How to rebuild a Rochester Quadrajet 4MV carburetor

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===G. Re-installation===
 
===G. Re-installation===
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====Carb gaskets====
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=====Early factory intake with exposed heat crossover=====
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[[File:Early qjetintake1.jpg|thumb|right|350px|Arrows indicate heat passage holes which can be blocked if desired]]
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On early (about 1969-back) Chevy BBC and SBC Q-jet intakes, the manifold was equipped with an exposed heat crossover below the primary side of the carb. This intake manifold requires a specific gasket and heat shield (below left) be used to prevent vacuum leaks and to resist the heat.
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[[File:Carb gasketsearly q-jet.gif|thumb|350px|left|Metal heat shield goes against carb; fiber insulator goes against intake]]
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The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations. <br style="clear:both"/>
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{{Caution}}Tighten the carb to the thick insulator-type gasket evenly but not too tightly. Because the gasket of that type has a lot of give to it, it can cause the base plate to warp- sometimes enough to cause the throttle shafts/blades to bind. In severe enough cases the baseplate can even crack.
 
{{Caution}}Tighten the carb to the thick insulator-type gasket evenly but not too tightly. Because the gasket of that type has a lot of give to it, it can cause the base plate to warp- sometimes enough to cause the throttle shafts/blades to bind. In severe enough cases the baseplate can even crack.
  
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{{Caution}}Do not overtighten the vacuum fitting used for the power brake booster (or any other application requiring a high flow, manifold vacuum source). The casting can crack if the male pipe threads of the fitting are overtightened.
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=====Later factory and aftermarket intakes=====
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In most cases a thick fiber insulator-type gasket will provide good sealing and help to keep the carb temperature down. Heed the caution above regarding not overtightening the carb.
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====Baseplate vacuum fittings====
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Not all carbs have a tapped vacuum port in the rear of the baseplate. When it is present, this port can be used for the power brake booster. Don't use it for the PCV system; that port is located in the front of the baseplate (primary side). The port is threaded with pipe threads, either 1/8" or 1/4".
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{{Caution}}Do not overtighten any vacuum fitting or plug into the baseplate! The aluminum casting will crack if the male pipe threads of the fitting are overtightened.
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Seal the plug or fitting with teflon tape or a good thread sealant. Either of these will also act as a lubricant, making overtightening a lot easier than if the threads were dry, so be careful.
  
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Seal the fitting with teflon tape or a good thread sealant. Either of these will also act as a lubricant, making overtightening a lot easier than if the threads were dry, so be careful.
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{{Note1}}Do not use teflon tape on any fuel line fittings. The filaments of teflon tape that is left over after a fitting is removed is hard to completely remove from the threads, and it will clog the needle and seat and/or jets if it finds its way into the carb.
  
 
==Tuning==
 
==Tuning==
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There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit.
 
There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit.
  
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Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting. This can be fairly easily removed if it's encountered. They block them in various places, sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets.
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Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting channels/orifices, to make the carb "generic" as opposed to being tuned for a specific, exact application. Generally speaking, there's no harm in doing this as long as it has been done correctly. Unfortunately, trying the carb on your running engine is about the only way you can know for sure, unless the desired orifice sizes are known ahead of time. Fortunately if need be, the lead can be fairly easily removed, or drilled to give bypass air.  
  
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The older 'tunnel' type (circles):
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The rebuilders block them in various places: sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets.
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The older 'tunnel' type bypass air channels (circles):
  
 
[[File:Bypass 1.jpg]]
 
[[File:Bypass 1.jpg]]
  
  
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The later 'tab' type, most look like the one here: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is):
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The later 'tab' type bypass air orifices, most look like the one shown below: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is).
  
 
[[File:Bypass2.jpg]]
 
[[File:Bypass2.jpg]]
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Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about.
 
Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about.
  
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==Changes to calibrations==
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==Carb gaskets==
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As far as jetting and calibrations, they have to be optimized for the engine anyway- and without knowing whether the carb was spot-on, lean or rich to begin with- and what the smoothing, etc. to the castings did to the flow and air/fuel ratio, there's no way anyone can say you will need to add or remove fuel, nor what circuits may be affected, or even if one circuit may want more fuel while another circuit wants less. So instead of worrying about any changes caused by the work you're contemplating doing, plan on taking care of all of it when the carb is set up for the engine and vehicle.
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[[File:Early qjetintake1.jpg|thumb|right|350px|Arrows indicate heat passage holes which can be blocked if desired]]On early (about 1969-back) Chevy BBC and SBC Q-jet intakes, the manifold was equipped with an exposed heat crossover below the primary side of the carb. This intake manifold requires a specific gasket and heat shield (below left) be used to prevent vacuum leaks and to resist the heat.
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Lastly, there's a lot more to be had in setting up the APT, idle/transition and main circuits, and the secondary tip-in than there is in dickering with the castings.
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===Drilling jets===
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The flow through a jet is determined by the surface finish (small consideration as long as it’s very smooth), the length of the orifice (moderately important, depending on how much the difference is), the entrance and exit angles and their finish, and the size of the orifice (major consideration).
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When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance. The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.
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The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range. 
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[[File:Carb gasketsearly q-jet.gif|thumb|350px|left|Metal heat shield goes against carb; fiber insulator goes against intake]]
  
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It is for these reasons there is no practical way for a person to casually drill a jet and have any way of knowing what the flow is going to be. Drilling out jets is a holdover from the dark days when guys were struggling to make “high performance” engines live, and if they got within 10% of ‘right’ they were at the top of the heap. Nowadays, thankfully, we all (should) know better and leave the drilling of jets and other dubious practices to the desperate, the uneducated, and the poverty-stricken.
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The heat crossover can be blocked either at the intake gasket, or by blocking off the ports that feed heat to the "U"-shaped channel in the intake with 1/4" pipe plugs. This mod will prevent the base plate from possibly warping and should help provide a denser air/fuel mixture due to the cooler temperature. Be aware that this can mean longer warm up times, and may require some changes to the calibrations. <br style="clear:both"/>
  
 
==Quadrajet tuning and repair parts==
 
==Quadrajet tuning and repair parts==

Revision as of 09:59, 25 February 2013

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