quadrajet idle adjustment, Fast Idle Adjusting Screw B-09249 B-09251 c D. E. 320. Adjust both screws equally and find the right mixture. Since the primary jet is larger, I would need a larger diameter metering rod as well but only the following Edelbrock M-series rods are now available:  0.048" / PN1941, 0.050" / PN 1943, 0.052" / PN 1945, 0.054" / PN 1947. If combustion occurs at the right time, the pressure peak in the combustion chamber will occur when it can exert the maximum torque on the crankshaft, thereby maximizing power and fuel economy.After fixing the plugged primary circuit problem, I started off with my APT turned in all the way (ie, 0 turns) and found that my fuel economy was poor and I had a lean surge. With every APT 1/2 turn, I would reduce my idle speed back to 2000 RPM. 323. The Rochester Quadrajet spreadbore 4bbl carburetor has a much smaller, centrally located fuel bowl that seems to work well for off-road vehicles. Since I also had a surplus Rochester Quadrajet that I removed my 1977 Pontiac in November of 1997 (for a propane conversion I completed in January of 1998), I thought I would give it a try in August of 2004. There is plug in the Quadrajet's cover that is removed to access the APT without having to disassemble the carb. He may be reached via the Corvette Forum and I would ask him for his paper before trying to tune your Quadrajet. Set desired speed with the Idle speed screw. Adjust the throttle idle screw to the desired idle speed. 1. Go back to the mix screws and see if you can improve on the vac reading. One of GM's better ideas was to include a way to fine-tune the part-throttle fuel mixture with the Adjustable Part Throttle (APT). This spring is not normally used on Idle Mixture screws. Try manifold vac. 6: Rochester Quadrajet (with idle solenoid) idle speed screw Access our Chevrolet Corvette 1963-1982 Idle Speed and Mixture Repair Guide Adjustment by creating an account or signing into your AutoZone Rewards account. Also used on Fast Idle screw on Rochester carburetors. SC drilled a cone into this plug so that it can be removed with an ordinary wood screw. area at cruise speed and at idle. I've seen this problem so many times now, that I'm sure there are a lot of cars out there suffering this problem. If you plan to fine-tune your Corvette's Quadrajet carburetor, one tool you may want to have handy is a handheld tachometer.This allows you to check and adjust the vehicle's idle speed while you wrench under the hood. The APT has a rectangular head, which requires a special tool. Hopefully, they will give me some insight. It's been doing its thing for 40 years on my '70 L48 350 Monte Carlo--forever known as … Fig. Determine secondary metering rod hanger length. Might need more initial timing to get your idle. I figure if someone might be able to steer me in the right direction on the idle speed screw, I'll then be able to dial in my idle mixture screws to see if I can rid myself of an annoying, "The man who lets a leader prescribe his course is a wreck being towed to the scrap heap.". & see what happens, your idle will come up & you'll be able to close the blades a bit. From my internet survey of APT adjustments, I believe that the APT may be turned out to a maximum of 8 to 8½ turns. Sticking with the 52 metering rod, I have the following results for a variety of main jets: Besides having the quickest 0-60 time, the #73 jet also had the best seat of the pants feel. After replacing a rusty gas tank in 2015, I ran the AFB again to ensure that any residual rusty silt didn't settle out in my newly rebuilt Quadrajet. Assuming you are starting with a freshly rebuilt carburetor, LG's Quadrajet tuning procedure has the following steps: Verify that that the float level is correct. The length of the non-metering portion of the metering rod was 2.01" and the length to the top of the power step was 2.026". If the part-throttle fuel mixture is nowhere close to being in the ballpark, no matter how rich the choke is set, the fuel mixture mixture will be too lean until the intake manifold is fully warm and will therefore require much gas-pedal pumping during warm-up. Another challenge I discovered was with carb idle and ignition timing. Add your vehicle in Manage My Vehicles. The OEM specifications for my engine are: The commonly recommended performance timing for my car is: By advancing my initial timing from 5° to 8° BTDC, the advance curve in my DC distributor (P3690788) provided the recommended advance (distributor: 11° @ 800-1200 RPM centrifugal w/ single red spring & 7° vacuum). It is quite common for the screws to be loose, producing not only a vacuum leak around the base of the fuel bowl, but violating the seal for the idle fuel transfer passages. OK,IT'S sure not the initial being retarded. Screw, fast idle … While I was driving the car in 2017 to get my APT setting right, I found that I had a persistent lean cruise and eventually reached point where I had the APT turned out 6 turns. Set the screw in the position where the engine’s idle sounds most even and smooth to … Holley sells these small jets in individual pairs or complete tuning kits. Open the throttle plates till the transfer slots form a square. Buzzbom, you said "My initial timing is 20 degrees with vacuum on ported". Leaner fuel mixtures tend to burn slower, which is why many people mistakenly believe that leaner mixtures are hotter. Although the primary jets interchange between all years of manufacture, there was design change in 1975 and the primary metering rods from 1965-1974 (PNs starting with 70) are 0.080" longer than those from 1975-1980 (PNs starting with 170). After raising the APT to 5½ turns, I felt that the engine had reached the practical limit of part-throttle enrichment so I used that as my APT adjustment starting point. To eliminate this surging, with my transmission in Park, I set my idle speed to 2000 RPM so that I would be off the idle circuit and on the primary circuit. It is used for idle speed setting and on fast idle screw. This article is about my experience with calibrating a M4MC Rochester Quadrajet to work properly on the 225 slant six in my 1965 Barracuda. Since this can be quite expensive due to the time required to remove & replace the carburetor while changing primary jets, I opted for the much cheaper time-trial method. So we run the same calculation for a.026” diameter rod inserted in the jet. Make adjustments 1/2 a turn in either direction to find the smoothest idle speed. A pin sticks out of the side of the piston, which hits the APT stop at high manifold vacuum. Dont restart it,just catch the carb when it starts dribbling and then back off the fuel line which will release/lower the fuel psi to see if that stops the dribbling . If turned out this far, the metering rods would be sitting very high out of the jet so that the economy fuel mixture would be fairly close to the power fuel mixture. I suspect that this improvement comes from the reduction in pumping losses with increasing throttle opening (lower manifold vacuum). Spring, Idle Speed / Fast IdleOur Part # 2041. http://garage.grumpysperformance.com/index.php?threads/q-jet-info.1144/ http://garage.grumpysperformance.com/index.php?threads/looking-for-q-jet-index.7763/ http://www.route66hotrodhigh.com/PowerTuneQjet.html http://www.gmcmi.com/wp-content/uploads/2014/05/Quadrajet-Carb-Primary-Metering-Rods.pdf http://www.corvetteforum.com/forums/c3-tech-performance/1023501-updated-q-jet-listing-shows-available-parts.html, http://www.cliffshighperformance.com/ https://quadrajetparts.com/ http://www.edelbrock.com/automotive/mc/carburetors/access-q-jet.shtml http://jetchip.com/product-category/performance-carburetors/rochester-quadrajet/rochester-quadrajet-parts/, http://garage.grumpysperformance.com/index.php?threads/q-jet-info.1144/, http://garage.grumpysperformance.com/index.php?threads/looking-for-q-jet-index.7763/, http://www.route66hotrodhigh.com/PowerTuneQjet.html, http://www.gmcmi.com/wp-content/uploads/2014/05/Quadrajet-Carb-Primary-Metering-Rods.pdf, http://www.corvetteforum.com/forums/c3-tech-performance/1023501-updated-q-jet-listing-shows-available-parts.html, http://www.edelbrock.com/automotive/mc/carburetors/access-q-jet.shtml, http://jetchip.com/product-category/performance-carburetors/rochester-quadrajet/rochester-quadrajet-parts/, #52K - 0.052" [Single Taper (0.052 - 0.046) & Step]. 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