Colin,
The OS 26 with pull ships like a strong FOX 35, 45 - 48" span, 40 oz ish weight. I had mine in a TF Tutor (original version) and ran 10 x6 prop, 10% nitro, 22% total oil (mostly synthetic). From my notes you want a wide blade prop (like an RSM or BBYO) around 9900 rpm. I needed 2 3/4oz of fuel for the PA pattern. Use a tank tall and skinny clunk works with the fuel pickup tube being 5/32" OD and drill out the cluck to 5/32" (this gives better fuel draw), use large fuel line and wire onto fuel nipple. Set the centerline of the tank inline with the center of the carb/venturi to start and adjust up/down to get equal lap times. Set up vent for standard vents (not uniflow) and muffler pressure.

Best,   DennisT

Colin,
The tall tank is just the regular cluck tank stood on its side. This minimizes change in fuel head being about 1 1/4” vs. 2 1/4” if set on its bottom. 4strokes don’t have as much pumping fuel draw as higher rpm 2 strokes that’s why they usually run better on conventional venting. One tip is to use 5/32” fuel pickup tube (drill out the cluck too) and larger fuel tube .

Best,  DennisT

Colin,
Change out the fuel pickup line to 5/32" (copper is best for nitro) and drill the cluck tip to same.

Best,    DennisT

Colin,
Due change out the fuel pickup line to 5/32" (copper is best for nitro) and drill the cluck tip to same.

Best,    DennisT

Thx Dennis, will do!

I used one of mine in a Sterling Mustang.
Flew great, took home many trophies in OTS and classic stunt.
I once beat a Super Tiger 60 powered Cardinal with the Mustang.

Bob Z.

Colin:
Don’t worry about competition - just go out and get as much enjoyment out of the sport as you can.
If you do decide to compete, you can take advantage of one of the four skill classes.
The important thing is to have fun!

And, keep us posted on your progress with the 4-stroke engines.
Keep in mind that the Sterling Mustang and Yak 9 are very close in performance.
Build them light and if you want, make the flaps non-moveable. I’ve done them both ways and couldn’t tell the difference.
Bob Z.

Colin.
For the short tanks the Sulllvan tubing is very thin walled and flexible but surgical tubing will be more flexible for the 2oz tank. Other option could be a round tank with a fixed copper tube and conventional vent pointed to the inside of the circle side of the tank, just make sure you mount the air side of the vent about the tank and on the inboard side of the fuse (this gives clean airflow to the vent).

Best,    DennisT

Colin,
Yes the Sullivan tubing is thinner walled and more flexible than the Dubro. Again use the 5/32" copper tube and drill out the cluck.

Best,   DennisT

I have a number of planes flying with the Surpass 26, hundreds of flights.
One plane flys with THREE of them.
I’ve had very good results with a plain VECO two-ounce metal tank, non-uniflow, over/under venting, no muffler pressure. The engines are profile mounted.
Up to now, I’ve been using Brodak 10/23, 50-50 castor/synthetic but I plan to mix up some 20/20, 18/2 synthetic/castor to try out.
The Surpass 26 is a very good four cycle engine, requiring minimal fussing and maintenance.
The OS four cycle plug works perfectly.
Best of luck!!
Bob Z.

I still have a question in the OS 26FS relating to fuel pressure (muffler pressure) versus standard over/under venting for a full speed CL application. A few have said they use no muffler pressure. However, the FS26 owner's manual shows the recommended fuel system set up and shows muffler pressure applied to the fuel tank. It seems muffler pressure just enhances fuel draw.

I plan to set up the carb to be about 1/3 open to start and allow for some brief warm up. Then ground adjust to desired full speed before takeoff.

The 26FS is basically designed as an R/C engine. Maybe fuel pressure not needed for a fixed speed CL application?

Thanks Dennis.

Concerning the speed adjustment, I got this idea from a flying buddy who has a few CL 4-stroke models. I installed a simple aluminum gear (a part from a hobby shop intended for an RC car) on the carb. The gear teeth are in contact w/ the choke rod and keep the carb where you set it. So I can set the carb partially open to start. Then rotate gear to full open before takeoff. My opinion only, and maybe not necessary as you say, but it is not a bad idea to start at a lower speed and allow for some brief warmup. With all the FS moving parts (compared to 2-stroke) a brief low speed warmup can't hurt.

  To add to what Bob is saying, an R/C carb that is wide open is way bigger than what a restrictor venturi with a spray bar through it would be and add to that all the clearance around the throttle barrel passing air. Closing down the throttle decreases the opening and adds some choke to help with fuel draw, and 4 strokes are all about fuel draw. I would test it at about 1/3 closed, and run muffler pressure. In my limited experience with both OS and Saito four strokes, this has always worked the best for me. It's more or less the "Bob Reeves" philosophy or running a 4 stroke engine and has worked pretty well for me. I applied what I learned running and flying my Saito .56 to an OS.70 and it responded the same as the Saito did. I had to use the carb closed off, as no one made any bolt on venturi for C'L stunt for it. Flying four strokes has been fun and is a viable power plant, and I hope to get my Top Flite Score recovered soon and get back to it, and maybe a couple of other models in the future, but I haven't sold off any of my 2 stroke engines just yet!
  Type at you later,
   Dan McEntee
 
   

Dan, not sure I fully understand. Are you saying you have had good luck with 4-strokes (on a CL application) with the throttle maybe 66% open (to simulate a venturi restriction), then adjust the needle for desired full speed?

Or if the engine is not running well at full throttle (due to air leakage), test and start to close the throttle to see if the engine performance improves?

Thanks in a advance.

   That is exactly correct. Search out threads on here by Bob Reeves. He and some others did a lot of research and experiments into this. You could also go to the Tulsa Gluedobbers web site and a lot of it is posted there. Bob has since passed away but he left a lot of information behind. Bob advocated for the low RPM/high pitch prop method of running 4 strokes. If you happen to have a C/L venturi on an engine, he came up with the idea t add a screw to the throat of the venturi that goes in on one side only, and by running the screw in and out, you effectively change the venturi choke area. Running it in makes it smaller and draws more fuel, out makes it bigger and draws more air. Not really a new idea, the speed limit combat guys have been doing the same thing for a long time to get their engines to run a constant speed.. As you turn the screw in, you turn the needle in next to get a max RPM setting just above 8000 RPM. That is what he said to do for a Saito 56/62. props are in the 6 and 7 inch pitch range. If the weather gets warmer, you can screw the blocking screw out just a bit to let more air in and reset the needle. When it gets cold out,  you do the opposite, screw in in to let in less air and reset the needle. It will kind of go against everything you already know about engines and props for 2 strokes, but it works. With an R/C carb, the barrel takes the place of the screw. Just play with it and get familiar with it. For a 4 stroke that small, i don't know if you need a new RPM range or not, but just experiment, go by lap times and line tension. before you try any maneuvers. The engine might not sound like it will pull your hat off your head, but just ignore that and go by lap time and feel. This set up with muffler pressure will make the engine run very consistently where ever it is flying. You will have to get used to that and if you have been flying 2 strokes for any length of time it will sound foreign to you.   The 4 stroke is a different animal, and resist all temptation to run it like a two stroke. This low RPM set up lets the engine get plenty of fuel, and thus it runs cooler. I have run several gallons of fuel through my Score and Saito .56 and all of the chrome is still shiny. I ran all of this by Gilbert Berringer from France. He is one of the top stunt pilots in Europe and helped bring the 4 stroke engine along. He has a large manufacturing company that makes aircraft wheels and brake systems, and comes to Oshkosh every year, along with Sun and Fun in Florida during the winter. He flew my Score several times and was impressed by it. He sets his engines up with a very small venturi to start with, no bigger that 6mm, he told me, and when you put a spray bar through that, the choke area is pretty small. I run Powermaster YS-20/20 for the most part, and it's getting pricey now, but i only use about 3.5 ounces per flight, so it lasts a while. Try some of the Powermaster synthetic blends in the 15% range. It would not hurt to add some extra synthetic oil to get the total oil content up closer to 20% like the YS blend. I have been trying not to add any castor, as there are a LOT more and smaller moving parts in these engines, and even smaller yet for your .26. If it sets up and runs the lower ROM ok, it might not be necessary. It will take a lot of attention on your part as you go through the motions. Everything from starting them to picking props and setting the needle will be different and again, the big thing is DO NOT run it like a two stroke!! The closest thing I have to your engine is a Saito Black Knight .30 but I have yet to pick a model for it.
  Type at you later,
   Dan McEntee

Colin,
Dan gave some very good information. Here is an engine review of the OS 26FS Surpass (http://sceptreflight.com/Model%20Engine%20Tests/OS%20FS-26%20Surpass.html). I like to run mine in the 9500rpm range and use a higher pitch (like 7") with a wide blade, diameter around 10". The point Dan made about the sound is important. With the muffler it is like flying electric, but the note is a little flat compared to 2 stroke (think Harley vs, Yamaha). You will notice that you may be flying faster lap times with the 4 stroke and but it will seem like it is slower because of the sound.

The Yak is a very good ship for this size engine.

Best,    DennisT