Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
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==Tuning== Some basic [http://www.73-87.com/7387garage/drivetrain/myqjet.htm tips and links], from from 73-87.com. ===Air fuel ratio=== The a/f ratio requirements vary with the demand; richer for high demand (low vacuum) situations, leaner for light throttle cruise conditions, and in between these two when accelerating at less than WOT. [[File:Af chart1.jpg]] ===Air/fuel ratio meter=== Using an air/fuel ratio meter makes tuning a lot easier. They are available from several manufacturers, [http://www.fuelairspark.com/Products/FS-%27Digital%20Air/Fuel%20Meters%27-0.aspx this] is an example from FAST, there are several others: Innovative, Edelbrock, Auto Meter, etc. There are also instructions available online for making an A/F meter, an example can be seen [http://www.scirocco.org/tech/misc/afgauge/af.html '''here'''], from scirocco.org. ===Basic carb adjustments=== [http://vetteworks.tripod.com/qjet.htm Rochester 4MC, 4MV, M4MC, and E4ME Carburetors] from vetteworks.tripod.com ===Idle circuit=== *[http://www.73-87.com/7387garage/drivetrain/qjetidle.htm Q-jet Idle Problems] from 73-87.com ====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 with the screws. Usually about 0.090" is sufficient. 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). ====Idle bypass air==== Similar to drilling the primary throttle blades, the idle air bypass orifices introduce air below the primary throttle blades. These orifices can be enlarged/reduced to help some engines idle better. The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller. There are different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them using a drill bit. Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off with lead shot tapped into the casting channels/orifices, to make the carb "generic" as opposed to being tuned for a specific, exact application. Generally speaking, there's no harm in doing this as long as it has been done correctly. Unfortunately, trying the carb on your running engine is about the only way you can know for sure, unless the desired orifice sizes are known ahead of time. Fortunately if need be, the lead can be fairly easily removed, or drilled to give bypass air. The rebuilders block them in various places: sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find these bypass holes. Often these rebuilt carbs will use cheap unmarked, unplated brass metering rods and might even have unmarked jets. The older 'tunnel' type bypass air channels (circles): [[File:Bypass 1.jpg]] The later 'tab' type bypass air orifices, most look like the one shown below: one orifice is in the left tab, the other orifice is just inside the bore (arrow on right). Some have two orifices like on the left (right tab would be where the circle is). [[File:Bypass2.jpg]] ===Power piston=== From quadrajetparts.com: <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 (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 power piston is stuck in the up position if you suspect it might be, like after a rebuild. ===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 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 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 use a softer rate PP spring. 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 pistons.jpg|400px]] |} Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n 1994 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 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/Delco PP springs come in packs of 10 springs unless you can talk your way into buying less. ===Adjustable part throttle (APT) setting=== 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. 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"/> ====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. The following is with the carb disassembled: 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. [[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. <br style="clear:both"/> 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): "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 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==== 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. These baseplate-located APT adjustment screws and wire assemblies are subject to being heavily carboned up and stuck in place, If the carb has been in service long, the baseplate needs to be removed and the APT assembly removed and thoroughly cleaned before being adjusted. Be sure to measure how far from the baseplate the wire spring protrudes BEFORE removing it so it can be returned to the same position as a baseline setting. {| |[[File:Apt baseplate plug and wire.jpg|thumb|center|400px|Red arrow points to baseplate APT plug location; yellow arrow shows wire that lifts the power piston so set the APT adjustment]] |[[File:Apt base plate.jpg|thumb|center|700px|A = APT adjustment screw; B= wire spring that lifts/lowers power piston as screw is adjusted (yellow arrow at left); C = power piston having bottom pin (pin not used with airhorn-adjusted APT)]] |} ====Aneroid or spool location==== There were also additional Q-jet designs used mainly in 1975-'76, that used an auxiliary enrichment system in addition to (or in some cases instead of) the primary PP system. One has an accordion-shaped aneroid "bellows" that lifted/lowered a single metering rod in a jet; it used no vacuum or power piston to operate it. The aneroid bellows changed the air/fuel ratio in accordance with the barometric pressure. Another was identical to the aneroid except the aneroid was deleted and was replaced by a filler spool that did not respond to barometric pressure; instead it was adjusted to give the correct air/fuel ratio according to ambient conditions only. These systems was found to be lacking, so were discontinued. {| |[[File:Aneroid.jpg|thumb|center|350px|Red arrow points to aneroid adjustment screw through hole in airhorn. Hole is plugged with a cup plug.]] |[[File:Aneroid2.jpg|thumb|center|360px|Aneroid shown with airhorn removed]] |} [[File:ANEROID OR FILLER SPOOL DETAILS.jpg|thumb|left|600px|Early APT, ca. 1975]] <br style="clear:both"/> The other system uses a vacuum operated PP like the primary PP, but it differs by using only one metering rod and one fixed jet to feed both sides of the primaries. These were abandoned soon after they were released; it was found the primary PP system could be tailored to meet the requirements without the added complexity of the auxiliary system. [[File:Qjet aux PP 75-76.jpg|thumb|400px|left|1975-'76 auxillary power piston equipped Q-jet. This one is from a Pontiac]]<br style="clear:both"/> ===Secondary tuning=== #[http://www.73-87.com/7387garage/drivetrain/damon.htm Performance Tune Your Q-jet Secondaries, Part 1] from 73-87.com #[http://www.73-87.com/7387garage/drivetrain/damon2.htm Performance Tune Your Q-jet Secondaries, Part 2] from 73-87.com ====Secondary air valve==== [[File:17080213 3519 CPX 1980 Chev, rear po, hot air choke.jpg|thumb|480px|left|Arrow indicates air valve limiter]] If the engine exhibits a flattening out at midrange rpm and everything seems to be in order, check to see that the secondary air valve spring tension isn't too tight, causing the air valve to not open fully. The secondary air valve only opens when the engine is under a load and the primary throttle blades are opened about half way or more. If the AV is adjusted so the spring pressure is too light, it can open too much, too soon- and the result is a bog. The AV shouldn't open when you jazz the throttle in PARK/neutral. Only when under a load, and at sufficient RPM do you want the AV to open. Also, there shouldn't be a noticeable transition into the secondaries. Many guys think the carb should "bang" into the secondaries, but what's happening is a bog when the secondaries open, followed by the engine 'catching up' to the carb- this gives the impression of acceleration. A timing slip will show otherwise. You can get a 'feel' for how it's adjusted by pushing the AV open (engine OFF) with a finger. It should shut without hesitation when you let off pressure. After you've been working with the Q-jet for a while, you will get a feel for how the AV is adjusted just by doing this. There is an adjustment for the AV spring. Should you want to try different adjustments on the air valve, use a 3/32" allen wrench to loosen the lock screw under the AV spring adjustment. 3/4 of a turn from first contact of the spring is the base setting, changing the setting 1/8 to 1/4 turn at a time will get it dialed in. The next thing is to check to see that you're getting full opening of the secondary air valve. Some carbs had the amount of the air valve opening set to a lot less than it's capable of. The fix is easy, just look where the red arrow (left) is pointing, that's the stop. It can be carefully bent to set the opening amount. I will use a Dremel to slot the linkage to make bending it easier- I prefer that over grinding the stop in case it needs further adjustment to lessen the opening amount, because the idea is to increase the opening until you either reach the max it will open or to where it doesn't help performance any more- in which case you'd decrease the opening back to where it was best. The linkage can be bent several times without breaking as long as the cut isn't made so there's not enough metal, and it's not bent too many times. <br style="clear:both"/> Conversely, if the carb is going into the secondaries smoothly, yet the engine seems to lay down even though everything is as it should be, the air valve may be opening too far; that is, the engine doesn't need all the flow the Q-jet can supply, at WOT. If that is the case, carefully adjust the stop to provide less AV opening until it accelerates cleanly and strongly to redline. ====Pulloff orifice==== Once that's squared away you can go about quickening the opening rate by enlarging the pull off orifice. One way to do this to go very large on the factory-installed orifice, then solder the opening shut. From there, enlarging the orifice is as easy as using a small numbered bit in a pin vise. If you go too large, a touch w/a soldering iron will close it back up. And plan on going too large during the tuning of the orifice size, because you will keep enlarging it until you go too far and get a bog, then you back up to where it was best (just like the opening amount of the air valve). Some pulloff diaphragms have a 90 degree bend in the vacuum tube. If there is enough length before the bend, the 90 degree can be cut off in order to reach the orifice. If not, the tube can be carefully ground down at the 90 degree bend enough to access the orifice with a drill bit, then enlarge the orifice oversize as explained above. Reseal the tube and tune it by changing the orifice size at the soldered over tip of the vacuum tube.
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