While a carburetor hasn’t graced a production vehicle’s engine in the United States for almost 25 years, they are still wildly popular in both enthusiast street and race applications today. For those who choose to run a carburetor on the street, there are several factors that come into play for a smooth experience, but with a little bit of care, a Holley carburetor give you a worry-free daily commute, while getting rowdy on demand.
In this quick video, Tom Kise of Holley goes over how to properly adjust the accelerator pump system on one of the company’s carburetors. As the name implies, the accelerator pump provides additional fuel upon acceleration of the engine, while the primary circuit catches up to demand. As such, a poorly tuned accelerator pump circuit can lead to laggy off-idle performance.
In the video, Kise uses a Holley double-pumper (which has a fuel feed at each end) as a demonstration tool, but assures us that a carburetor with a single accelerator pump would be adjusted in the same manner.
“What we’re really trying to achieve here is the proper relationship between the adjusting bolt on the accelerator pump arm, and the actual accelerator pump lever,” says Kise. “Ideally, we’d like that to be zero-lash and zero preload at idle. We also want to check it at wide-open throttle and make sure that there is at least .015-inch of travel left to make sure we aren’t bottoming out that pump diaphragm.”
Bottoming out an accelerator pump diaphragm can lead to a damage over time, or even rupture the diaphragm itself, which can cause a fuel leak, and the associated headaches which come along with that.
The first step is to adjust the accelerator pump actuation arm and the pump’s lever. To do that, you can use an open-ended wrench and a nut driver, or a pair of wrenches. Once you have all the slack taken out of the engagement with the system closed, making sure not to impart any preload on the lever, you then insert a .015-inch feeler gauge, and crank the carburetor to wide-open throttle.
“You want to make sure that the diaphragm isn’t bottomed out, and that there is some travel left in the arm,” Kise says. “One of the ways you can visually check, is to look to see if there is a gap between the adjuster nut and the arm. If there is a gap, you are bottomed out, and you definitely don’t want that.”
Once you have the accelerator pump arm adjusted properly, you can move on to the other areas of adjustment in the accelerator pump circuit.
“We have an assortment of accelerator pump cams available, which are the multicolored cams that the tail of the accelerator pump rides on,” says Kise. “You can change those cams to adjust the overall timing of the secondary pump’s actuation, as well as its aggressiveness.”
Then, there is the more complicated process of changing the accelerator pump discharge nozzles. While beyond the scope of this video, Kise does offer a few quick tips on how to size those jets, if you’re still having an issue once your accelerator pump arm has been correctly adjusted.
“Once you get the accelerator pump arm adjusted and you find you have an off-the-line hesitation, if it’s less than a second’s worth, you can go up one size on the accelerator pump nozzle,” Kise says. “If it’s a second or longer, I’d go up two sizes. Conversely, if those changes don’t work, you might need to go the opposite direction. If adjusting the accelerator pump nozzles both directions doesn’t seem to affect the stumble at all, it’s probably not being caused by the carburetor.”