The "sweatiness" of the polyurethane tubing may not be as bad as it seems. As far as I can tell (Grant, you know more about this then me

), the plasticizer most likely to be used is a phthalate, either butyl benzyl phthalate or di-2-ethyl hexyl phthalate. These are used in plastic manufacturing to allow the molecules to link to one another but allow them some movement, sort of like a lubricant. Excess plasticizers are always released from plastics. That's why new PVC has such a strong distinctive smell, that's the volatile plasticizer evaporating.
Anyway, all plasticizers are highly soluble in just about all organic solvents. Petrol/gasoline is a typical organic solvent. I have to dig out my extra polyurethane tubing and see if it's sweaty, but I'm betting that all I have to do is run some petrol (or some other solvent, alcohols should work just as well but be less dangerous) (and cheaper too!

), wipe off the outside, and the tubing will be just fine.
If the plasticizer does get carried into carburetors and somehow get deposited, it should be washed off immediately with a fresh charge of petrol. The small quantities involved should not have any effects on combustion either.
Ken