Yellow "Varnish"

Fuel/ Vapour stain? You running MOGAS?

More importantly, you have a leaking gasket. How many hours since your carbs have been serviced? 

Hi Byron -

Yes to MoGas

Hi Ronald -

The engine has just turned over 80 hrs total. So almost new.

Due to an issue with a very small amount of fuel being intermittently ejected from the float bowl, the carbs went back to Rotax, about 30 hrs ago. Not sure that you would call that a service but they checked them out, made a small adjustment and returned them as good.

hi Sean

Fuel stains are common.  With Avgas it is normally blue from the dye used with the 100LL and it happens all the time.  Gaskets and fuel vapor from the vent tend to give up small amounts that can accumulate.  Some aircraft the flow over the body of the carb simply dissipates it and some do not.  Your cowl may tend to create some areas around the carbs that allow this buildup.  If some isopropyl cleans it up then I would not worry.  Auto fuels have different dyes, depending on where in the world you are and the regulations where you are.  Fuel vapors commonly find the low pressure area and will evaporate quickly from the vent tube.  You could try a longer vent tube and vent them into a static air area or into a larger catch bottle with a drain.  Note that the Rotax air intake canister they make the vent lines are ported into them with small drains at the bottom of the canister.  This is very similar to what is done with the 914 vents also in that the airbox before going into the engine will be a safe place to put the vent tube.  

Cheers

Thanks RW,

The varnish/stain only appears on one carburettor, the left.

My vent lines are connected to a homebuilt air box - similar but the the same as an OM Rotax. The airbox has a drain, each side, that terminates in a catch can located low in the cowling, ot far from the cowl exit air vent.

I will see if the staining returns and how long it takes to do so. A would expect that a quick return, will indicate a leaking gasket.

It may not be related however this side of the engine has recently shown a tendency to high EGT (Question elsewhere in the Forum). In my limited experince this is as the result of a lean mixture - loss of fuel???