I have been involve in this hobby since 1976 (13 years old) and most of my life was running FAI fuel (80% methanol and 20% castor). Then I come to RCcars and here I tryed different types. The best fuel is defenetly O'Donnell. It doesn't leav any residue nowhere-cranckshafts are always clean and no rust or other spots, even if I leave engine without washing for more then month-I never clean my engines.
About build up of varnish on botton and top of piston-vernish is engine killer. It works as insulation and don't let survaces to cool down. They have to be removed as they start build up.
Top of the piston and combustion surface on botton must to become dark brown on new engine after 8-10 tanks and it should be not a build up, it just very very tiny layer. From most of the fuel I did indicated real build up and after I have tryed O'Donnell I don't have this any more, this is why I don't clean my motors-simple no need it.
It is very easy to see the difference between "good" or "bad" brown build up-"good" is shiny and "bad" is not. "Bad" build up will come out easy with just by trying wipe it off by finger and "good build" up will not come out at all, unless sand paper used.
Edward
Edward

so any brands better than o'donnel ?

don't get wrong impression I'm using o'donnel 25% and I'm happy to use it because it's crystal clear and it doesn't give me sticky or gummy leftover. The smell of the smoke is okay too, it doesn't stink my nose and eyes.

No, You got me wrong-O'Donnell is the best fuel I saw for over the many years. I am highely recomend it to everybody.
It doesn't build up any warnish, it doesn't contain any water ( like many other brands do)
Edward

It doesn't happen to my motor what you described O ' donnell did to yours'. The main bearing and crankshaft in my motor rusted badly while on this stuff after two weeks. It was a brand new motor. This stuff runs real hot, motor bogs down after a couple of tanks. It sounded rich with lots of smoke but I am still burning plugs.My motor will quit for unknown reasons. Tuning most of the time. Problem went away after switching fuel.

Strange. I believe O'Donnell only produces a slight amount of smokes even when the engine tuning is still rich...

exactly the point, you noticed only slight amount of smoke even when engine tuning is still rich, am I suppose to lean it when it is still rich ? with so little smoke running rich, will the smoke trail disappear after leaning? What does it say about its oil content? less oil less smoke.

Many times burned glow plugs doesn't mean too lean. In most cases t will need adjust combustion chamber volume. By switching fuel it might be different nitro content and it is reason why it stops byrning glow plugs-might be less nitro and it is OK with little too small combustion volume.
About rust-it is totaly strange. Are you sure you use fairely new fuel which was propoerly closed and never absorbs moister?
Edward

Edward, nitro content was the same. Motor was new , I picked up a new gallon from the same shop, can't figure how long it was lying there.

Sound is totaly creasy. I am using O'Donnell for last 6 month and went through a lot of engines and different brands-never got even small peice of rust. For 6 month I went through about 16 gallons-always everything is clean and shiny inside. Some motors been on hold for 2-3 month and ones in while I open them to see and result is just great.
Is that black or white label? I use black type label.
Edward

try to buy your fuel from a shop where inventory does not sit on the shelf. even if it is still sealed, it can go bad after a while. temperature changes will cause mositure to develop on the inside of the container (just like a gas tank). I recommend a few drops of marvel mystery oil down the carb barrel and in the head- then bump it over quick on the starter to disperse the oil in the crankcase.

We only run our engines on Runner Time now. Noticed a considerable performance gain and the Palmaris really likes it. No fancy stuff in it. Just plain clear grade AAA Nitro. Not the cheapest but worth the extra money for a gallon.

Isn't Runner Time produced by the same company that makes Tornado fuels?

BTW, I think Cosmo RC fuel is only available in HK and mainly in Japan if I'm not mistaken... Good stuff apparently.

Is it okay to switch fuel ? my LHS don't have o'donnel fuel at the moment and I'm running low...... I don't want to borrow fuel from him. Is it okay if I switch to Tornado fuel ? Both are rated 25% nitro. Any precaution ?

Hey all, You are all talking about good fuel and I am surprised that no one has mentioned Wildcat fuels. In my opinion the best.
I have been running 30% in an RB x-12 Rody for probably 60 races. The engine always starts instantly NEVER flames out and has been very fast. I have only gone through 3 glow plugs in a year and a half.(MC-59) favorite none turbo glow plug. Until last week I had never taken the engine apart, and I let it sit for 5 months last winter with no after run oil and it has been sitting this year since October. No rust and only had residue on top of piston. It will take something very drastic to happen for me to switch fuels. I really enjoy not having to worry about my engine.

That's because not all fuels are accessible to everybody around the world. In the asian market, I don't think anybody has tried Wildcat fuels. Perhaps that unfortunate.