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Performance & Hotrod Business May '14

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70 n Performance & Hotrod Business n May 2014 or 800 CFM on a small-block Chevy with a 305-ci engine. Just because the factory 305 came with a Q-Jet spread-bore carbure- tor, they think they will use a similar size. This never truly works, as there is a huge difference between a square-bore vs. a spread-bore carburetor. It is easier to choose a smaller carburetor and then richen the calibration for your particular cubic-inch engine combination. It is easier to change the calibration to make your carburetor richer than to have a larger carburetor and try to make it run and perform better on a smaller engine." Tom Kise gives us more little known information: "You can use a vacuum sec- ondary carburetor on an engine with low vacuum. Vacuum secondary carbs operate on air velocity through the venturi, not on engine vacuum. A stock engine like a high-performance engine will have close to 0" HG vacuum at wide-open throttle. The air speed through the venturi creates the lift (vacuum) required to open the second- ary throttle. When dealing with a vacuum secondary carb, keep in mind that they will not open in park or neutral. The engine must be under a load. Place a paperclip on the secondary rod horizontally and slide it up to the base of the secondary diaphragm housing. Drive the vehicle under full load and check the position of the clip. If it is pushed down they are working. "Also keep in mind that they may not open if the carb is too large for the engine or if the spring is too stiff. To increase the secondary opening speed, install a lighter spring. If the carburetor is too large for the engine, it may not open the secondaries all the way, because the additional airflow is not required. "Also, unless the application is race-spe- cific with conditions that warrant plugging, the power valve should be left installed. This is especially true in a street vehicle. They will add fuel under load and remove it when not required, resulting in better fuel economy and drivability. If you must plug the valve you will need to jet up 6-8 jet sizes to compensate." JET Performance Products' Nicholas adds: "There have been claims for over 50 years about the carburetor that gets 100 mpg or the one that gets 200 mpg and all the conspiracy theories that go along with why it was never mass-produced. The fact is there is simply no such thing as a carburetor that can get that kind of mileage, never has been, and never will be. These crazy mileage claims from a carburetor break the first law of thermodynamics; if you're really inter- ested, you can look it up." Dangerous Practices Stop and think about it: Carburetors are full of gasoline and gasoline is flammable. Not paying enough attention can cause some very unintended consequences. From Demon's Witmer: Always start the engine and check for leaks before closing the hood. Better to repair a leaking fuel line fitting or a malfunctioning float level than to witness the paint being burned off an im- maculate hood. The two most common offenders are, first, HOTROD CARBURETOR SECRETS Left: Newer carbure- tors are designed for today's fuel choices. (Courtesy Holley) Below: In the idle con- dition the transfer slots should give the appear- ance of a small square when viewed from underneath the base plate. (Courtesy Demon Carburetion) PHBMAYp58-91.indd 70 4/2/14 1:58 PM

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