By Mike Bumbeck
2/8/05

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?

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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?