Engine varnish development

Thanks Yabby

On my part of the world fuels sold nowadays are blended almost exclusively with synt oil, so for my COX engines I must mix my own fuel to make sure it is castor based. I am not sure if the castor I buy is degummed or not.
The varnish build up is otherwise very rare in the engines that I fly exclusively in RC service, be it a TeeDee 010 or anything up to 051.
The 09 TeeDee RC acts somehow different, but its cylinder and muffler are well exposed to the air, so cooling should not be a problem I guess. Relatively new piston and cylinder combo with only a few hours runtime on it. Still good pinch at TDC, these are classic tapered old stock items...maybe it is the taper bore, combined with the Kamtechnik head and medium cold plug plus the muffler  that makes it more susceptible to varnish build up by causing some overheating??

My flights are near 8 minutes long with 2oz fuel on board, the engine mainly running at full throttle.

I am thinking of removing the silicon exhaust extender to help better exhaust ventilaton, even though I was under the impression the slanted extension tip acts like a venturi and actually provides rather depression than overpressure in the muffler...unfortunately I have timing shims for 049 size cylinders only, so removing the SPI is not an option with the 09 cylinder.

Wow, thats a really weird problem that it is happening on the 09 only in the main. I assume you use the same fuel in all of them, so that really eliminates the fuel issue. I personally think the de-gummed castor is one of those Cox model engine myths. Lol. maybe its true, I just dont know. :-) But if the same fuel in other engines doesnt cause the problem, I tend to think its not the fuel. I blend my own fuel also because it fast, easy, cheap (half the price), and I know exactly whats in it. Ive got a graduated (engraved) 1 litre pyrex container. Add 250 mls of Nitro, 250 mls of castor, 500 mls of methanol, pour into 1 litre suitable bottle, done!

SPI with mufflers is a genuine problem as it sucks old oily fuel back in. But!!, SPI on race bikes with specifically designed pipes/chambers is a performance boost as they are designed to just sniff enough of the exhaust into the crankcase to preheat the mixture in the crankcase which helps it atomize, flow, basically goes bang better! :-) But, in general having SPI with a muffler not designed for the purpose is not a very good thing. Would it cause the problem you are seeing? I honestly dont know.

If you ran the engine statically without flying in a test bench does it happen when you do that? Are you able to remove the exhaust extension in a test bench situation and see what happens, and then remove the muffler completely if necessary and see what happens.

I assume the castor is turning into varnish due to heat...? One of the things castor does and does do very well is carry heat away from the engine as it is expelled out the exhaust.

I have found I need extra gaskets with the Kamtechnic heads, which I use on most of my engines. I guess one change you could make would be to try a standard cox glow head and see what happens. Again you could do this on the bench if you live in an area where that sort of thing is ok. You would think if you have several hours on an engine, then its not the engine being too new.

A very out of the box thought. Is the crank plenty free and not too tight. A slightly tight crank will certainly produce a very lot of heat. Tight engine parts make more heat than the bang part. :-) Which is what I think you were getting at about the older tapered bore. But you wouldn't think that would cause the problem, especially after several hours of running.

Really interesting problem!, but Im sure very frustrating and annoying to you!

The only thing I can suggest and the way I would trouble shoot it is to do the following one at a time on a test bench to try and reproduce it and then fix it:
1. run the engine on a test bench with same fuel, muffler, glow, time period, etc. multiple times to make sure it can be reproduced on test bench. If you cant reproduce it on the test bench then its something to do with when its in the plane and airborne, which would be very bad for resolving it.
2. check the crank is not too tight, if you change it by putting drive plate back on with more end play, test as above again.
3. Remove the entire muffler system and see if you can reproduce it.
4. use a standard Cox glow plug
5. use a non tapered piston/cylinder combo

I very recently had a problem with a surestart 049 I newly assembled myself, and it would always get really hot and sag in revs and stop running if I tried to make it run anywhere near normal revs. Turned out that the standard Cox plugs using brand new gaskets would not seal when you screwed them in. I tried three brand new Cox plus and new gaskets. Still happened. I could turn the engine over by hand and see little bubbles between the cylinder and head. Put a Kamtechnic head on it and the entire problem went away. The head sealed properly, the engine didnt get as hot and ran fantastically.

Be very interested to hear if you are able to resolve the cause and what you find it to be.

Nice video and nice plane. One day I might try RC myself with Cox engines.

I hope I havent broken any rules by rambling on too much

Yabby

Thanks Yabby for the great advice, I will check the engine on the bench with and without the muffler, and with COX stock 09 head, to see what differences emerge compared to the current setup with Kamtechnik, muffler etc. I do not have un-tapered 09 cylinder though, and it is better to complete the experiment with the actual, tapered cylinder on the engine, because i will keep it for the everyday use .

The engine is otherwise quite healthy with no crankshaft issues like tight or loose fit. The cross hatches of honing still visible on the cylinder wall..

P.S.: There are no rules here that I would know of, restricting the length of very useful posts, so please feel free to share your IC engine experience with the team here.. If there were some rules like that, I would have been banned already with my sometimes lengthy entries where my didactic explanation is to balance and add clarity to my Hunglish