Hey Blue,

Yeah I just used a vacuum hose, like Chuck said and you can definately hear the differenace.

Be sure the engine is not backfiring!

I'll try the 56 mm.

Chuck, 72 onwards (last year for zeniths?) they removed the vent and replaced it with a bolt & nut.

From http://www.jaimekop.com/CarbManual/Page04-05/index.html
"The venting valves were eliminated on 1972 models in order to conform to federal requirements. Only internal venting is provided on these models. "

Thanks for everybody's help.