June '18

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JUNE 2018 THE SHOP 31 tinued over the years, so you may have problems finding a vacuum advance that will match the vacuum level at idle of some vintage carburetor-equipped engines when you are using manifold vacuum for the vacuum advance. TUNING THE VACUUM ADVANCE SYSTEM Most of the vacuum advance units that are still available today were designed to work with a blend of gasoline that no longer exists, so they tend to supply too much vacuum-based spark advance for today's reformulated gasoline. There are some adjustable vacuum advance units on the market that either limit the amount of advance or change the vacuum level needed for it to operate, but in my experience sometimes the stop will adjust itself over time, so I prefer to use a mechanical stop to limit the advance from the vacuum advance unit. The spark advance from the various vacuum advance units that were used back in the days of leaded gasoline varied from 10 to 30 degrees of spark advance. If the engine gets too much spark advance from the vacuum advance system, the engine will experience misfire prob- lems. We tend to limit the advance from the vacuum advance system to 10 to 12 degrees on the vehicles we work with after the initial timing and mechanical advance systems have been optimized for the needs of the engine with today's reformulated unleaded gasoline blends. We have come up with this 10- to 12-degree target using both drivability tests and looking at engine misfire rates (HC reading) and engine efficiency (CO2 reading) from a five-gas exhaust gas ana- lyzer. We prefer a vacuum advance corrector for distributors that uses a General Motors points-style vacuum advance (sold by Ole's Auto Parts in San Bruno, California), but if you are working with other vacuum advance systems you may need to fabricate or MacGyver some type of stop to limit the advance from the vacuum advance unit. THE RESULTS The first and most important step in unlocking all the power and fuel efficiency an engine has to offer is proper tuning of the ignition system. In my experience, for most of the vehicles that are brought to us with drivability or engine performance problems, ignition spark timing curves are the culprit. Once the ignition spark advance system is prop- erly tuned to match the needs of the engine with the blend of gasoline it is burning, you are well on your way to supplying your customer with an engine that performs and drives as well or better than it did when the vehicle was new. The next step in the tuning process is proper tuning of the air/fuel mixture curves the engine is burning. HENRY P. OLSEN is a carbu- retor specialist who has been rebuilding and tuning carbu- retor-equipped engines since the late 1970s, including helping to solve drivability prob- lems with the Speed Demon series of carbu- retors for the Barry Grant carburetor company. He is also involved in operations at Ole's Carburetor & Electric Inc., the family-owned auto parts business in San Bruno, California that was started by his father in 1954. Performance Over 200 Applications In Stock & Ready to Ship! Easiest Way to Upsell Tires and Wheels! BODY LIFTS

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