[BrianB74SD]

I'm a novice tuner who is hitting a wall, running out of ideas, and kindly need some help. I tune my Q-Jet by "giving it what it asks for" at the track as opposed to dyno tuning. It's a mild 4000lb car that runs 12.9/105mph. Steady idle of 14Hg in Drive and no vacuum leaks that I can observe. Total timing is 36 degrees.....16 initial/20 mechanical. Best ET and MPH is always with 78 jets and DG secondary rods on a "G" Hanger. Even richer rods like CCs slow it down and richer in the middle AX's also slow it down. Carb is factory 800cfm unit with only a 1/4" base gasket...no spacer. Float level is 1/4" and needle & seat is .135. I use a RobbMc mechanical pump with functioning return line. If I take fuel out and use 76s or 75s, ET and MPH suffers. If I put 78s back in, all is well. Choke pull-off release time is one second. Engine is 455 Pontiac with mild hydraulic roller 224/230 .552/.528 112 lobe separation. Rest of drivetrain is TH400 with 3:08. Stock cast iron intake but oversized cast iron exhaust manifolds and "X" pipe. I tune for "cooler" weather, which is what I prefer to run in. DAs around 600 feet to 700 feet is what the car likes. It seems to just want more and more primary jet but I'm having a real difficult time thinking even larger primary jets than 78s are needed to go faster/quicker. I'm starting to run out of larger jet options. Should I slowly add more timing? Have I plateaued with my set-up? Thank you.

 

[vpd66]

What model number Quadra jet are you running??

 

[BrianB74SD]

7044270. It's the same as the 1973 versions only the factory downsized the primary jets in mine to 70s and used thinner primary rods for part throttle in order to meet increasing EPA emission constraints. '73 versions used 76 jets and 51 primary rods I believe.

 

[vpd66]

That is why I asked what number carb you are running. The later the Qjet the leaner they made them to meet emissions. Also when the EGR valve came into existence it also made them lean the carbs jetting out. The first thing you should do is make sure you have the idle circuit and transition circuits right. Fuel mixture doesn't quit flowing from these circuits when you get into the main circuit like on a Holley. Instead it adds to the total fuel/air mixture. So if your lean on your idle mixture then you would need to go to a larger main jet/smaller needle combo to make up for the lean idle mixture. This is a place to start. Do you have a wide band AFR gauge???

 

[BrianB74SD]

I don't have a gauge. Guess I better put in on my wish list.

 

[vpd66]

Idle curcuit is the place to start and a wide band is a very good tool to help you get it right. Once you know you idle circuit is good there are other ways to tune the main fuel circuit.

 

[BrianB74SD]

I see. I may need to thoughtfully enlarge the idle pickup tube restriction and the down channel restriction and possibly the upper and lower idle air bleeds to offset the need for larger primary jets?

 

[lmsport]

Seems fairly impressive as is

 

[BrianB74SD]

I appreciate that. I'm just wondering if I'm "at the end of the line" with my set-up or if there's a little more power lurking in there somewhere.

 

[vpd66]

I see. I may need to thoughtfully enlarge the idle pickup tube restriction and the down channel restriction and possibly the upper and lower idle air bleeds to offset the need for larger primary jets?

Yes that is the place to start. Did you rebuild the carb yourself?? You can make the idle circuit fully adjustable. They sell threaded idle tubes and you can tap the down channel restriction and use small brass set screws for jets. This makes your idle circuit fully adjustable.

 

[75gmck25]

The carburetor tuning sounds reasonable from my limited experience with Quadrajets, but with a TH400 and 3.08 gears is seems like it takes quite a while to get into the right rpm range for your cam. If you still drive it on the street I would move up to about 3.73 gears, and maybe even go lower for track-only use.

On my ‘75 truck I moved from a similar carburetor up to a newer 17080213 because it was about the last version without any electrical controls. The newer carbs are supposed to be an improved design, but I was mostly just looking for the external APT adjustment capability.

I had a shop weld an O2 bung in a short section of exhaust pipe and then expand the ends. I then cut an equivalent section out of one pipe under the truck and put the new pipe in for the AFR gauge sensor. It was the easiest way to add the O2 bung without needing a welder. I am using an Innovate gauge.

 

[BrianB74SD]

My car and carb only had 12,000 miles on it when I bought it so I can't say I rebuilt the carb....only tuned/refurbished it. It's somewhat rare so the idle tube and down channel mods vpd66 outlined sound intriguing because they can be reversed. I'm using a Jim Hand Special Continental torque converter that flashes to 2800 to help get into torque range of cam that has a range of 1800 to 5800 according to Comp Cams. Trans shifts at 5700 at WOT. It is and will always primarily be a street car but I have been thinking about future 3:42 gears. My goal with this car is to improve/modernize each of its systems (fuel/ignition etc), get the most from it, and drive it like I would have if I bought it new in 1974. I like 75gmck25's ideas for the AFR gauge. Great feedback from all! Thank you!

 

[Bonneville462]

I know with the Quadrajets I've had, they all needed the same thing, the secondary's correctly adjusted.

Most times people tighten that little spring on the secondary's. That little spring is just to keep the secondary flappers shut when not in use. If you want the flappers to open faster, remove the vacuum hose and open the crimped tubing end up. Sometimes I cut the whole crimped end off..

If your getting a stumble when the gas is opened up, it needs crimped a little more. Remember a little goes a long way with these carbs..

 

[BrianB74SD]

Yep. That and getting the power piston spring right.

 

[vpd66]

Yep. That and getting the power piston spring right.

There are different springs that are rated by how inches of vacuum they start letting the power piston move. All you have to do is put a vacuum gauge on your motor at idle then pick a spring that is rated lower then the vacuum you have at idle.

 

[vpd66]

The carburetor tuning sounds reasonable from my limited experience with Quadrajets, but with a TH400 and 3.08 gears is seems like it takes quite a while to get into the right rpm range for your cam. If you still drive it on the street I would move up to about 3.73 gears, and maybe even go lower for track-only use.

On my ‘75 truck I moved from a similar carburetor up to a newer 17080213 because it was about the last version without any electrical controls. The newer carbs are supposed to be an improved design, but I was mostly just looking for the external APT adjustment capability.

I had a shop weld an O2 bung in a short section of exhaust pipe and then expand the ends. I then cut an equivalent section out of one pipe under the truck and put the new pipe in for the AFR gauge sensor. It was the easiest way to add the O2 bung without needing a welder. I am using an Innovate gauge.

I agree his gear is a little tall but if he is driving this car on the street then he most likely wants to stay with the taller gear. 400-455 Pontiacs are very different motors then small block Chevys. They are torque monsters and can pull taller gearing then SBCs. They also aren't very good high rpm motors. So keeping the taller gear in and using the torque these motors have is the way to go. I wouldn't run more then 3.23 gears.

 

[95 Truck]

If your carb is a rare one then you might want to find a newer style and have at altering the idle circuit and setting this unit up for car.

 

[vpd66]

I wouldn't say its a rare carb 73-74 Pontiac 400-455. If I was to start without a carb I would build a later model one and make it fully adjustable.

 

[BrianB74SD]

My carb is the stock one for my '74 SD-455. I do have a spare Pontiac #7043266 I may use to avoid drilling bleeds in the air horn and/or main casting of my SD carb even though the 266 carb is 750cfm v. 800cfm.

 

[vpd66]

My carb is the stock one for my '74 SD-455. I do have a spare Pontiac #7043266 I may use to avoid drilling bleeds in the air horn and/or main casting of my SD carb even though the 266 carb is 750cfm v. 800cfm.

Unfortunately GM,Rochester,Carter, or whatever company manufacturing Quadrajets didn't break down the application to that detail in the carb number. The best book (and most detailed) on looking up and defining Quadrajet carb numbers is one I got back in 1986 When I worked at a Chevy dealer. It is a AC Delco carburetor parts book. The number they stamped into it only defines the Year, Company of application (Chevy,Pontiac, Olds, and etc), engine size, and transmission (only states manual or automatic). There are a few carbs that were listed as "Hi Performance" but thats it. I know a whole group of Pontiac guys and have done many carbs for them and they alway say their carb is a "ram air" carb or whatever. I get the book out and show them that there is no special designation for a "ram air carb". So yes your carb is the original 74 SD-455 carb but it is the same numbered carb they used on all 73-74 400-455 engines. What I really would like to find is a book that tells you what calibration (jets,needles,air bleed sizes, and etc) they used in each numbered carb, but so far I've never seen such a book.