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CarburetorsCarb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
So I went through my shop the other day and found no less than 5 Q-Jets. 3 of them were the 850 cfm version everyone calls the 800 cfm. The ones I have were built and used on 305 and 350 trucks and vans between 1977 and 1984. My first victim is a 1984 model 17084226 that came off an 84 G20 van with a 305. It is destined to run a healthy 350 Vortec.
Stock it had 72 jets, 50m primary rods, DR rods on a M hanger. It was also calibrated fairly lean in both the idle tubes and the idle mixture screw holes. It was drilled for idle bypass holes but they were pretty small as well given the mild nature of the factory 305 and factory 305 cams.
I rebrushed the throttle shafts, stepped up to 77 jets and down to a 48M rod, installed a 8 in/hg power spring, fattened the secondaries up to a CE rod and P hanger. Drilled the accelerator pump fischarge holes to 0.035". I cut 0.150" off the accelerator pump plunger taking it from 2.65" to 2.50" then replaced the spring between the plunger and accelerator pump cup with a strong factory spring. I drilled the idle air bypass holes to 0.09375". I drilled the idle down tubes to 0.040" and slightly enlarged the idle channel restrictions to 0.060". Then I drilled the idle mixture screw openings to 0.09375". I adjusted the APT screw 3 turns from the bottom. I drilled the secondary pull over enrichment holes to 0.055" and opened the POE well restrictions to 0.040". Then drilled the air bleed tube for the secondaries main discharge tube to 0.036". I added primary pull over enrichment tubes to the airhorn by first drilling the passageway for them higher into the airhorn casting, then intersecting the new passageway with a 0.025" hole, finally tapped the Primary POe tubes into place that extend down to the fuel bowl. Primary pull over enrichment discharges on the top side of the primary venturis behind the choke plate. Allows you to run a leaner low load primary setting and still get additional fuel around 1/2-2/3 throttle on the primaries before you open the secondaries.
I will be testing this carb on my 383 that is currently on the run stand. Had to tune up that engine though as the cap/rotor and plug wires were all more than 10 years old. Gave me a nasty little shock setting the timing. Had a plug wire that was burned through by header heat and also a plug wire tower on the cap that was cracked.
I cleaned the carb in a heated ultrasonic bath of simple green and water and then followed that with vinegar and water.
Cleaned parts.
First step is to bend the primary linkage stop tab to give The primary throttle blades a 90° opening, followed by bending the secondary pickup tab to get the secondaries to 90° opening.
Then I used a pin vice or a power drill depending on the drill size to drill everything out that needed enlargement.
I also used a flat center punch and an old check ball to create a new seat for the accelerator pump discharge ball as well as the redrilled float seat.
Mini torch helps in the removal of the idle down tubes.
Will get some more pictures on the 2nd carb build. I am bad about not documenting everything.
New cap, rotor and wires for the test engine. 17083226 q-jet is on it now, getting replaced with the recalibrated 17084226.
Sounds like you know what you are doing? What may I ask is your experience and background? If I EVER get to mod* the stock 403 Olds / AT smallblock, in my 1979 "Y84 Bandit" Trans Am, could you set up one of those Rochester M4ME's and sell it to me?
* flattop pistons to go from 8-1 to 9.5 to one, MILD Cam upgrade, port heads, true Dual Y Pipe, recurve distributor.
Sounds like you know what you are doing? What may I ask is your experience and background? If I EVER get to mod* the stock 403 Olds / AT smallblock, in my 1979 "Y84 Bandit" Trans Am, could you set up one of those Rochester M4ME's and sell it to me?
* flattop pistons to go from 8-1 to 9.5 to one, MILD Cam upgrade, port heads, true Dual Y Pipe, recurve distributor.
I am a parts guy that has a passion in cars. Been working with them for close to 20 years. I drove some 80s vehicles in the early 2000s because I couldn't afford a new fancy car or truck at the time. Have played with quadrajets since then as well as TBI and TPI. With the help of Doug Roe and Cliff Ruggles have learned alot about them and how they function. I still probably build 4-6 carbs a year. Recently did a M2ME dual jet for a 79 Camaro LG3 305 for a local woman that worked at the GM Arlington plant in the late 70s and 80s that bought the car new with her employee discount. It is a true survivor car.
I could find a good M4ME and build it for you. I also do plating on them. Wouldn't be something I could do immediately. Work and projects have me well occupied at the moment. Oldsmobile cores are fairly different. Do you have access to a good oldsmobile M4ME to build? Would be best to start with a 350 or 403 olds carb because the linkages, fuel inlet and vacuum ports will be correct for the setup. Oldsmobiles have a funny throttle linkage and a forward facing fuel inlet as well as a small power brake vacuum fitting. I can make these carbs idle smoothly down to around 5-6 in/hg vacuum so a mild cam is not an issue. Typically a "RV" sized cam pulling around 15 in/hg vacuum requires only a little idle recalibration and some additional fuel via the jets and metering rods. Not sure I would want to go much bigger on a garden variety oldsmobile 403 with their windowed mains. Years ago I had a solid main 403 but they only seemed to use them in the motorhomes and those are scarce now. Few were built and even fewer have their stock engine. A hot setup years ago was a DX diesel block with a 425 crank and an 1/8" overbore. Made for somethibg like 460 cubic inches in a small block olds shell.
If you need a Q-Jet done to the show quality, Lars, Sean Murphy or Cliff Ruggles are the guys to contact.
It is finished!
I filed the base flat for stop and bent the backside of the secondary lockout pin for a positive stop to prevent the secondaries from over centering and starting to close again. Choke plate and shaft are removed but the electric choke thermostat is in place along with the high idle cam and the secondary lockout. This allows for an easy warmup and prevents you from blowing the engine up while it is still cold. Choke pulloff releases in 1.5-2 seconds. I filed the secondary air door stop to give 1.27" opening from the rear of the casting to the top of the air door. Exactly the dimension of the performer rpm 850cfm carbs. I notched the leading edge of the airdoor behind the secondary POE openings. Float needle seat is drilled to 0.149 and created a new seat with a steel ball, a punch and a hammer. I broke out my APT adjustment tool. Will adjust the APT once I have it running on the engine. I first get it fired up and warmed up. Adjust the idle speed and mixture screws for lean best idle. Then with the idle speed screw spring removed, adjust the idle screw to get about 2,500 rpm. With a vacuum gauge connected I adjust the APT until I reach peak vacuum then screw it down enough to drop off about 1 in/hg vacuum. At this point a light blip of the throttle should still give smooth throttle response. I call this lean best cruise. The fuel mileage will be notably better with the idle mixture at lean best and the cruise mixture at lean best. If you go too lean you will get a stumble and a flat spot as you open the throttle. That 1" drop usually puts the air/fuel mixture in the 16-16.5:1 range. Remember the APT is the lean stop, under load the mixture will get richer as the vacuum drops and the power piston overcomes the vacuum and rises. At this point I shut the engine down, remove the idle speed screw, re-install the spring and readjust to the previous idle speed. This 383 engine idles comfortably at 700 rpm or less. With the air cleaner installed this baby looks like your well worn nothing special Q-Jet since I skipped plating things on this one for a sleeper look. Actually skipped fully cleaning the exterior of the choke thermostat and choke pulloff even.
I also broke out my abrasive cleaner, cleaned the plugs, regapped to 0.060" to take advantage of my Accel wires, MSD cap and rotor as well as lightly used Accel super coil and module I dug out of hiding in a box that was last run on my old G20 in about 2004.
I am actually planning a back to back test for efficiency at light load. I have this fresh Q-Jet as well as a fresh Edelbrock carb. I will adjust the idle mixture on both using lean best. Also have several sizes of primary metering rods for the Edelbrock to dial in the low-load cruise mixture as best as possible. Will adjust both to run at 2,500 rpm and run them out of fuel. Will supply each with 1 quart of fuel and run them at 2,500 rpm while timing the runs. Also plan to test the Q-Jet with and without a pair of knock off flowmaster 40s. I have a feeling the best fuel consumption will be with the Q-Jet and mufflers. I am also thinking of doing a manifold vs ported vacuum idle test as well just to see how much difference it makes in fuel consumption with the idle adjusted to the same 750 rpm with and without vacuum present to the vacuum advance, I bet it is signifigant..
Its ready for some run testing... I have to work tomorrow at 12:30. If I wake up early enough I might get it fired up tomorrow. Otherwise I will do it Friday morning on my day off. But have to pickup a sofa from a furniture store and drag it 80 miles away for my mother Friday afternoon.
18" vacuum at idle, is that what I see? Which cam again?
About that. Its a Comp 218/228 on a 108 LSA with 1.7 rockers. 0.578 lift. Rhoads V-Max lifters cold lashed 0.014" from the bottom of their travel. Its weird but how its done. Adjustment should be 0.020" HOT but deducting 0.006 off the cold does the same thing..
I am actually going to rebuild the HEI tomorrow. The advance weight pins are heavily worn. Secured a pair of replacements for $10. Going to throw a recurve kit and an Accel adjustable vacuum advance on it. Get it running perfectly.
I also have the Rhoads V-MAX lifters in my 302 Chevy running on a SOLID FT camshaft. Jack Rhoads says it's OK.
Weird adjustment procedure, I agree. I don't fully understand it, but the lifters work. They doubled my vacuum at idle vs. conventional solid tappets.
I estimate that Comp cam you have with conventional lifters should pull about 13.5" vacuum in a 305.
The advance curve kit weights need to be "glass" hard like stock GM weights. Use the same way to test as they do on Forged In Fire - a file should dance across the surface without biting in. The ONLY curve kit that I have ever found that actually has hardened weights is Moroso. I have more than a few cheap curve kit weights that are worn, and you wouldn't believe what that does to your timing curve. You will go crazy trying to troubleshoot this problem.
Last edited by NoEmissions84TA; 04-08-2021 at 09:23 PM.
I also have the Rhoads V-MAX lifters in my 302 Chevy running on a SOLID FT camshaft. Jack Rhoads says it's OK.
Weird adjustment procedure, I agree. I don't fully understand it, but the lifters work. They doubled my vacuum at idle vs. conventional solid tappets.
I estimate that Comp cam you have with conventional lifters should pull about 13.5" vacuum in a 305.
The advance curve kit weights need to be "glass" hard like stock GM weights. Use the same way to test as they do on Forged In Fire - a file should dance across the surface without biting in. The ONLY curve kit that I have ever found that actually has hardened weights is Moroso. I have more than a few cheap curve kit weights that are worn, and you wouldn't believe what that does to your timing curve. You will go crazy trying to troubleshoot this problem.
I tend to use GM weights and a GM center section. Just use the aftermarket stop bushings and the springs. From memory I usually use a 369 center section and 41 weights as they offer a decent advance curve that is limited to 22° centrifical.
Going through a couple of bins of used weights and center sections I have in one of my organizer trays found a combination to try. Should limit the centrifical advance to 14° crank. I found a 363 center and a set of 053 weights. The 053 weights take about 1° off the mechanical advance. The 363 center gives about 15° at the crank advance. I will bump the initial to about 16° and move the vacuum advance to ported. Will idle around 16° of timing and reach 30° around 2,800 with two light weights. Total advance should reach around 40 @ 3,000 with no load. Will verify results with my timing light. Installing the setup now with the new weight pivot pins.
Runs really nicely with the 353 center, 053 weights and medium springs from a Mr Gasket recurve kit. Installed a Standard VC236 which is the standard vacuum advance on many later 70s and early 80s 305 and 350 cars, trucks and vans. Installed it with a limiter bushing from the Mr Gasket kit. Set the initial timing at 16°. The centrifical advance adds 15°. Starts at about 1,500 and full in by 3,600. The vacuum advance starts about 6 in/hg and is full in by 12 in/hg with 15° added. At 3,600 rpm total timing is 46°. The idle timing is steady as the rock of Gibralter. I did the fuel consumption test I mentioned I was going to do between the carbs. Unfortunately the front crank seal decided to start puking before I got to the muffler test with the Edelbrock. The mufflers added 30 seconds run time to the Quadrajet while dropping 1 in/hg vacuum. Expect similar results with the Edelbrock because the vacuum was the same 23 in/hg @ 2,000 with both carbs. The vacuum advance was a no go on this engine. It HATES not having a functioning vacuum advance at idle. It runs rough and just sounds like crap. I took a smart water bottle, measured out 1 qt of water with a measuring cup then marked the bottle and replaced the water with fuel after drying the bottle with compressed air. With the fuel line and float bowls empty, spun the engine over until I primed the carb and then fired them up. The Q-Jet ran a full minute longer and that is not including the 20 seconds or so the Edelbrock was running on one float bowl. I ran both with a 3/4" open center spacer under them. Same ignition timing settings. Same manifold vacuum port on the carbs for the vacuum advance. Both running at 2,000 rpm. Both with the idle mixture set by lean best. I ran the Q-Jet with the APT 5 turns out and then 2.5 turns out. 2.5 turns made over 2 minutes run time difference between the settings on the Q-Jet. Headers definately ran hotter with the leaner setting though. With the Quadrajet and the Edelbrock throttle response is flawless. I winged both to nearly 6,000 rpm on the primaries practically instantly.
The vacuum advance was a no go on this engine. It HATES not having a functioning vacuum advance at idle. It runs rough and just sounds like crap.
I will have to check my notes, but IIRC my engine DOES NOT like any vacuum advance at idle.
You said you used a limiter bushing on the vacuum advance canister. I thought that was not possible with stock canisters. Did you mean the notched timing limiter PLATE?
Last edited by NoEmissions84TA; 04-09-2021 at 09:42 PM.
The vacuum advance was a no go on this engine. It HATES not having a functioning vacuum advance at idle. It runs rough and just sounds like crap.
I will have to check my notes, but IIRC my engine DOES NOT like any vacuum advance at idle.
Guess I should have worded that better. Mine barely idles without the vacuum advance connected and when you raise the idle speed to compensate it is very brash and rough sounding like the cam is 10° larger. The headers get very hot almost instantly as well.
The vacuum advance was a no go on this engine. It HATES not having a functioning vacuum advance at idle. It runs rough and just sounds like crap.
I will have to check my notes, but IIRC my engine DOES NOT like any vacuum advance at idle.
You said you used a limiter bushing on the vacuum advance canister. I thought that was not possible with stock canisters. Did you mean the notched timing limiter PLATE?
Put one of the plastic bushings over the end of the shaft inside the slot under the lever on the pickup coil. It slightly moves the at rest position as well as the stop position. Brings on the advance slightly slower and limits out slightly lower and sooner. Make sure to re-adjust the initial after doing this as it will change by about 1° from the slight preload on the vacuum advance so to speak.
Yes, the bushing limits both ends; the timing plate limits only the rest position, and you also have to change the initial to compensate, like you said above.
Yes, the bushing limits both ends; the timing plate limits only the rest position, and you also have to change the initial to compensate, like you said above.
Yep and definately stuff like that I wish I knew 20 years ago as well.
03-29-2021, 12:04 PM
Thank you for this post. I will be following closely. 
