How to rebuild a Rochester Quadrajet 4MV carburetor
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=Introduction= | =Introduction= | ||
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This article is written with the intention of informing, guiding and assisting the first-time builder who has never dealt with a carburetor before. It merges detailed, step-by-step directions, complete with images, links to online and print resources, and a multitude of helpful tips and pointers collected from the members of [http://www.Hotrodders.com Hotrodders] forum, all of them in their own words. | This article is written with the intention of informing, guiding and assisting the first-time builder who has never dealt with a carburetor before. It merges detailed, step-by-step directions, complete with images, links to online and print resources, and a multitude of helpful tips and pointers collected from the members of [http://www.Hotrodders.com Hotrodders] forum, all of them in their own words. | ||
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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. | 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. | ||
| − | If a permanent fix is wanted, the wells can be resealed | + | If a permanent fix is wanted, the wells can be resealed with 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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To do the peening, you can clamp a piece of square stock into a vice to use as an anvil to support the screw head on, then peen the ends of the screws with a small jeweler's hammer or ball peen. You don't need to simulate the factory way of staking; the main idea is to disrupt the threads on the end of the screw so they cannot fall into the engine. The loctite will do the rest. | To do the peening, you can clamp a piece of square stock into a vice to use as an anvil to support the screw head on, then peen the ends of the screws with a small jeweler's hammer or ball peen. You don't need to simulate the factory way of staking; the main idea is to disrupt the threads on the end of the screw so they cannot fall into the engine. The loctite will do the rest. | ||
| − | If the ends of the screws look too long, they can be ground down | + | If the ends of the screws look too long, they can be ground down with a dremel to the approximate length of the stock screws before peening them. |
Be very careful if you end up trying to find a replacement for the secondary shaft. They are not all the same. I'm curious. Why did you remove the shaft if you aren't bushing it? | Be very careful if you end up trying to find a replacement for the secondary shaft. They are not all the same. I'm curious. Why did you remove the shaft if you aren't bushing it? | ||
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==Tuning== | ==Tuning== | ||
===Idle discharge ports=== | ===Idle discharge ports=== | ||
| − | 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 | + | 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 with the screws. |
| − | 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 | + | 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 with 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). |
===Power piston springs=== | ===Power piston springs=== | ||
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Edelbrock has some Q-Jet tuning parts; they show only 4 different PP springs, but they only come in a "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. There are also other suppliers of PP springs and other various tune up and repair parts; see the links below this section. | Edelbrock has some Q-Jet tuning parts; they show only 4 different PP springs, but they only come in a "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. There are also other suppliers of PP springs and other various tune up and repair parts; see the links below this section. | ||
| − | *GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well | + | *GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well with a high vacuum, smooth idling and/or wide LSA-type cam. |
*GM PP spring p/n 7036019 has a tip in of about 8 in/Hg. | *GM PP spring p/n 7036019 has a tip in of about 8 in/Hg. | ||
*GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift. | *GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift. | ||
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===Adjustable part throttle (APT) setting=== | ===Adjustable part throttle (APT) setting=== | ||
| − | Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing | + | Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing with long duration or overlap cams. This adjustment is referred to as the APT or "adjustable part throttle" setting. |
There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. | There are two common types of APT adjustment locations: later carbs use a plugged hole in the airhorn, earlier carbs use a plugged hole in the baseplate. | ||
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Before removing the adjustment screw from the carb body (airhorn-adjusted APT), count the turns it takes to lightly bottom the adjustment screw and write it down before removing the screw- if it needs to be removed; often it can just be left in place. Use this setting as a baseline at reassembly. | Before removing the adjustment screw from the carb body (airhorn-adjusted APT), count the turns it takes to lightly bottom the adjustment screw and write it down before removing the screw- if it needs to be removed; often it can just be left in place. Use this setting as a baseline at reassembly. | ||
| − | ====Airhorn adjustment location==== | + | ====Airhorn APT adjustment location==== |
It is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | It is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | ||
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You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted. | You may read that 2 to as much as 3-1/2 turns CCW from lightly bottoming the adjustment screw is a starting point. What may also be done for a starting point is for the top of the adjustment screw to be just a small amount above the carb body casting without a gasket in place. The carb [I]may[/I] be richer than needed in that position (or not, depending. But in that position the engine can be run safely and further fine tuning can be done, and with the following modification to the airhorn to allow easy access to the adjustment, it can be quickly and easily adjusted. | ||
| − | [[File:Apt airhornhole plug.jpg|thumb|400px|left|Q-jet showing APT (yellow arrow) and plug removed (red arrow)]]After setting the adjuster even with the body casting, remove the aluminum plug in the air horn casting using a punch and ball peen hammer (''carefully''- support the casting so when you tap out the plug there's no undue pressure put on the casting), then tap the hole for a screw-in plug. That way you can easily adjust the power piston | + | [[File:Apt airhornhole plug.jpg|thumb|400px|left|Q-jet showing APT (yellow arrow) and plug removed (red arrow)]]After setting the adjuster even with the body casting, remove the aluminum plug in the air horn casting using a punch and ball peen hammer (''carefully''- support the casting so when you tap out the plug there's no undue pressure put on the casting), then tap the hole for a screw-in plug. That way you can easily adjust the power piston with the carb on the engine. |
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If you are experiencing an "off-idle stumble", you can turn the screw up at 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. | If you are experiencing an "off-idle stumble", you can turn the screw up at 1/4-1/2 turn at a time until it is eliminated. But be sure to adjust your idle mixture screws properly first, as they can also cause an off idle stumble. | ||
| − | ====Baseplate adjustment location==== | + | ====Baseplate APT adjustment location==== |
Earlier Q-jet carbs have a baseplate APT adjustment location. A hole centered between the idle mixture screws (plugged at the factory) has an adjustment screw that raises/lowers the power piston to set the APT adjustment. | Earlier Q-jet carbs have a baseplate APT adjustment location. A hole centered between the idle mixture screws (plugged at the factory) has an adjustment screw that raises/lowers the power piston to set the APT adjustment. | ||
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[[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> | [[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> | ||
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| + | ==Casting smoothing== | ||
| + | Generally speaking, reworking the castings of a carb without any empirical way of measuring changes is like porting a cylinder head without a flow bench. What looks good may in fact be the exact opposite of what the carb needs, and without knowing what is happening as you make changes, the results can be worse than if you had done nothing at all, so if you decide to do anything, do not change the shapes, angles of attack, or any other parameter that was a designed-in feature of the carb. Keep things to a minimum- smoothing obvious flaws and mismatches, and leave the rest alone. | ||
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| + | ===Primary=== | ||
| + | On the primary side, the booster rings and the venturi can be carefully smoothed where there may be a casting mismatch or casting flash or et cetera. If there's a mismatch between the baseplate and main body, that can be taken care of- but this is not generally a problem, nor has the Q-jet historically had any significant amount of casting irregularities. | ||
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| + | ===Secondary=== | ||
| + | There's no point in doing anything (other than for personal satisfaction) to the secondary side casting unless there's some huge gaping mismatch, etc.The fit of the base plate gasket can be more of a flow problem than any casting irregularity; if the gasket intrudes into the throttle bore, removing THAT will help more than anything. | ||
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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. | ||
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| + | ===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. | ||
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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. | ||
===Quadrajet tuning and repair parts=== | ===Quadrajet tuning and repair parts=== | ||
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