...Have you had a recent 'Back-Fire' through the carb? Then check blown/Leaking Power Valve. Also check for a fouled spark plug...Intermittent or Miss-Firing. This will lead to Raw Fuel in the Exhaust. Last, the Idle 'Mixture' Screws are Open too far, And/Or, the Choke is Not Opening Fully.
Double Pumpers Always Run Rich out of the Box! They are tuned that way! In as, on 'Mechanical' Secondary Carbs, the Internal Passages are LARGER for Increased/Faster Fuel Flow, and Can NOT Be TRIMMED Down. The flow must be increased because the SECs Open almost Instantly. So we are driving around in Traffic with a Carb that is always running Rich! Changing the Jets will not make a difference, Nor any Adjustments...It's The Passageways.
In My View...they are for the Racetrack Only! A 650 CFM is NOT Too Big. And Yes! A Vac Sec of the same size Will Fix Your Problem, as far as Raw Fuel.
I took a 600 V.S. to the Desert Ran Clean NO Smells, Got Home, Put the 750 Vac Sec Back On, Now the Fumes are Back....just a Little.
The Plus is, with the Carb LESS Rich...the Throttle Response will be More Crisp!!
Be advised, there are Dozens of ways that cause UNburned Fuel...even in the Timing.
Good-Luck with It!
MJ