How to rebuild automotive carburetors
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==Introduction== | ==Introduction== | ||
Sooner or later, every hotrodder runs into the steps of making induction and carburetion choices while building a performance engine. | Sooner or later, every hotrodder runs into the steps of making induction and carburetion choices while building a performance engine. | ||
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Often the best place to start is to baseline the carb back to factory specs. This eliminates any chance that a previous owner's changes will mess up the way the carb works on your engine. | Often the best place to start is to baseline the carb back to factory specs. This eliminates any chance that a previous owner's changes will mess up the way the carb works on your engine. | ||
| − | [ | + | *[[Media:HolleyCarbList.pdf|'''Aftermarket Holley carb numerical listing''']] by carb number shows the factory specs as well as the part numbers for rebuild kits and gaskets. |
| + | *'''[[Holley HP series carb specs]]''' by carb number | ||
| + | *To compare metering plates to jet sizes, see the info [[Media:08 Holley Catalog pages - 90.pdf|'''here''']]. | ||
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[[File:Holley pv stamping1 id.jpg|thumb|left|400px| Holley power valve vacuum rating]] | [[File:Holley pv stamping1 id.jpg|thumb|left|400px| Holley power valve vacuum rating]] | ||
The Holley carb uses a power valve to supply additional fuel when the engine vacuum drops (and presumably the engine load increases). The amount of fuel it supplies is equal to about 7-8 jet numbers. If the rear power valve (if present) is removed, the threaded hole it was in has to be plugged. The jets have to be enlarged 7-8 jet sizes to compensate for the loss of fuel flow it provided. <br style="clear:both"/> | The Holley carb uses a power valve to supply additional fuel when the engine vacuum drops (and presumably the engine load increases). The amount of fuel it supplies is equal to about 7-8 jet numbers. If the rear power valve (if present) is removed, the threaded hole it was in has to be plugged. The jets have to be enlarged 7-8 jet sizes to compensate for the loss of fuel flow it provided. <br style="clear:both"/> | ||
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| + | If you want to check to see if the PV rating is too high and allowing it to open during idle, turn the idle mixture screws all the way in (count the turns so you can return them after the test). Then start the engine. If the engine won't idle, the power valve is not open. | ||
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| + | If you want to check if the PV is open during idle, turn the idle mixture screws all the way in (count the turns so you can return them after the test). Then start the engine. If the engine won't idle, the power valve is not open. | ||
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===Holley idle transfer slot=== | ===Holley idle transfer slot=== | ||
The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the Left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, Right (approximately as long as it is wide): | The drawing below shows the transition slot as seen with the carb held upside down. The drawing on the Left shows an overexposed transition slot. Baseline the throttle blades to give a transfer slot that looks like the image below, Right (approximately as long as it is wide): | ||
| − | [[File:Holley | + | [[File:Holley t-fer slot.jpg]] <br style="clear:both"/> |
{{Note1}}Other carbs have similar transition circuits, and they need to have the throttle blades in the 'sweet spot' same as the Holley. If the throttle blades are open too far, a poor idle and off-idle transition can be the result. | {{Note1}}Other carbs have similar transition circuits, and they need to have the throttle blades in the 'sweet spot' same as the Holley. If the throttle blades are open too far, a poor idle and off-idle transition can be the result. | ||
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| − | [[File:Oring holley sec.jpg|thumb|350px|Arrow indicates cork O-ring that seals the vacuum chamber to the main carb body]] | + | [[File:Oring holley sec.jpg|thumb|left|350px|Arrow indicates cork O-ring that seals the vacuum chamber to the main carb body]] |
Use a fresh cork O-ring to seal the secondary vacuum chamber assembly to the carb body. Using a light smear of Hylomar on the cork gasket helps it to seal and stay pliable. A small rubber O-ring may be used instead of the cork gasket to seal a Holley secondary chamber. If you try this, be sure the O-ring is sealing, w/o being ''too'' thick. | Use a fresh cork O-ring to seal the secondary vacuum chamber assembly to the carb body. Using a light smear of Hylomar on the cork gasket helps it to seal and stay pliable. A small rubber O-ring may be used instead of the cork gasket to seal a Holley secondary chamber. If you try this, be sure the O-ring is sealing, w/o being ''too'' thick. | ||
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| − | Be sure the vacuum chamber is assembled correctly. The diaphragm can | + | [[File:Holley sec checkball.jpg|thumb|450px|Secondary vacuum chamber check ball]] |
| + | Be sure the vacuum chamber is assembled correctly. The diaphragm can be torn or perforated; if that happens there will be no or very slow to activate with incomplete opening of the secondaries. Also, the check ball can get left out or can stick, or the check ball seat can be rough, allowing the secondaries to activate too easily. So be sure the check ball is present and the seat is well formed. | ||
| − | The seat can be re formed by using an old check ball and a small punch. Drop the check ball into the check ball passage, then with the vacuum chamber solidly supported, lightly tap the old check ball with the punch and a small hammer. This re forms the seat. | + | The seat can be re-formed by using an old check ball and a small punch. Drop the check ball into the check ball passage, then with the vacuum chamber solidly supported, lightly tap the old check ball with the punch and a small hammer. This re-forms the seat. |
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| − | '''NOTE:''' There are small | + | '''NOTE:''' There are small grooves in the vacuum chamber check ball seat. This is normal, so do not try to use this method to remove the grooves. They need to be there to allow the secondaries to work correctly. |
A tuning trick is to use a deeper or shallower groove to change the opening rate. A tool can be made from a modified phillips head screwdriver to cut deeper and/or more grooves in the seat. Also, a lighter (aluminum) check ball can be used. | A tuning trick is to use a deeper or shallower groove to change the opening rate. A tool can be made from a modified phillips head screwdriver to cut deeper and/or more grooves in the seat. Also, a lighter (aluminum) check ball can be used. | ||
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If you use your lips or a vacuum pump to pull a vacuum on the chamber (chamber off the carb), there should be some resistance felt; the diaphragm shouldn't be too easily retracted because the check ball should be restricting the vacuum somewhat. That tells you the check ball is there and is able to help modulate the secondary opening rate. | If you use your lips or a vacuum pump to pull a vacuum on the chamber (chamber off the carb), there should be some resistance felt; the diaphragm shouldn't be too easily retracted because the check ball should be restricting the vacuum somewhat. That tells you the check ball is there and is able to help modulate the secondary opening rate. | ||
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| + | ==Motorcraft== | ||
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| + | ===Model 2100/2150 2bbl=== | ||
| + | *[http://grantorinosport.org/BubbaF250/carb/carb01.html Info on theory, rebuilding and tuning] | ||
==Rochester Quadrajet carb== | ==Rochester Quadrajet carb== | ||
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===See also:=== | ===See also:=== | ||
*[[How to Rebuild a Rochester Quadrajet 4MV Carburetor]] | *[[How to Rebuild a Rochester Quadrajet 4MV Carburetor]] | ||
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| + | ==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). | ||
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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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| + | There is no practical way for a person to casually drill an orifice and know what the change in flow will be. Drilling orifices by hand is mostly an anachronism. Nowadays, better options are almost always readily available. Only rare and/or extreme circumstances dictates such practices. | ||
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| + | Highly educated engineers work with mathematical formulas to design correct sized orifices. During the formative years of our contemporary automotive racing environments, carburetors were not always made with a full range of ready made fuel and air bleed orifice replacements so carburetor tuning 'magic' was done with a drill. Because of the difficulty in obtaining precision, for those reasons listed above, a general recommendation to purchase an assortment of pre-calculated metering orifices (fuel or air bleeds) is emphasized. Nevertheless, carburetors exist that do not have readily available interchangeable orifices, and these warnings are made to caution the ambitious tuner. | ||
===Quadrajet Tuning and Repair Parts=== | ===Quadrajet Tuning and Repair Parts=== | ||
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==Carter carb== | ==Carter carb== | ||
| − | [http://www.mymopar.com/downloads/carterafbtuning.pdf Carter AFB Selection and Tuning Guidelines] from Federal Mogul | + | ===AFB=== |
| + | *[http://www.mymopar.com/downloads/carterafbtuning.pdf Carter AFB Selection and Tuning Guidelines] from Federal Mogul | ||
| + | *[[Media:Afb rebuild guide.pdf|How to Overhaul and Tune your Carter AFB or Edelbrock EPS Four Barrel Carburetor]] | ||
| + | *[[Media:Carterafbtuning.pdf|Carter AFB Selection and Tuning Guidelines]] | ||
| + | ===Thremoquad=== | ||
[[Image:Thermoquad.jpg]] | [[Image:Thermoquad.jpg]] | ||
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[[Category:Carburetors]] | [[Category:Carburetors]] | ||
[[Category:Engine]] | [[Category:Engine]] | ||
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