How to rebuild a Rochester Quadrajet 4MV carburetor
(Minor clean up of portions attributed to Cobalt327) |
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===C. Tools=== | ===C. Tools=== | ||
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=====Parts Organizer===== | =====Parts Organizer===== | ||
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=====Tools===== | =====Tools===== | ||
| + | There are a few tools that will make rebuilding the Q-jet easier: | ||
| + | * A 1/16" punch for removing the accelerator pump arm roll pin | ||
| + | * An adjustable T-square for setting float level | ||
| + | * A "pin vice" and a set of small numbered drill bits for measuring and making (or enlarging) small diameter holes. They are bought typically in sets numbered from #61 (0.0390") to #80 (0.0135"). Numbered bits are also available larger than #61- all the way to #1 (0.2280"). At this point, lettered bits take over. Interspersed throughout these are also the fractional and metric sizes. | ||
| + | [[File:Numbered bit index.jpg|thumb|300px|left|Small numbered drill bit index]][[File:Pinvice.jpg]] | ||
| + | [[File:115 pc drill set.jpg|thumb|300px|left|115 piece drill index]] | ||
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| + | Having a complete 115 piece drill bit index is a very handy tool to have- it allows making, measuring and gauging holes as well as setting Holley and other float levels where the bit can be fit under the float, or above it when setting the float drop. | ||
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:*Socket wrench set | :*Socket wrench set | ||
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:*Screwdrivers - various sizes | :*Screwdrivers - various sizes | ||
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| − | :The carburetor is held in place by two long bolts visible on top of the air horn and two nuts on the back of the carburetor on top of the throttle cable bracket. Loosen these up, remove them, and set them aside in the parts organizer. | + | :The carburetor is held in place by two long bolts visible on top of the air horn and either two nuts or two bolts on the back of the carburetor on top of the throttle cable bracket. Loosen these up, remove them, and set them aside in the parts organizer. |
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<blockquote>''"It is not uncommon for the retainer to fail to positively secure the PP in its hole in and of itself. The solution is to take a chisel and gently stake the edge of the hole in one or two spots against the retainer." ^''</blockquote> | <blockquote>''"It is not uncommon for the retainer to fail to positively secure the PP in its hole in and of itself. The solution is to take a chisel and gently stake the edge of the hole in one or two spots against the retainer." ^''</blockquote> | ||
| − | + | Be sure that when it's all said and done that there's positively no friction from the retainer on the power piston shaft. It has to be absolutely free to move without any hindrance. | |
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[[File:GHP-Q3621-400w.jpg|thumb|250px|left|Metal power piston retainer clip (used on early Q-jet)]] | [[File:GHP-Q3621-400w.jpg|thumb|250px|left|Metal power piston retainer clip (used on early Q-jet)]] | ||
| − | [[File:GHP-Q3622-PP RETAINER BUSHING SPLIT.jpg|thumb|250px|center|Replacement power piston retainer bushing; split type.]] | + | [[File:GHP-Q3622-PP RETAINER BUSHING SPLIT.jpg|thumb|250px|center|Replacement power piston retainer bushing; split type.]] <br style="clear: both"/> |
=====Leaking Plugs / Main Wells===== | =====Leaking Plugs / Main Wells===== | ||
| − | + | A common problem on the early Q-jets especially, is leaky main wells. The 'cure' for this has long been to use JB Weld, etc. to seal them over. This is just a stop-gap measure, I have seen personally how the epoxy will degrade over time and quit sealing as it did at first. | |
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| − | + | If a permanent fix is wanted, the wells can be resealed w/new plugs that are peened over to hold them in place, or the wells can be drilled and tapped for short screws that will cure them for good. The primary wells seldom leak, the secondary wells on the early carbs do tend to leak more than the later carbs. | |
Also please see post #48 on [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-4.html page 4] on the thread [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558.html ''"Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level")''] | Also please see post #48 on [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-4.html page 4] on the thread [http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558.html ''"Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level")''] | ||
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Check the base plate to be sure the throttle blades are exactly 90° when wide open. There are some who believe having the secondary throttle blades go "over center" helps with A/F distribution in some cases using certain intake manifolds. While this may be true, until you have a good grasp on the engine tune, and unless the plug readings indicate a need for this, keep them at 90°. | Check the base plate to be sure the throttle blades are exactly 90° when wide open. There are some who believe having the secondary throttle blades go "over center" helps with A/F distribution in some cases using certain intake manifolds. While this may be true, until you have a good grasp on the engine tune, and unless the plug readings indicate a need for this, keep them at 90°. | ||
| − | The linkage adjustment is fairly easy to do, it only takes a pair of pliers and a bit of patience to get | + | The linkage adjustment is fairly easy to do, it only takes a pair of pliers and a bit of patience to get them set perfectly. |
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=====Base/ Throttle Plate===== | =====Base/ Throttle Plate===== | ||
| − | '''Note:''' | + | '''Note:''' Sometimes the Q-jet base plate may be missing a screw or two (there are only two on most carbs; three at the most). While the base plate is "helped" being held on by the two long front intake manifold mounting bolts and the two long screws at the rear of the carb, it's still a good idea to replace the missing screws. Just be sure to not over tighten these long bolts. If they're over tightened, it can warp the carb permanently. |
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| − | '''Note:''' | + | '''Note:''' When removing the accelerator pump arm roll pin, place a ~0.050" feeler gauge (or similar thickness material) behind the accelerator pump lever pin, so when you drive the pin towards the air horn lip it doesn't butt tightly against it; instead there's room to get a thin straight blade screwdriver in behind it to lever the pin back into position. |
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| − | '''Cautionary Note:''' | + | '''Cautionary Note:''' 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. |
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| − | '''Tuning Tip:''' | + | '''Tuning Tip:''' The idle discharge ports that are controlled by the idle mixture screws can be anywhere from around 0.050" to 0.080" diameter. If you find a lean idle condition that doesn't seem to respond to turning the screws CCW, you can gauge the port size using your small numbered drill bit index, then enlarge it one size at a time until you regain control w/the screws. |
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| − | + | This is providing that the throttle plates are not open too far due to a large cam- which will over-expose the transfer slot, just like on a Holley. The cure for that is the same as you'd do w/a Holley- start by using more ignition advance. This allows the primary throttle blades to be closed down some to lower the idle speed, which increases due to the advanced timing. Often, this is all that's needed, so be sure to do this FIRST, then enlarge the ports only if still needed. Normally, going larger than 0.0100" isn't needed and if the idle screws are still not responsive at this size, the problem lies elsewhere- like needing idle bypass air (or more bypass air). | |
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