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<blockquote data-quote="swampratt" data-source="post: 3101839" data-attributes="member: 15054"><p>No you are getting it ..but regular and dual stage do not flow the same amount of fuel when opening.</p><p>Basically 2 opening rates on the dual stage to keep things a bit leaner and it never enriches the circuit as much as the single stage.</p><p></p><p>The power valve circuit is needed in a carburetor application most of the time any way.. you can delete the power valve circuit but the main circuit will become overly rich because you must add a larger jet to make up for deleting a power valve circuit.</p><p>Usually .008-.010" in larger jet size.</p><p></p><p>Lets go about this differently. Simple version.</p><p>Carburetor on an engine as you know will pull air through the carb venturi and the air is vacuum this vacuum is called venturi vacuum.</p><p>This vacuum will basically pull gasoline or "fuel" through an orifice. like drinking from a straw. </p><p>The jet associated with that venturi must be calibrated to flow the correct amount of fuel to make the air fuel ratio correct.</p><p></p><p>This we know ..I will not get into other passages that help emulsify the fuel.</p><p></p><p>But if you are cruising or say pulling a hill and keeping vacuum up in the 11+ range you will be only needing the fuel from that jet.</p><p>This is called the main circuit.</p><p></p><p>After vacuum drops lower and lower from you applying more and more throttle you just can't flow enough fuel from that jet.</p><p>So the engine will go lean ..This is where the power valve circuit comes in. it adds the needed extra fuel to keep the engine in a good AFR range.</p><p></p><p>If you added a large enough jet usually 10 thousandths larger than what would give you a nice 14.7 AFR you would be overly rich during part throttle application.</p><p></p><p>The dual stage power valve flows less fuel than a regular power valve under this situation where you are not under full throttle but you are far enough into it to need a slight enrichment.</p><p></p><p>Get all the way into the throttle and it will flow extra fuel.</p><p></p><p>A Carter AFB or Quadrajet and many other carburetors use a brass metering rod that sticks into the jet.</p><p>This rod is fat in the upper portion and towards the tip it is smaller diameter.. say .075" at the fat portion and .047" at the thinner portion.. as vacuum drops you need to richen the mix and the rod will spring up due to a piston /spring assembly and the thin portion is now in the jet.. allowing it to flow more fuel. Get out of the throttle and the fat portion goes into the jet.</p><p></p><p></p><p>No dual stage available for that.</p><p>This is all really easy to see and grasp if the carb is in front up you.</p><p></p><p>Some carbs at very low RPM will not be in the main circuit but will be in the idle circuit.. giving very good MPG figures.</p><p></p><p>One carb that allows higher cruise rpm before it gets into the mains is the carter AFB/ edelbrock 750.</p><p>You can actually get a tick better MPG from it vs the 600 cfm.</p><p>Yes i said that. larger carb more MPG.</p><p>It comes back to the venturi vacuum pulling fuel from the jet. The smaller venturi passing the same volume of air will pull harder on the passage to the jet allowing the main circuit to come in much earlier.</p><p></p><p>I have many of them.</p><p>With the 600 edelbrock In park with engine running you can slowly open the throttle and before 1500rpm fuel will come out of the boosters from the main jet.</p><p>The 750 edelbrock will get closer to 2500rpm before it pulls fuel from the booster.</p><p></p><p>There are many guys running 500+ BBC engines and OD transmissions and stupid big cams like 260@ .050 and large single carbs with large venturies and pulling 15+ MPG on the highway.</p><p></p><p>Incredible.. but not so much really those large engines are barely working down low at say 1900rpm it is easy to maintain highway speeds and the whole time they are in the idle circuit.</p><p>Yep more MPG than the 350 buzzing along at 3000 rpm with small cam and small carb.</p><p></p><p>Head scratcher .</p></blockquote><p></p>
[QUOTE="swampratt, post: 3101839, member: 15054"] No you are getting it ..but regular and dual stage do not flow the same amount of fuel when opening. Basically 2 opening rates on the dual stage to keep things a bit leaner and it never enriches the circuit as much as the single stage. The power valve circuit is needed in a carburetor application most of the time any way.. you can delete the power valve circuit but the main circuit will become overly rich because you must add a larger jet to make up for deleting a power valve circuit. Usually .008-.010" in larger jet size. Lets go about this differently. Simple version. Carburetor on an engine as you know will pull air through the carb venturi and the air is vacuum this vacuum is called venturi vacuum. This vacuum will basically pull gasoline or "fuel" through an orifice. like drinking from a straw. The jet associated with that venturi must be calibrated to flow the correct amount of fuel to make the air fuel ratio correct. This we know ..I will not get into other passages that help emulsify the fuel. But if you are cruising or say pulling a hill and keeping vacuum up in the 11+ range you will be only needing the fuel from that jet. This is called the main circuit. After vacuum drops lower and lower from you applying more and more throttle you just can't flow enough fuel from that jet. So the engine will go lean ..This is where the power valve circuit comes in. it adds the needed extra fuel to keep the engine in a good AFR range. If you added a large enough jet usually 10 thousandths larger than what would give you a nice 14.7 AFR you would be overly rich during part throttle application. The dual stage power valve flows less fuel than a regular power valve under this situation where you are not under full throttle but you are far enough into it to need a slight enrichment. Get all the way into the throttle and it will flow extra fuel. A Carter AFB or Quadrajet and many other carburetors use a brass metering rod that sticks into the jet. This rod is fat in the upper portion and towards the tip it is smaller diameter.. say .075" at the fat portion and .047" at the thinner portion.. as vacuum drops you need to richen the mix and the rod will spring up due to a piston /spring assembly and the thin portion is now in the jet.. allowing it to flow more fuel. Get out of the throttle and the fat portion goes into the jet. No dual stage available for that. This is all really easy to see and grasp if the carb is in front up you. Some carbs at very low RPM will not be in the main circuit but will be in the idle circuit.. giving very good MPG figures. One carb that allows higher cruise rpm before it gets into the mains is the carter AFB/ edelbrock 750. You can actually get a tick better MPG from it vs the 600 cfm. Yes i said that. larger carb more MPG. It comes back to the venturi vacuum pulling fuel from the jet. The smaller venturi passing the same volume of air will pull harder on the passage to the jet allowing the main circuit to come in much earlier. I have many of them. With the 600 edelbrock In park with engine running you can slowly open the throttle and before 1500rpm fuel will come out of the boosters from the main jet. The 750 edelbrock will get closer to 2500rpm before it pulls fuel from the booster. There are many guys running 500+ BBC engines and OD transmissions and stupid big cams like 260@ .050 and large single carbs with large venturies and pulling 15+ MPG on the highway. Incredible.. but not so much really those large engines are barely working down low at say 1900rpm it is easy to maintain highway speeds and the whole time they are in the idle circuit. Yep more MPG than the 350 buzzing along at 3000 rpm with small cam and small carb. Head scratcher . [/QUOTE]
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