Hi everyone. Just started building a “Blue Pants” for classic. Originally planned on a PAW .19 but I scored an OS 20fs today. I’m thinking I’d like to use this engine on it. Question though, is it worth trying to find a Venturi for it or will it work ok with the throttle locked open? It works fine on smaller 2 cycle engines I’ve used in sports models. Will a 4 cycle still start ok with the throttle wide open?

Any advice would be appreciated. Thanks.

I had some videos on my ringmaster with the FS20 posted around here somewhere. Plenty of power.  Very worth the conversion.  The black master airscrew 9x6 is the magic prop.

Hi Daniel

I have used one in a Veco Tom Tom and a flite streak, 9 x 6 or 9.5 X 4.5 prop, after many hours I had to decarbonise the head, almost totally filled with carbon at the exhaust port, fitted new bearings while in there, great engines, ridiculously economical.

It may have been the first foustroke flown in stunt at a NZ Nationals many moons ago

I'm not sure if I'm one of those eggheads to whom you're referring but if so, your comment is total BULLS***. If not, let's press on.

The OS 20 FS is a great little engine but there are a few shortcomings. 

The main problem is the fit between the throttle barrel and its respective bore - I have measured variances between .001 and .0015. This may not seem like much but it allows enough of an air leak to cause erratic running.
Here's my cure: I rotate the barrel 180 degrees and then gently tighten the stop screw to lock it wide open.
I then remove the throttle arm and replace it with a large washer. The washer is sealed with RTV and leaks are eliminated. No more erratic runs.
The second item of importance is the valve adjustment - it is VERY CRITICAL, more than any FS I've run. A thousandth too wide and you'll notice the difference in power. 
Regarding mufflers for the FS 20, I bought both of mine new and neither came with a muffler. 
The 26 comes with one and, as I recall, they refer to it as a "pressure adapter" or something of that ilk.
The problem is, the outlet hole is .143 inch diameter, yielding an area of around .016 inch squared.
This pressurizes the tank nicely but at a noticeable reduction in power. 
I open it up to .185 inch which increases the area to around .027 inch squared.
The power increase is quite noticeable and the runs are perfect. 
I have used the 20 with and without mufflers in a number of aircraft including the Ringmaster, the Sterling profile Mustang, the Flite Streak and now my (soon to be renamed  ) Cardinette.
Profile mounted, both the 20 and 26 work well without a muffler. When inverted, the muffler (and pressure) have yielded the best results.
I have always run non-uniflow metal thanks and I've been pleased with the results.

In closing, I will state that this short article relates some of my experiences with the 4S engines I've been using off and on for the last 20 years.
Your results may differ and I'd welcome comments.

Bob Z.

COMMENT: the Surpass 40 is THE WORST engine I've ever owned!!!!!