted, at least looks to me that the overflow is pointing up with the yellow cap, the pick up seems to be the working as the uniflow vent and I imagine the uniflow vent is what feeding the engine.

Chris Wilson mentioned a very important point, if the edge of the tank is out board from the NVA the engine will go lean when flying and if the speed goes up it will go leaner and  go faster and faster.

No matter how much foam or type of padding I tried I could not get rid of the fuel foaming problem with this exact metal tank. After trying many things, I found the solution for my problems: I mounted a Sullivan RST 4.0Oz tank inboard, plumbed for uniflow as per John Miller.  I use a foam cut in a wedge shape to help isolate the vibration and set the tank in an angle to give me a clean engine cut at the end. I secure the tank using Scotch  3/4" clear tape.

Here is a video of my Ringmaster, please the first time you watch this video turn the sound off, the sound will make it seems going faster then it really is. I like to fly the RM faster then normal, my laps are around 4.4s, but the important thing is that my setup is doing a pretty good job to keep the maneuvering speed in control. I can reduce the speed on the needle, but this is how I like it. It keeps the lines very tigh. Try to observe that the maneuvering speed is not so fast and under total control



Martin

Try a 10 x 4 three blade Tornado , if you can find one . The ten ton p-38 with two OS 35S drags itself around ok on them . F2B schedule ,Lotta Wt .

Thanks for all the info guys, and very nice flying Martin! (are you in the national team?!!) The issue of needle valves keeps coming up, where can I get ST nvas please? I want to use the OS35 next year and would like to try that on a future build.
Phil

Hi Phil,

I would give Randy Smith a call and get one of his NVAs for the OS.  They are a MUCH improved version of the ST NVA.  Really.

Bill