Zeniths can develop 'body warp' that then need rectifying by dismantling and rubbing down, new 'Zeniths' are sometimes copies made overseas of varying quality. Webers allegedly give better economy (not by much) but this could possibly due to (some say) strangling the engine. I have a Weber on my lightweight and it goes OK. ACR do a SU conversion which could be worth considering if you have the cash.