This past weekend my OS 40 Surpass lost power after flying over 5 minutes. It continued to run at what I'd call an idle and stayed in the air over a total 12 minutes. The history of this engine is sketchy; I inherited it from a friend that had acquired it as a used engine. He said he never had a problem with it. I have been using it on a 54" 50? ounce Prowler profile with a AFC 11x5 prop. I've been using 15% N with 23% 50/50 oil fuel. From the beginning I've used a plastic 4 oz tank set up for uniflow pressuring the overflow, not the vent. There has not been one hiccup with the engine since I began using it in early 2025, It's become my favorite go to model and engine.

After returning when I started it, it would not stay running. I tore it down and noted the valve lash was set correctly, TDC, cold using the feeler gage provide with the engine. The rear crankcase cover was secure, and I do not note any wear in the main bearings. The carb throttle is wired open as it has since I began using it. The carb interior is clean and needle valve threads in and out and fits securely to the engine intake side of the head. I did note a good deal of carbon build up on the exhaust valve shaft and on the interior at the valve exit. I removed the valves cleaned and attempted to lap the valve seats using 1200 paper. Pushrods appear fine and it appears the cam is working fine. I reassembled the engine with oil and restarted it while changing the tank pressure to the uniflow vent and capped the overflow. I should add I did not pull the piston, but I noted the sleeve looks, now evidence of overheating or scoring. I will add the compression is weak even after I placed a few drops of oil in the cylinder.

 It would start easily and would continue to run but it appeared to be well underpowered and would not run smooth. I change to 15% 4 cycl fuel with little castor and installed a 4 cycl plug in it. It ran a full tank, the NVA adjustment was sensitive in that one click to lean would kill the run. I also noted it was blowing wet fuel out of the exhaust. In the past the engine exhaust would expel a minimal normal amount of oil.  Any thoughts?

Just a thought.

The OS FS engines, per the owner's manuals, are designed for 20% total oil content in the fuel.

I have been running VP 10% and 15% AIR fuel (18% oil - blend 80% syn/20% castor) with a bit of light syn added to boost the total oil to 20%. I have been running this fuel on OS and Saito FS engines without issues (so far).

I would think the 23 % oil (50/50) is too viscus not allowing the engine to be properly lubricated, over time slowly leading to abnormal wear.

I am also not a fan of starting a FS at full power (CL application). I have been using the stock carb and prefer to start them at say 1/3 throttle for a brief warm up period at low speed, then go to full speed for launch. I have my FS's set up with a manually adjustable throttle system. I know a lot of pilots just wire the carb full open or set up the engine with a venturi, but I prefer to not run my FS's that way. Just my opinion. 

Colin

Colin,

I've ordered a replacement head and valve assembly from Bob Brooks. I have a strong suspicion it is a burned valve and damaged seat problem causing the poor compression, low power and heavy wet exhaust. I feel this because after attempting to lap the valves the run did improved slightly, we'll see.

I agree with you on the fuel, so in the future I'll only use the near full synthetic with minimal castor oil. This should also help eliminate the heavy carbon build up on the exhaust valve.

I'll let you know the results.

Steve

Steve,
Do you have contact information for Bob Brooks?

Best,   DennisT

Lapping valve seats is not done with paper. Plenty of information about valve lapping on the web. This would be the first thing to correct. Leaky valves will cause a lot of trouble. If you burned a valve in a .40 glow engine, it would be the first.

Too much oil isn't a problem except for coking and the required cleaning. If you were running it rich to set RPM, this makes it worse. Odds are carbon wasn't allowing valves to seal. Too much oil won't cause a lack of lubrication in these. Excessive blow-by from the case breather could indicate a stuck ring.

Quote from: Dennis Toth on May 23, 2026, 11:16:30 AMSteve,
Do you have contact information for Bob Brooks?

Best,   DennisT

shtterman@aol.com

(954) 234-0863

Colin

Greg,  I knew trying to clean up the seats using paper wasn't an effective way to lapp them. I'll check out if I can find something here. There isn't much to hold on to if I want to attempt to rotate a valve (1/4" diameter face) and I would think standard lapping compound would be too coarse. I did notice when priming using my finger over the carburetor I felt a pressure?? Would this indicate a leaky intake valve.

Here it is, the method to reseat the valves...
You can reseat the valves with Dupont #7 white compound. I put a little compound on the valve, put it in the head, take a piece of fuel tubing and put over the valve stem, chuck the other end in an electric drill, and with a slight tug on the drill spin the valve. It will clean up the seat and the valve for a nice seal.  Be sure to clean all the compound out before reassembly. Pipe cleaners work well on OS valve guides.

Now my question is do we use DuPont's No. 7 Rubbing compound or Polishing compound?

Update after polishing the valve seats:

Greg, Spare Parts,

I decided to try lapping the valves using the Dupont #7 white polishing compound and discovered the exhaust valve is leaking. To verify this, I placed the engine in a stand and using machine oil I flooded the valve guide tube. Holding my finger over the exhaust outlet, creating a back pressure in the cylinder I discovered on the compression stroke air bubbles escaping in the oil around the valve stem. The intake was not showing any leakage.  This could be a problem with the valve seat or the seat in the head. I noticed there is a thin circumferential groove in the valve seat, no doubt from contacting the seat in the head. It appears the valve and possibly the head is compromised. Do you agree?

Steve

I wrote this last night and forgot to post it.

White works if it's carbon or varnish, real damage will take a long time. I've used the orange rubbing compound also. OS is a bit tricky because the valves are softer than the seats, at least in the few OS I've observed with wear. Sanye (ASP, Magnum, and related) and Enya seem to have harder valves than seats. OS makes more sense in terms of service for the user, in that new valves would refresh the engine, whereas a new head is costly. Hard to say what really caused the wear as there were all second hand engines. Occasionally, valve guides can wear enough to cause trouble with valve sealing, I've mostly seen this in Enya. Or, like the Magnum FS-30 I have, was supplied from the factory loose and never sealed correctly to begin with.

I use permanent marker to check contact. I made mandrels to lap seats instead of using the valve itself. Use a loupe to inspect the seat and valve. Once you have a groove it's likely the mating part is not in great condition either.

Wish I took a photo of the valve seats that appear to be a brass(?) sleeve into the head that has been chamfered on the edge to mate to the chamfer on the steel valve. The white compound is not very aggressive, so I did apply some finger pressure while spinning with the drill motor several times and replacing the compound. The surfaces shined up but I don't think any material was removed to reseat or improve the fit. I was concerned about removing the material. The valve stems in the guides appear to have no play and they have been clean up removing carbon using 1200 grit paper. I did notice some improvement in compression but it's still inadequate with leaks as described. I'm now looking for a replacement head and valve assembly. New valves alone seated in using a more aggressive compound might be the answer, however.

Steve

Most (not all) OS have bronze seats cast in to the head. OS guides hold up well.

At some point, you have to decide to repair or replace.