Carb assembly

I've always put them in dry. The manual doesn't call for any lubrication of the piston.

When you disassemble the carbs there always appears to be a thin grey/black oily film on the pistons, they never seem to be dry.

Hi Mike

You're right, anything on the piston (chrome part of the slide) could trap dirt and cause sticking and wear.  As Kevin pointed out the slide parts tend to pick up material after long periods.  i suspect it may be your fuel type or environmental conditions. They should be clean and dry.  I have never seen anything from Rotax or from Bing that shows any lube.  

Cheers

I'm sure this was a deliberately applied lubricant. It was not a thin film, it was only on the slide rod, it was something very slippery and the carb had only done ca 15 hrs since the rebuild (after which it had been stored for some years.)

I shall clean it and reassemble dry.

I would follow the manufacturer's documentation unless you have a good reason to deviate from it. I have run mine dry for the best part of 1000 hours without and evidence of wear on the piston.

What exactly are you referring to when you say "the slide rod"?