Maurice

Why is a Double pumper better? I don't understand the nitty gritty of carbs in the first place, but could somebody explain to me in 6th grade language why a double pumper is better than a single pumper carb? The part that is confusing me is that it isn't "pumping" gas from the tank to the carb, that is the fuel pumps job. So if the fuel is being pumped at a certain rate to the carburator, what do the "pumps" pump at the carb?

ZL1powr

Maurice... As you press on the accelerator there is a pump on the carb that shoots an extra volume of gas into the throtle bore to keep the engine from going lean when you suddenly open the throtle. A double pumper has a seperate pump for the primary and secondary circuit. The second pump is required for mechanical secondary's since you control how fast the throtle opens. A vacuum secondary carb dosn't require a second pump since its opening rate is controled by engine demand and dosn't create a lean condition if the carb is properly jeted. I wouldn't say that a double pumper is always better than a vacuum secondary carb. It depends on the aplication.

73 LS-4

Double pumper simply refers to there being an accelerator pump in both the primary and seconday bores rather then just the primaries in a vacuum secondary carb. So it actually has nothing to do with how the fuel gets to the carb. The secondary accelerator pumps are needed with a mechanical carb becouse a 4 butterflies can be opened wide open at the same time and the accelerator pumps from the front two barrels would be inadequate to deliver enough fuel by themselves to prevent a bog due to lack of fuel, therefore the secondaries also have an accelerator pump. Now with a vacuum secondary carb the secondaries or the air valves to the secondaries in a quadrajets case open gradually, therefore the fuel demand isn't as instantaneous and the secondary accelerator pump isn't nescessary.

:cheers:
Pat Kunz

Maurice

Thanks, I do believe I understand now.

So, what part of the carb makes it a 650/750/850? I am speaking about Holleys also. Is it the main part in the middle and the size of the holes?

Thanks in advance.

GDaina

The amount of of air flow.....the larger the number, ie 850, the greater the air flow....also larger throttle bores and plates. In essence, more air and fuel delivery to the motor....but before u start thinking...."Ah...just what I need, LARGE CARB, 850 CFM....UM....YUM YUM....lottsa air, lotsa gas....more power...." u got to match the cfm rating to your engine's demand....larger ain't better in carb applications.

Flareside

Maurice, have you seen the streams of fuel that squirt down into the front two throttle bores when you move the throttle lever? That's the primary pump working. On a double pumper, you would see the same type of squirt down the back two bores when you moved the rear throttle lever.