Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
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====Trouble spots==== Please keep an eye on the following areas during dis-assembly and rebuild. =====Accelerator pump assembly===== If the accelerator pump wasn't working correctly, be sure to attach the linkage to the pump arm hole nearest the pivot to get the biggest pump shot. You can try the smaller shot and if that works well, use it. There are several different accelerator pump rod lengths used depending on the carb, if it's the wrong length this can cause the pump to work poorly. If the springs are reversed and the return spring that should be under the pump assembly is used as the override spring on the shaft, and the override spring is used under the assembly, that can cause it to work poorly. Sometimes the fuel filter spring gets mixed up with the accelerator pump springs as well. If the piston seal is wrong it can have a hard time pushing fuel and/or allowing the pump to refill. There's a ball that acts as a check valve under a screw in the float bowl, driver side. Be sure the ball is in place and sealing as it should. The check ball seat can be re-formed by carefully tapping an old check ball against the seat using a small punch and light mallet/hammer. Sometimes the accelerator pump bore gets glazed where it doesn't seal against the piston very well. The bore can be lightly honed using 600 grit wet or dry. I use a wooden dowel w/a piece of cloth wrapped around the dowel and the emery paper wrapped around the cloth. Then make like a boy scout starting a fire or like you're lapping a valve. You don't want to remove the finish, just break the glaze ever so lightly and to smooth the bore. =====Stripped or loose/damaged fuel inlet threads===== This is an all-too-common problem on the Q-jet. Because the casting is relatively thin and the threads are so large (7/8-20 for most 1971-back carbs, 1"-20 for later carbs), using an oversize fuel inlet to cut new threads is to be avoided if at all possible. The better solution is to use a helicoil-type thread insert. For best results this requires a mill and a fixture for holding the carb body perfectly in alignment for the tap to cut the threads. The tap has to be modified to also cut the seat at the bottom of the threads for the inlet gasket to seal on. [http://www.cliffshighperformance.com/services.html Ruggles] and [http://quadrajetparts.com/rochester-quadrajet-parts-quadrajet-inlet-fittings-service-fuel-inlet-thread-repair-heli-coil-installation-inlet-carburetors-pr-356.html Gessler] will do this operation, cost varies. If using a helicoil repair is not possible for whatever reason, there are self tapping fuel inlets available, like from [http://www.gesslerheadporting.com/gesslerheadporting/ghp.nsf/822dcaaaa26c6da985256dd80023623d/a661ef84a4b8f9f6852574fd005f54f8!OpenDocument Gessler]. =====Loose power piston retainer===== According to Hotrodders forum member '''''TommyK''''': <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. [[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.]] <br style="clear: both"/> =====Leaking float bowl well plugs===== A common problem (mostly) on the early (1968-back) Q-jets especially, is leaky wells. The early leak-prone plugs can be identified by being brass cup-type plugs as opposed to aluminum. The late-type plugs are shown below, left- front plugs arrows, rear plugs circled. The 'cure' for this has long been to use JB Weld, etc. to seal them over. This is just a stop-gap measure, the epoxy will degrade and quit sealing. The well plug stop-leak pads that come in many rebuild kits are a joke (below, center and right). They will seal the leaking rear plugs for a few heat cycles before shrinking down to a fraction of their former thickness. The result is the carb will initially run as it should, ''quickly'' followed by the same symptoms of a leaky plug returning. Do not waste your time with them. If you have a leaking plug, repair it right. [[File:Q jet well plugs.jpg|400px]][[File:WELL PLUG 1.jpg|295px]][[File:WELL PLUG 2.jpg]] The tell-tail signs of leaking well plugs is if the carb float bowl empties overnight. This will usually leave a small hit of fuel in the bottom of the accelerator pump well, but unless the engine starts and runs well when cold (or has an electric fuel pump), the engine will often stall after starting, then will take a lot of cranking for the mechanical pump to refill the bowl and the engine to restart. On the post ~1968 Q-jets, the [B]secondary[/B] plugs are spun in then peened, so to speak. The [I]main[/I] plugs are pressed and sealed w/a soft metal around them- and they can and do leak on some carbs, regardless of the year. Epoxy is not the way to fix a leaking main well plug- tapping them for a machine screw works much longer- as in forever. Epoxy will eventually delaminate from the metal and the leak will resume.Ruggles mentions using Marine Tex epoxy to seal the threads of replacement well plugs, but he states that using epoxy alone to repair a leaking well plug is akin to "cutting your arm off and putting a Band-Aid on it". FWIW, there is an epoxy by Devcon (made in the USA, look for "Devcon Home"). It comes in the older 2 tube packages and the newer double plunger syringe type. On the back it clearly states it <u>*RESISTS UNLEADED FUEL*</u>, salt, gasoline, oil etc. Full set time 8-12 hrs/2500 psi. However, be aware there is no empirical evidence that this epoxy will work for the life of the carb without any delamination or degradation. So, 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 plugs that will seal them for good. The primary wells seldom leak, the secondary wells on the early (1968-back) carbs do leak much more often 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")''] [[File:Quadrajet- Detriorated Epoxy on Wells.jpg|thumb|center|450px|Deteriorated epoxy on a Quadrajet 4MV]]Please see Post #76-78 and #90 on [http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757-6.html page 6] of the thread [http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757-6.html ''"1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive"''] =====Throttle shaft bore wear===== The primary bore of the aluminum throttle plate is where the majority of wear is going to be. The shaft has an anti friction coating (the green colored material seen on the shaft and some linkages) but even so, the shaft wears directly on the aluminum of the throttle plate, so the primary shaft will also show some wear- but being steel on aluminum, the shaft wear is going to be much less than seen in the bore. There are bushing kits to fix the throttle shaft bores that use a brass oilite-type bushing if the bore is worn too much. How much wear is too much? There will be some play in ALL Q-jets (even new ones); that's why determining how much is ''too'' much is a little tricky. Basically if it idles good and the idle is steady even when the throttle shaft is moved fore and aft, it's good to go. But if you get an erratic or unsteady "hunting" idle, or if the shaft moves fore and aft a considerable amount- especially if there's enough wear that the throttle blade can contact the throttle bore- the throttle shaft bore is obviously worn out and needs bushed or the entire throttle plate can be replaced. Wear on the throttle shaft bore is mainly caused by the tension the throttle return spring exerts on the shaft/bore, especially if the return spring is installed wrong (more on this below). Wear on the primary shaft bores of the throttle plate can be avoided by properly orientating the return spring. While theoretically the best way is to use a bellcrank ("Best", below), using a throttle return spring in front of, and in line with, the throttle cable or linkage ("Better", below) is simpler and very nearly as good as a bellcrank assembly: [[File:Throttle return good.jpg|thumb|800px|left|Good hrottle return spring orientation]] <br style="clear:both"/> Throttle return springs used as shown below should be avoided: [[File:Throttle return bad.jpg|thumb|800px|left|Bad throttle return spring orientation]] <br style="clear:both"/> The secondary throttle shaft rarely ever need bushings; even brand new there's quite a bit of clearance between the shaft and throttle plate. If the secondary shaft/throttle plate bore so bad that bushings are needed, the rest of the carb is likely worn very badly; another core or at a least another throttle plate should be considered. =====No drilling/Teflon bushing fix===== [http://67-72chevytrucks.com/vboard/showthread.php?p=4688535 No Drilling / Teflon Bushing Article] shows how to re-bush the primary shaft with no drilling by cutting a teflon sheet into small strips, as shown below. The teflon sheets are available from smallparts.com as well as many other sources online. If this is attempted, be sure that the amount of wear is measured so the right thickness of teflon sheet is used. [[File:QJet 3.jpg|500px]][[File:Teflon_bushing.jpeg|530px]] This fix leaves the body original, however a correctly done repair will not detract from the value of a carb in most cases, in fact it adds to it due to the permanent nature of fixing it with a brass/oilite-type bushing as opposed to using a material like teflon that will wear out much sooner than a bushing. =====Throttle blade screw removal===== If primary throttle shaft bushings are to be installed, the throttle shaft has to first be removed. After removing all the linkages and choke parts from the end of the shaft, the throttle blades will need to be removed. First, the ends of the original screws have to be ground away because they're "staked" at the factory (see image below, left). You may notice the ends are split a slight amount or rounded over to make them impossible to loosen, back out and fall into the intake. All the screws that hold the throttle blades, air valve, and choke flap are staked in this manner. [[File:STAKED QJET SCREW.jpg|border|250px|left]] Should a screw get broken off in the shaft, it can be removed by whatever method available; often drilling is done but be careful to get centered on the broken screw. A drill press is the way to do this best, with the throttle plate clamped down. There's no need to worry about matching the screw sizes side-to-side, in other words, if only one screw broke off, use the other original threaded hole along with the larger hole made for the broken screw. Just use the next larger size and use loctite as well as "peening" the ends of the screws. The original screw thread size is 3-48, however the holes can be opened up enough to use a larger screw like a 6-32 thread if need be. =====Throttle blade screw replacement===== As mentioned above, the screws can be enlarged to 6-32 if need be. The factory stakes or peens the screws to retain them from falling into the engine, you should do the same (the end of the screw has to protrude past the shaft to do this). To do the staking, 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; the blue formula works good. Be careful to not hit the ends of the screws unless the head of the screw is supported against something solid. You don't want to bend the shaft. [[File:Vise grip.jpg|right]] Alternatively, you can use a pair of clamping vise grips to get the jaws on both ends of the screw. This can disrupt the ends of the screws enough to keep them from falling into the engine. If the ends of the screws look too long, they can be ground down with a dremel or small file to the approximate length of the stock screws before peening them. =====Cleaning orifices===== ;Please see posts numbers 23 through 28 on page 2 in the thread ''[http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-2.html#post1374844 Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)]''. One area that needs special attention are the secondary well fill orifices. Most first-time rebuilders don't even realize they exist. The location is shown below. [[File:Sec wells1.jpg]] Do not enlarge the orifices unless you know what you're getting yourself into. If these orifices are made too large the secondaries can run too rich. =====Throttle blades===== 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 them set perfectly. =====Airhorn warpage===== The warpage of the air horn is discussed in Ruggles' book. For additional discussion, please see posts 49 through 53 in the thread, ''[http://www.hotrodders.com/forum/rochester-quadrajet-4mv-carburetor-removal-disassembly-rebuild-rookie-level-190558-4.html "Rochester Quadrajet 4MV Carburetor: Removal, Disassembly, Rebuild (Rookie Level)"]'' =====Cleaning idle tubes===== The procedure for removal, cleaning and re-installation is covered in Ruggles' book (chapter 5, "Rebuilding the Carb", pages 77-78). It is also discussed on the thread ''[http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757.html "1977 Chevy K20: Engine Suffers from Hesitation/Power Loss When In Drive"]'' ([http://www.hotrodders.com/forum/1977-chevy-k20-engine-suffers-hesitation-power-loss-when-drive-213757-4.html page4], post # 53, 58, 59).
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