ORIGINAL: opjose
While it is almost unavoidable to get Castor stains on engines, you DO need to be aware that stains can point to a problem that newbies tend to overlook...
Namely that cap screws tend to loosen when the engine has been run for a while.
That lets the castor seep from the engine/cylinder head and other areas.
If you see castor stains on your engine you are advised to run the engine until warm, then remove one cap screw at a time, put thread lock on the screw ( after cleaning ) and partially re-tighten them down. Then torque them back down evenly alternating screws to get a good seal.
The engine should NOT continue to produce castor stains unless you spill fuel on the engine itself.
So DO pay attention to castor stains, they may represent a problem you may have overlooked.
Excellent point
and indeed its always wise for every one to check head tightness form time to time but especially when the engine is new even when one is using fuel with no caster and without which the varnish cannot serve as a pointer to a loose head.
Just one more additional thought. When an engine has been carelessly stored and it has used fuel with 'some caster' this same varnish which yes of course will gum up the moving parts but at the same time this varnish can serve as a preservative and even minimising the occurance of rust. Freeing up a gummed engine is not a hugh issue and can usually be done with the application of heat in various forms.
John