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How To Adjust The Accelerator Pump Arm on a Holley Carburetor

By: Tom Kise04/04/2018 < Back to Blog Home

Today I'm going to take a couple of moments to talk about how to properly adjust the accelerator pump arm on a Holley carburetor. In this case, we have a Holley double pumper and I'm just going to focus on the secondary side. Your carburetor may be a single pump, so you only have one accelerator pump, but it really doesn't matter because the procedure is just going to be the same. 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. Ideally, you want that to be zero at idle. By zero I mean there shouldn't be any gap, and it shouldn't have any pre-load. That being said we also want to check it at wide open throttle and make sure we've got a minimum of a 15000ths of a gap, as far as available travel left, and that's to make sure we don't bottom out that pump diaphragm.


To make that adjustment, your going to need to use a nut driver or a couple of wrenches, in this case, a 3/8th and we want to adjust this arm until we just take the slack we've got out of it, without creating any pre-load on the lever. Again the reason we don't want this thing pre-loaded is if it was pre-loaded partially down, it's going to limit the overall travel of the accelerator pump lever and what that's going to cause is a reduction in the amount of fuel that we can deliver and we can end up causing a flat spot or a lean hesitation problem. Once we get that set to zero we're going to install the 15000ths feeler gauge between the pump lever and the pump arm and go to wide open throttle and we want to make sure that we're not bottoming out the diaphragm, we should still have a little bit of additional travel available.


One of the ways you visually tell if you don't have enough movement in it and you are bottoming it out is when you look at it at wide open throttle. If you look at the relationship between the adjusting nut and the top edge of the pump arm, you may see a gap in there and that's something you definitely don't want to see. If you are bottoming out that accelerator pump at wide open throttle, you can damage it, even rupture the diaphragm and result in a fuel leak that could be a fire hazard so it's real important to make sure that your adjustment is correct. In relationship with the accelerator pump arm, you've got a couple of other components of the accelerator pump system that you can also go in and tune. We have an assortment of accelerator pump cams which are the multicolored cams that detail what the accelerator pump rides on. You can change those to change the timing or overall aggressiveness of the accelerator pump shot and you can also tune with the accelerator pump discharge nozzles on the top. We're not going to go into in-depth tuning on this but a couple of quick tips that can help you out, so once you get the accelerator pump arm adjusted if you find that you have an offline hesitation and it's less than a second, you can go ahead and increase the accelerator pump nozzle and go up one size. If it is a second or longer I would initially go up two. If you are hesitation is fuel related and this pump arm is adjusted correctly when you change the nozzle size, you should see an improvement or it may go the opposite direction. If you see an improvement and it didn't take it all out all the way, go up a little bit more, if you increase the nozzle size and it gets worse, go ahead and put it back the way it was and try going in the opposite direction. If you change that nozzle size and it's not making it any better or it makes it worse whether you go up or you go down, chances are your hesitation is not due to something with the accelerator pump circuit, you could have a vacuum leak, timing issues, or something else going on with you overall combination. For more helpful Holley tech tips and videos, go to Holley.com

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