OS FS 91 SII w/ pump question:

Engine sounds fine on the ground. idles nice and accelerates responsively. In the air its a different story. when cruising level flight at quarter throttle the engine seems to be running fast. I throttle up to 3/4 and pull up the engine sputters and RPM / sound of the pitch drops. worried the engine will quit I level the plane and the engine sounds fine at 1/4 throttle. I have the engine carb set at default. the Low at 1/8 turn richer. Am I running Rich or Lean? when the plane is cruising at 1/4 throttle not much exhaust smoke. when I throttle up I see a lot of exhaust smoke and sputtering sound. The engine does fine on the bench. I point the engine up and it sounds good. I'm turning a Master Airscrew 14x6 K series and Cool Power 20/20 4 stroke pattern fuel.

It sounds like the high end is a bit lean.

The plane leans out in flight versus what it does on the ground.

Sounds like it's just a bit out of tune but being a pumper there can be a couple of other little problems depending on the age and condition of the engine. If you go into the glow engine forum there is an OS site for factory support, you can ask Bill Baxtor. If the engine is used and been stored the diaphragm can be a bit hard or there can be some castor in the pump. Just little things but it only takes something like that to jump up and bite you. Bill should be able to tell you or give you some ideas to try. When in doubt I always ask Bill.

I use the same combo on allot of my 60 size warbirds, some 14/6 some 14/8 Mas props, there great little engines,

I know some will cringe but take your tach and run the engine up to max rpm adjust it with the high speed needle so your peaking you should get about 9700-10100 rpm give or take a hundred or so, then richen the engine up about 3-500 rpm, on my engines with low run time 500 on the ones that have run a while 300 is ok

when in flight the engine will unload the prop and there will be plenty of power,

also before I ever run my OS 4st engines even new out of the box, I check the valves to make sure there correctly adjusted, none of the brand new ones have been out, but a couple that I have bought off ebay were way out.

after some tweaking there solid runners, its a once a year thing so I do mine at the beginning of the flying season and there good to go, most don't even need to be touched if there low time motors, but if the valves are out they can affect performance.

The engine is back on the bench for testing tomorrow morning. Low needle is set 2 turns open from closed (factory default) and I'll dial in the High and slightly richen it. I'll recheck the rocker clearance before I do this of course. I'll take a quick peak at the pump too.

The pump is easy to check but the diaphragm is a problem when it starts going bad. They last for years but if the engine has been sitting for years it could have gotten hard or gunked up. It's also one of those things that OS will not sell you, it's a send it in and we will charge you through the nose to replace. Those two screws and spring are beyond us to install correctly.

In my experience using these engines:
If it's new or nearly new you just have to set it right meaning needle adjustment.
If you bought it used and it has sat around for awhile on someones bench it always seems to be the pump and as before mentioned in this thread I agree that the only thing you can do is send it in to have the pump repaired/replaced by an authorized OS engines repair facility.
I steer away from buying them used because they always seem to have pump issues.
No more OS pumped engines for me.
Good Luck!

The pump is a replace only part. EXCEPT if you know a friend of mine here on RCU that decided buying a new pump for there price was silly and was able to take one of mine apart and replace the pump diaphragm with a home made one. My old 1.20 pumper works great again. I {Me} poked a hole in it getting out some old gunk. If the pump is pumping it's good. Just remove the glow plug, place the main fuel line in some fuel and roll the engine over, it should pulse out with some force. It really sounds like the engine is just out of tune but the pump and regulator can cause a problem. This is more of a heads up warning of something it can be. If you still have problems then Bill Baxter is the guy to talk to in the glow engine forum.

I had a friend tach the prop and it was turning at 9000. To me it sounded really high and fast but to him he laughed and said " you are running rich". He got the engine to its max 10200 and he richened it just slightly. I gave that a go and took her up. Wow - what a differance. Smoke trail has become a minimal. I'm just worried about that. Also, where the High needle is set at is almost half turn open from fully closed. Is that too lean? But maybe its ok for this engine?

4strokes are a bit different than 2 you dont need a bunch of smoke out the pipe and your running high nitro fuel, I run mine on 10-15% and they run great on that, the pump really gets the fuel to the engine so you need to adjust it so the engine is running at optimal RPM's like you friend said.

a OS 91 4st engine running strong at 9700 RPM's is just about right