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post #1 of 4 (permalink) Old 02-07-2011, 06:35 PM Thread Starter
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Holley accelerator pump diaphram

I am rebuilding a Holley 850 DP. The carburetor had two 50cc accelerator pump diaphrams when I tore it down. My confusion is this. The kit has one 50cc diaphram and two 30cc diaphrams. I went back to O'Reilly just to make sure they hadn't ordered the wrong kit and they double checked the kit # for a Holley #4781 and it was right.

What are the chances someone has replaced the 30cc diaphram with a 50cc? Has anyone ever heard of that happening. Would the carburetor function properly with primary and secondary 50cc accelerator diaphrams? Should I just order another 50cc diaphram and rebuild it with two 50cc diaphrams or what.

Thanks for the help.
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post #2 of 4 (permalink) Old 02-07-2011, 06:51 PM
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Re: Holley accelerator pump diaphram

If it was working ok before teardown, it might be good to go back with what came out.

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post #3 of 4 (permalink) Old 02-08-2011, 07:54 AM
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Re: Holley accelerator pump diaphram

Upgrading to 50 cc accelerator pumps is a common way to handle off idle bogs.
If the carb was a bit on the large size for the engine it was used on it might have been necessary to upgrade in order to make the car accelerate quickly and smoothly. It is also common to change to larger shooters (or drill out the stock units) to go along with the larger pumps. that way you can force more fuel in the engine until the rpm brings the flow up to where it is ok.
Mind you - this is a patch at best and results in a very rich off-idle mixture. The proper fix would be to select a carburetor that was better sized for the engine and tune the IFR to assist the off-idle fuel supply in relation to actual demand.

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post #4 of 4 (permalink) Old 02-08-2011, 09:27 AM
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Re: Holley accelerator pump diaphram

what size engine is the 850 DP on ? carb could be the right CFM for smaller cubes that run at high RPM , reason for two 50cc pumps
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