How to rebuild a Rochester Quadrajet 4MV carburetor
(Add image) |
(Minor clean up) |
||
| Line 353: | Line 353: | ||
From quadrajetparts.com: | From quadrajetparts.com: | ||
| − | <blockquote>'''Testing a power | + | <blockquote>'''Testing a power piston'''<br> |
| − | You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (down | + | You can easily test your Rochester Quadrajet power valve spring to see if it is too strong. At idle, the engine vacuum needs to keep the power valve fully seated (the fully down position is the leanest position). Take a normal plastic drinking straw and place it in the top vent of the airhorn, by leaning the straw on a slant you should be able to feel the power piston as you press the piston down and up. With the engine shut off, place a mark with a marker on the straw showing to top height of the vent. When you start the engine, the straw should pull down and stay down. If you see the straw move up and down, you know that the power piston valve spring is too strong for the vacuum produced by the engine at idle and needs to be replaced with a lighter (weaker) spring. Also if the power piston is moving up and down at idle, the engine rpm will also be unstable and will rise and fall.</blockquote> |
| − | This will also tell you if the | + | This will also tell you if the power piston is stuck in the up position if you suspect it might be, like after a rebuild. |
===Power piston springs=== | ===Power piston springs=== | ||
| − | A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs | + | A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs aren't always mentioned when Q-jet tuning is being discussed, but need to be addressed just like if it were a Holley PV. The same principals apply, as far as tuning for low vacuum cams, etc. |
| − | At high vacuum, the vacuum exceeds the PP spring rate and the | + | At high vacuum, the vacuum exceeds the PP spring rate and the primary metering rods are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops, like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the pull from the engine vacuum, which allows the spring to lift the primary metering rods up to their smaller diameter, or "rich" position. |
| − | If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will | + | If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will use a softer rate PP spring. |
| − | Below is an image showing a variety of different springs | + | Below, left is an image showing a variety of different springs. The springs vary in length, coils per inch, and wire diameter (0.012"- 0.020" in the collection below, as an example). |
| + | |||
| + | The image to the right shows three different types of power piston. The power piston on the left with the rod extending from the bottom is used with the baseplate mounted APT adjustment. | ||
{| | {| | ||
|[[File:Power piston springs 002.jpg|400px]] | |[[File:Power piston springs 002.jpg|400px]] | ||
| Line 371: | Line 373: | ||
|} | |} | ||
| − | Edelbrock has | + | Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n 1980 spring kit. These springs are also available in their "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 with a high vacuum, smooth idling and/or wide LSA type cam. | *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. | ||
| Line 379: | Line 381: | ||
===Adjustable part throttle (APT) setting=== | ===Adjustable part throttle (APT) setting=== | ||
| − | Another | + | Another adjustment that can be made to a Q-Jet is the position of the tips of the primary metering rods in the jets. By setting the position of the power piston up or down using the adjustable part throttle adjustment, the primary metering rods can be made to sit higher or lower in the jets at max vacuum. This brings in enrichment sooner or later, as needed. This 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. |
| + | ====Airhorn APT adjustment location==== | ||
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. | ||
| − | + | The later style adjuster is shown with a red arrow pointing to it, below. The yellow arrow points to the hole where the power piston is located. | |
| − | + | ||
| − | + | ||
| − | + | ||
| − | + | ||
[[File:Apt screw details.jpg]] <br style="clear:both"/> | [[File:Apt screw details.jpg]] <br style="clear:both"/> | ||
| + | ====APT adjustment==== | ||
This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway. | This adjustment is called the "adjustable part throttle" setting, or just "APT" for short. Using those terms will help you find additional info on the subject should you want it. The setting procedures will be for a Quadrajet 4-barrel- but they are basically the same for the Rochester Dualjet 2-barrel as well, which is the primary side of a Q-jet carb, anyway. | ||
| Line 395: | Line 395: | ||
If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it. | If you don't have the tool that fits the adjuster screw, carefully slot the screw yourself so you can use a small straight blade screwdriver to adjust it. | ||
| + | |||
| + | 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. | ||
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. | ||
| Line 401: | Line 403: | ||
<br style="clear:both"/> | <br style="clear:both"/> | ||
| − | Now, you can use the choke or your hand hand to partially block some of the air | + | Now, you can use the choke or your hand hand to partially block some of the air entering the carb. Set the APT to where there is a slight increase in RPM as the carb is partially choked, while running at about 2000 RPM. Don't choke it so much that it stumbles or tries to stall, just enough to see if- and how- the RPM changes. The slight increase in rpm indicates the ATP is set just at the edge of being lean. Further adjustments can be made as needed, be sure the engine is always fully warmed up and that the idle mixture screws are spot on before adjusting the APT, and readjust the idle mixture screws after each APT adjustment as well. |
| − | Another method (from SMI, IIRC): " | + | Another method (from SMI, IIRC): "Simply turn the screw DOWN (CW) 1/2 turn at a time until you experience a "lean surge". A lean surge will feel like someone is moving the throttle, or you get a hesitation on light throttle acceleration). Then turn the screw UP (CCW), until the lean surge goes away. |
| − | If you are experiencing an "off-idle stumble", you can turn the screw up | + | If you are experiencing an "off-idle stumble", you can turn the screw up 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 APT adjustment location==== | ====Baseplate APT adjustment location==== | ||
Your Privacy Choices