Note: Unless you have an EFI engine or induction system, your classic truck has a carburetor. This month, the focus is carburetor design and trouble spots. Understanding how a carburetor functions will enhance your tuning and troubleshooting skills.

Note: To prevent hesitation as the secondary throttle valves open, the Quadrajet provides a small supply of fuel from the secondary accelerating wells, drawn by manifold vacuum. This is not the accelerator pump system, but rather an auxiliary fuel supply that helps the transition to the secondary metering rods and air valve opening.

What Did You Learn This Month?
Night School" would not be complete without a quiz! Don't worry about your test-taking skills or grades. This is an open-magazine, true or false test. Answers can be found in the "Night School" text, photos or captions. Have a good month!

True or False Questions:
1. In its ideal combustible state, gasoline has a "stoichiometric" air/fuel ratio of 14.7:1.

2. Manifold vacuum decreases as engine load increases. Decelerating creates high manifold vacuum. Manifold vacuum is high at an engine idle.

3. Venturi effect does not rely on manifold vacuum to deliver fuel. In a carburetor bore, air speeds up as it moves through a restricted area, or venturi. This speed change creates a low-pressure zone in the area of the venturi discharge tube. This low pressure causes fuel to flow through the discharge tube's nozzle and into the carburetor bore.

4. Ported vacuum serves the off-idle fuel circuit. Ported vacuum is also the signal for a distributor's vacuum advance mechanism.

5. Hesitation or stumble at throttle tip-in can be caused by a defective vacuum advance in the distributor. A weak accelerator pump can create this same symptom during acceleration.

6. Idle mixture screw adjustments have little effect on engine performance at cruise speeds.

7. The cold start system, typically a choke mechanism, makes the air/fuel ratio lean. Cold engines need a leaner fuel mixture to run smoothly.

8. If the engine still ran before your carburetor rebuild, there is no reason to adjust the linkages. Linkage adjustments have little effect on carburetor performance.

9. The Quadrajet was popular for a wide range of General Motors engines. In outward appearance, Quadrajets look similar. A Buick V-6 application would have the same jets and metering rods as the Quadrajet on a 500ci Cadillac V-8.

10. A worn throttle shaft or throttle plate bore creates vacuum leaks. You can test for a worn throttle shaft or plate before removing the carburetor from the engine.

Answers:
1 true, 2 true, 3 true, 4 true, 5 true, 6 true, 7 false, 8 false, 9 false, 10 true

SOURCE
Summit Racing
P.O. Box 909
Akron
OH  44309-0909
800-230-3030
www.summitracing.com
Echlin/NAPA Stores
www.napaechlin.com