How to rebuild a Rochester Quadrajet 4MV carburetor
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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. | ||
| − | + | ==Changes to calibrations== | |
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. | 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. | ||
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. | 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. | ||
| − | + | ===Drilling jets=== | |
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). | 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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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. | 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. | ||
| − | 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. | + | 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. |
==Quadrajet tuning and repair parts== | ==Quadrajet tuning and repair parts== | ||
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