Back in August last year it was a warm evening, maybe 80 degrees or so, I started my OS40FP-S and set the needle for 9,510 RPM with a Top Flight 11-6 wood prop and Omega 10/22 fuel. It ran fine as usual until a few minutes into the flight when it very suddenly bogged and sputtered around the circle so slow that I could hardly keep the plane in the air (a Mathis Stuka profile). This had never happened before.

Next day I flew it again, with a new OS#6 A3 plug and different fuel from a newly opened jug. Also replaced the fuel tubing, tightened the klunk tank stopper, and checked backplate, muffler and engine mount screws. It then started easily and ran fine except I could not get the RPM higher than 8,800 RPM! And subsequent runs have been the same. Anything above that it goes lean before it hits 9,000. It also seems to make a crackling sound which it never did before. This engine has only 77 flights on it, I bought it NIB and broke it in very carefully (at least I thought so). It had always been a good consistent runner.

What is going on with this engine??? Did I wreck the piston/sleeve fit by setting it too lean on a hot day?    What else could be wrong?

Obviously I am not an expert on engines so I am relying on you guys for a diagnosis, any thoughts or advice are appreciated, as always.

Thanks,

Darryl Wurtz

Hey Dan, thanks for those insights and yes I forgot to mention that the compression still seems to be normal as far as I can tell. The varnish thing had not occurred to me but that makes sense, and I do have the APC props you mentioned on hand so I will definitely give those a try. Bench run won't be for another month or two though, we are in the dead of winter here and I won't even mention how cold it is.

BTW off-topic, but I sure enjoyed that video of your Cox Stuka flight!

Darryl

Test is ' leak down ' . fueled , put it at top dead center. or the piston right up. even .

Turning it after 5 or 10 seconds , it should have compreshon still, assisting the piston down . if idll hold it for 30 seconds , life is marvelous .

Yep. I think the props the problem .  4 inch pitch is the go on the FPs , or otherwise close .

Some say ' the crackling ' noise , is when its ' right on ' and going bl**dy marvelous . cept with the 6 in pitch youll be at 150 % speed, maybe .

If it flicks ove bouncey , finger & thumb at hub, too. Youve nought to worry about .

Cheap & nasty fuel can wreck them , but getting a bit hots not the end of the world .
Do some seaches on " FP 40 " too.

Have you checked for junk in your spray bar?

Last night I did the leak-down test and it holds compression for 30 seconds +, and it does feel bouncy when flipped. Next, I checked the spray bar, couldn't see anything but flushed it out anyway. I found the instruction sheet that came with the engine and I see it recommends on O.S. #8 plug, which is medium hot so I will try a cooler glow plug and also maybe going to 5% nitro as well.  Found 9 inch and and APC 11.5x4 props in my prop stash.

I will have to wait for warmer weather to bench run to see the result, but thanks again guys for all the comments and tips, much appreciated!

     I understand the waiting for warmer weather. I shoveled and moved snow this morning in -2 degree weather!!   You could take the engine apart and clean everything with some solvent and scotch brite or steel wool, but running it with some good synthetic oil fuel a few times at peak RPM a few seconds at a time with the smaller prop does the same thing if you are not comfortable in working on the engine. It's been discussed a few times before over the years and is nothing new. The hot plug you were using should not be a problem either. It may have been damaged in that runaway you had and could be the root of your problem now. Try the OS #8 during the bench run, but don't be afraid to go back to a new hot plug like you were using. When you over load the engine with too much prop and it over heats, it leans out and gets hotter. If there is any carbon build up in the combustion chamber , glow plug or on the piston crown, it will glow red also and cause the cackling and preignition that has been mentioned.  When the weather gets hot you can go back to the 10% nitro also. Running the flatter props at a higher RPM does take some getting used to. Your flight will go something like this if everything in correct.  Start engine, set RPM in the 9000 to 9200 range, when you launch the airplane it will accelerate out, the engine will unload and the RPM will jump a bit, and by the time you get a lap around again centrifugal force will increase the pressure of the fuel feed and the engine should go a bit richer in flight. Check your lap times or have someone else else check them to see if you are where you are comfortable and have some good line tension. Depending on the plane, and line length, anywhere from 5 seconds to 5.5 seconds is pretty common for most people. You can fine tune from there. If all is right and tight with you fuel system, you should here a slight break in the engine during maneuvers. If your are happy with the result, fly the snot out of it. It can probably be fine tuned depending on what you like. At this point getting some more help will be beneficial, but you should have a pretty good base line now for what the set up should be.
   Type at you later,
    Dan McEntee

Last night I did the leak-down test and it holds compression for 30 seconds +, and it does feel bouncy when flipped. Next, I checked the spray bar, couldn't see anything but flushed it out anyway. I found the instruction sheet that came with the engine and I see it recommends on O.S. #8 plug, which is medium hot so I will try a cooler glow plug and also maybe going to 5% nitro as well.  Found 9 inch and and APC 11.5x4 props in my prop stash.

I will have to wait for warmer weather to bench run to see the result, but thanks again guys for all the comments and tips, much appreciated!

      If it has compression and moves freely, there is almost certainly nothing much wrong with it internally. I would look at leaks around the head or backplate, or clogs the intake system. At most I would clean it in case of varnish (crock pot/antifreeze, NOT with steel wool, or anything abrasive) but that seems unlikely using that fuel. 

     If a particular type of plug worked before whatever happened, it will work afterwards, too.

    That is more pitch than is wise for performance reasons, but it won't damage the engine - I assure you I and many others have used much more demanding props (in my case, a Rev-Up 13-5 (really 4.5) at "incandescent lean" settings) and while it wasn't great to fly, it certainly didn't break it.

    It's something very simple, but that doesn't mean finding it is easy. Clean the needle/spraybar, check the backplate and head screws (that they are *snug*, don't crank down on them with a cheater bar), put it on the test stand with a 11-4 or something like that, and fire it up. I would do it ASAP so if you have to keep going you will know it, but, to be honest with you, if it was cold out, I might be able to other things to do.

     Brett

An 11-4 wood prop, as Brett recommends, is much safer than trying to run a .40 on a 9 inch prop.  Sure, a 9" will unload the engine, and allow it to run high RPM to clean out carbon.  But a 11" low pitch is just as good, and it will not fracture your finger, in case you don't use a Chicken Stick.