ORIGINAL: ameyam

What you are getting is bake-on castor varnish. Basically, when you run the engine, the fuel (which contains castor) spurts out of the carb and on to the hot cylinger etc. Since castor has a high flash-point (in other words, it burns at a much higher temperature. This by the way is why it is used as a lubricant- it remains intact and continues to lubricate when the other components of the fuel burn. That is another story) it does not burn but forms long polymeric chains that bake on to the surface in the form of a varnish. While this is normal, it may mean you are running the engine very rich.

Tuning the engine being another issue, it is normal for the brown staining to appear. Just ensure they stay brown and do not go black. When that happens, the varnish has oxidised & carbonised and will intefere with heat transfer from the engine surface i.e. your engine may overheat (though I have seen many engines operate well even in this condition). The best way to prevent this is to regularly (may be once a month) scrub down your engine with a tooth brush and soap water (you can also use the windex / Colin etc that you use to clean the airplane). Make sure you completely dry the engine afterwards with a hair drier or heat gun otherwise the MS parts will begin to rust. Scrubbing regularly only prevents the buildup of the varnish, you may have difficulty in removing it later though a wire brush and a bit of elbow-grease may help

If you already have difficulty removing the stains, just keep an eye on them. If they start to go black, you need to wash your engine. There are various ways to do that but the way I do it is to remove the needle, carb, backplate and glow plug and immerse the engine for a couple of days in plain alcohol. Some also use the crock-pot method (they boil the engine in anti-freeze). You may need to do a bit of scrubbing afterwards. Just ensure that you dont open up the head as then you may need another gasket

As always, prevention is better than cure

Ameyam

Just ensure that you dont open up the head as then you may need another gasket