By Mike Bumbeck
For those of us who like to turn
screws instead of messing with cords, laptops, and electronic
fuel management systems, there's no more tried and true form
of fuel delivery than a good 'ole Holley-style carburetor.
While much has been said about the legendary double-pumper,
the purveyors of lore often forget about the little brother
of the 4779 - the vacuum secondary 750 CFM 3310. The beauty
of the 3310 is the vacuum secondary circuit, which when calibrated
correctly, delivers fuel and air according only to engine
demand. In theory it's a perfect system - the right amount
of fuel and air whether cruising down the boulevard or motoring
down the track.
In practice things work out a little differently. The spring
that controls the secondary rate of opening is difficult to
replace, requiring removal of the vacuum housing itself to
access the screw that lies under the lip of the main body.
A quick-change kit helps, but you still have to remove the
top of the vacuum housing to switch out springs. Another drawback
of the 3310 is that the secondary metering plate is non-adjustable
when it comes to that staple of screwdriver fuel delivery
modification -- jet changes. It's not difficult to hit the
fuel delivery limit for the secondary circuit and since there's
no adjustment you're seemingly out of luck. Or are you?
It's time to take that old 3310 back off the shelf or out
of the box and breathe new life into it. If you don't have
an old core around, the good news is that you can barely walk
three feet into a swap meet without tripping over a blanket
or tarp covered in old carburetors and they can usually be
had on the cheap. When you go to pick one up the very first
thing you should look for is excessive play in the throttle
shaft assembly. Worn or bent throttle shafts and butterflies
can cause a myriad of vacuum headaches and are a good sign
to throw that one back into the pond. If the throttle shafts
and butterflies look good, then do your best bargaining and
get that baby home.
The goal is to increase performance and flexibility by taking
a three-pronged approach. The first is to increase airflow
and throttle response by replacing the carburetor body. The
second is to add jetting flexibility to the secondary fuel
circuit. The last is to add external tuning adjustment to
the secondary opening rate with the installation of an externally
adjustable vacuum secondary housing. With the addition of
the metering plate jet swaps will be a few screwdriver turns
away and dialing in the right amount of total air-fuel delivery
according to engine demand will be as easy as turning a screw
on the external vacuum housing.
With a few hundred bucks worth of parts, a lot of cleaning,
scads of scraping, and hopefully not too much swearing, you
can build an extremely versatile carburetor for use on the
street or out at the track. With some real world tuning this
carburetor will be there through cam changes, head swaps,
header installs, exhaust reconfigurations, and even engine
swaps. And besides…what else are you going to do with
all that snow out there?