In my experience, smelling fuel during a climb typically means that one or more of your carb floats is overweight. This causes fuel fumes to vent out of the carb bowl vent tube and into one's nostrils.
When new from the ROTAX factory, the carbs are equipped with thin cork bowl gaskets. These cork gaskets don't seem well suited for repeated use and can be awkward to properly reinstall. ROTAX parts dealers (e.g., Lockwood, CPS, etc.) supply a thicker fiber-based gasket which works well -- no leaks for me.
I have heard that the rubber (silicone or neoprene?) gaskets are not happy with mogas/ethanol fuels.
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B) - N124DH flying since March 2014 - 560+ hours (as of Oct 2017)!
through June 2018.