Historical - Fuel Metering in SI Engines

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MolaKule

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Before the advent of fuel injection systems various carburetor designs were used. For V8's, the carbs of choice were either the Rochester 2-Barrel or the QuadaraJet 4-Barrel. In the QuadaraJet,carburetor there was a small piston assembly sitting in a fuel well. Q1: What was the name of this small piston assembly? Q2; What was it's function with respect to the metering rods? This question is NOT open to any Engineering Discipline. Remember, your response goes into a queue so there will be a delay in the appearance of your response.
 
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I always called it the power valve ,it lifted metering rods to richen the mixture at part throttle.The secondary top butterfly lifted the rods when the secondaries were opened.
 
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We called it an accelerator pump. It squirts extra fuel in when the metering rods are raised to give the fuel time to catch up with the extra air when the secondaries are actuated. To help with the fuel delivery delay.
 
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It's the accelerator pump. (or that is how I learned it). It squirts gas into the engine when you step on the gas pedal. Without it, the engine would hesitate when you step on the gas.
 
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As others stated, accelerator pump. Gives fuel as the air flow increases so fast when the secondaries open, prevents a lag when they open. Holley had a double pumper carb with 2 pistons. The quadrajet spreadbore, when right really got decent mileage and ran well. I've rebuilt a few, but its been a while. Not everyone could get them right.
 
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The small piston sitting in a fuel filled well is called the accelerator pump. The small piston sitting in the middle of the fuel bowl is the power piston. your question is unclear. Hence 2 answers.
 
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There is a metering rod spring that prevents movement of the rods relative to the hanger in the primary power piston. Having rebuilt a lot of Qjets in the day, there is nothing to prevent the primary power valve piston from oscillating except the mass of the piston and the size of the passage feeding it vacuum. Well that and occasionally varnish from fuel builds up there. I never noted excess wear of the piston or bore, so it must not oscillate significantly. Rod
 

MolaKule

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Thanks for the many responses and I too have rebuilt quite a few Quadrajets myself for older Suburbans. According to E. Obert in, Internal Combustion Engines and Air Pollution,, Section 11-10 where he describes Carburetors for High Output Engines and specifically he discusses the Quadarjet he says, "To prevent wavering and oscillations of the metering rods, a dashpot, a piston in a fuel well arrangement, is included." I would have preferred the term damper. Now the closest thing I could find to what is described as a dashpot assembly is Part 15 in the exploded view below for the 4MV of 1966.. https://quadrajetparts.com/-a-7.html
 
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In the quadrrajet, there are 2 distinct metering systems, the primary and secondary. The secondary also incorporates tapered rods, but they are not controlled by vacuum, they are controlled by a flapper where the choke would normally be located. The flapper is actually a mass air flow metering device. The flapper on some engines had a damper in the fuel well. That system also has a tip in fuel feed system, a vacuum dashpot that also controls the choke unloader, one of the reason the secondary is locked out when the coke is on. And the flapper also has a mostly hidden torsion spring that is part of the system to prevent secondary tip in bogging. So the answer is also different depending on the primary or secondary system. It is an ingenious and poorly understood carb, And all that knowledge is pretty much useless nowadays.
 
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