I have an 800 CFM Quadrajet on a 455 with a TA 230 - 240 (50 thousands duration) cam, 9.5 to 1 compression ratio, ported stock intake, ported stock heads with Stage 1 valves with ported exhaust manifold. According to the build sheet the Quadrajet has primary 77 jets, primary 45B rods and the "idle air by-pass" has been plugged. The car idles and operates very rich. Have never noticed black smoke, but the tail pipes are very sooty, and the exhaust smells rich. Mileage is a little short of where it should be. Am planning to change the jets to 75 to reduce the difference between the jet to the rod to an even 30 thousands, but was wondering if the idle bypass should be opened back up. According to Doug Roe's "Rochester Carburetors" the idle air bypass reduces nozzle drip, which sounds like a good thing. Should I open this back up?
Hi Loyd...... How much Vacuum at Idle?.... Idle screws all the way out?...... My thinking is to stagger jet it with a 77 and a 79, then move up on the metering rods till it gets flat feeling then back up a bit......I think the air by pass mod is to stop a stumble under light acceleration, but I thought you were suppose to make them bigger?......I haven't fooled much with drivablity problems so I maybe way off base..... o No: Mileage......Now thats funny....... :grin: :grin:
Hi Keith Idle at vacuum was about 13 inches the last time, I checked. The idle screws overall do not seem to make much difference, but I have taken them in then out a turn and a half, maybe two. If 47B rods were available I would leave the 77 jets in, but Summit does not even list the 47 rods anymore. Know where I could get a pair? Yeah I know, mileage is not really that much of a concern when you drive a 462 cubic inch engine, but I am concerned about all that fuel washing down the cylinder walls. Besides the 200 4R with the 3.42 gears should yield better than 12ish.
The fixed idle air bypass, is standard on big block carbs. It's purpose is to allow extra air to bypass the throttle blades, so that the engine can idle with a smaller throttle opening. This is a good thing, especially for an engine with a cam. I'm sure you've heard of guys drilling holes in their throttle blades to accomplish the same thing. Who plugged them up? Not a good idea, I think. I have a John Osborne prepped 7042240 800 carb. It has 76 jets, with 46B primary rods, and BG secondary rods. I know John didn't plug my bypass up.
Loyd, I run nearly the same set-up and if you haven't switched to a lighter enrichment spring, you're probably into full enrichment at part throttle. I'm talking about the spring that makes the primary rods pop up. Vacuum holds it down and with a cam like that you're not starting with alot to begin with.. o No:
I'll check through my stash of carb parts, I might have some metering rods. I checked with my GM parts guy, he was going to look them up and see if there still out there new.... I'm thinking your vacuum is a little low but that could have something to do with the rich idle. Mandys car idles with 17-18 in at 800 rpm with a KB 118 The idle mixture screws not working is telling me you need more bypass air. Have you looked at the thottle blade position to see if your in the transfer slot at idle?.. I'm thinking Thunder may be right that your pulling on the metering rods at idle, you can put a screw driver in the hole on top and hold them down (very carefully) and see if it clears up the idle.
I've got a set of 46C rods in my stash, don't know whats the difference between the B's and C's are. I checked with GM, the 47B rods were discontinued in 1985, the 48B's are still out there so I ordered a set, only $7.00 ea.... o No:
Thank goodness it has finally warmed up enough to tinker in the garage again. Obtained some 75 jets to go with the 45B rods that are in the carb. I was advised to take things a step at a time. That is replace the 77 jets to get the difference between the rods and jets down to 30 thousands. Quadshop Jim at the BOP swapmeet today verified that 30 thousands difference is a good ceiling for street performance and driveability. It that does not work the next step will be to open the plugged air bypass. Quadshop Jim also advised inserting a soda straw above the power piston while the car engine is off, then gently pushing down to mark the lowest power piston position on the straw. Remove the straw. Then start the car and let warm up, before placing the straw above the power poston to see if the vacuum is adequate to pull it to near the lowest position. Looking around the garage I found a short length of 1/4 OD copper tubing left over from another project. It fit the vent tube perfectly (with adequate clearance to slide easily) and thus settle on the arms of the power valve and not slide off. Gently pushed the tubing down and marked the lowest position. Then noted where it raised to with the engine off and no vacuum. When I started the car the tube slid down to the marks, bliping the throttle did not effect its location. With a fast idle you can see the front dashpot start to release. When I shut the engine off the tube return to the highest position. I know that does not cover part throttle transitions, but at least at idle it is where it should be. Will report when the 75 jets are installed and the effect on the idle and normal street operation.
Got the 75 jets in and the bung welded in for a wide band sensor. Therefore I can now monitor the air fuel ratio with the Innovative LM1 meter. The Chicagoland Chapter had our dyno day on Saturday (April 9th), which was the first chance to try out the device. At idle it was 14 and change for the air to fuel ratio which is pretty good. First pull with the dyno yielded a smooth transition from light load, but the AF ratio was in the high 15s. Engine pulled 305 HP at the back tires. Pulled the AY (.0567") ....#1953 rod out and replaced with a CK (.0527") ....#1952 Second pull had a giant bog in the transition from light load to pull, with slight increase in HP, but AF ratio was in the low 15s. Pulled the AY CK (.0527") ....#1952 rod out and replaced with a CE (.0410") ....#1951 Third pull had the same bog in the transition when the throttle was mashed, but she pulled 327 hp with AF in the high 13s. How do I fix the bog in the transition and where do I need to go from here. Obviously it likes the CE rods better and they stayed in.
Loyd, The front dashpot/vacuum break should not release at any idle speed, fast or not. It is leaking, and not holding vacuum, replace it. THis will make the engine bog when transitioning from primary to secondary operation. You can check it with a mityvac or equivalent. It should hold vacuum.
You're headed in the right direstion on the A/F ratio; keep going. At WOT you want to be at 12 or so. (13-14 at part throttle)
