Carburettor O rings

This sounds like poor quality "O" rings to me that are too dry having the elasticity component diminished hence the cracking either that or they are old and therefore knackered.
With good "O" rings a light lubrication is all that is required.
Try putting them on from the pointy end this always works for me running them over the threads will not damage them in any way.
 

Use some rubber grease, put on in the direction Paul says and then if you have one use a blunt seal pick to lift a portion up level with the larger diameter and then use fingernail to tease it over all the way around. In over 30 years I have not had a problem doing them that way and not known anyone else having issues fitting them. 

... I've always used sharp ones with no ill effects that I can see. A small hole in an O ring isn't going to let anything appreciable past it.