Rochester 4 Barrel Quadrajet Carburetor Jet Size
12-29-2016, 11:27 AM
#1
Rochester 4 Barrel Quadrajet Carburetor Jet Size
Happy Holidays:
Planning some Performance upgrades this Spring for my 1985 Trans Am 305 H LG4 with Rochester 4 Barrel Quadrajet Carburetor with Electronic Choke and Blue Solenoid.
What Size Jets Came Stock on this Quadrajet ?? The Carb was Rebuilt some time ago and want to know the starting point and go from there.
Performance Upgrade Plan List:
1.Perform a CAM Upgrade - looking at the GM Performance Kits that contains all the of the parts needed for the upgrade.
2. Rebuild the Rochester 4 Barrel Quadrajet Carburetor with Electronic Choke and Blue Solenoid to match the Upgrades.
Thanks Much !
Planning some Performance upgrades this Spring for my 1985 Trans Am 305 H LG4 with Rochester 4 Barrel Quadrajet Carburetor with Electronic Choke and Blue Solenoid.
What Size Jets Came Stock on this Quadrajet ?? The Carb was Rebuilt some time ago and want to know the starting point and go from there.
Performance Upgrade Plan List:
1.Perform a CAM Upgrade - looking at the GM Performance Kits that contains all the of the parts needed for the upgrade.
2. Rebuild the Rochester 4 Barrel Quadrajet Carburetor with Electronic Choke and Blue Solenoid to match the Upgrades.
Thanks Much !
Last edited by 1985blktransam; 01-11-2017 at 11:05 AM.
12-29-2016, 02:00 PM
#2
Supreme Member
Re: Rochester 4 Barrel Quadrajet Carburetor Jet Size
Indirectly, there used to be a way to determine jet sizes by using the carb number stamped on the carb body. I don't know if this can be easily done these days.
Record the carb number, go to a Rochester parts manual and it would list the part numbers for the carb parts. Find the part number for the jets in question; sometimes the description would list the jet size.
IIRC, the jet size number is relative. IOW, a No. 76 didn't refer to a diameter or anything else. It just means that it's larger than a No. 74.
When I was rebuilding carbs, I didn't bother removing the jets from the carbs. They were usually gorilla tight and would often get buggered up from attempts to remove them.
Record the carb number, go to a Rochester parts manual and it would list the part numbers for the carb parts. Find the part number for the jets in question; sometimes the description would list the jet size.
IIRC, the jet size number is relative. IOW, a No. 76 didn't refer to a diameter or anything else. It just means that it's larger than a No. 74.
When I was rebuilding carbs, I didn't bother removing the jets from the carbs. They were usually gorilla tight and would often get buggered up from attempts to remove them.
12-29-2016, 04:03 PM
#3
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Re: Rochester 4 Barrel Quadrajet Carburetor Jet Size
In general, you don't calibrate Q-Jets by the same methods you do Holley or Carter (Edelbrock).
There is a tapered rod going through each jet, that the mixture control solenoid raises and lowers about 7 times a second or so. The duty cycle that drives that, i.e. the % of the time that the thick part of the rod is in the jet orifice as opposed to the thin part, determines the mixture.
You have a flat-tappet setup if you still have the 85 motor. Not sure GM sells any flat-tappet cams that are appropriate to a LG4 anymore.
Incidentally, what is the 8th digit of your VIN? G or H?
However all that may be, there are "stops" inside the carb, that adjust how far up and how far down the rods move. Plus, there's an adjustment for idle mixture (the screws in the base plate basically don't do anything) that you can set after you get the stops right. The calibration process is, you adjust the rich (upward) stop for best power, such as on a chassis dyno. Then after that is set, you adjust the lean stop downwards (lean) until the car begins to hesitate and stumble on a throttle tip-in, then adjust it back up until that goes away, then adjust it back up a scosche more. It may take a couple of iterations of those until it's perfect. Then once those are set right, you adjust the idle air bleed control for about a 50% duty cycle (in gear if auto, out of gear if stick) using an old skool dwell meter, and tune in small increments from there for perfect driveability. Stick cars tend to want the dwell reading to be a bit on the higher side, like 65% duty cycle, otherwise they tend to jerk and not be smooth letting the clutch out with light throttle.
The jets that come in one of these carbs are fairly large. I had my original L69 carb on a 400 with a Comp 282S cam for some years, and while I never did get it really quite "perfect", it was pretty damn good: wasn't near as far off as most people's old poorly maintained stock 305 peanut-cam cars. I'm not thinking you're likely to exceed the limits of adjustability of what's there by doing what you're proposing.
The things keeping the LG4 slow are, in order of importance:
Exhaust - EVERY SINGLE PIECE from the heads to the street
Gears - your 3.08s are meant for grocery carts, not hot-rods
Converter - although if you really have 3.08 you probably have a 5-speed? (unless it was a swap)
THEN AND ONLY THEN the cam - The Comp XE256 is well and widely known to work well in the LG4, and the XE262 in the L69
Heads - install 1.94" intake valves and port the bowls (NOT "gasket match")
Air cleaner - you have a Firebird so that's a bit of a challenge
Notice, the carb is not on the list AT ALL.
What cams are you looking at? What ELSE are you willing to do to the car FIRST to get rid of the things that are choking the motor?
There is a tapered rod going through each jet, that the mixture control solenoid raises and lowers about 7 times a second or so. The duty cycle that drives that, i.e. the % of the time that the thick part of the rod is in the jet orifice as opposed to the thin part, determines the mixture.
You have a flat-tappet setup if you still have the 85 motor. Not sure GM sells any flat-tappet cams that are appropriate to a LG4 anymore.
Incidentally, what is the 8th digit of your VIN? G or H?
However all that may be, there are "stops" inside the carb, that adjust how far up and how far down the rods move. Plus, there's an adjustment for idle mixture (the screws in the base plate basically don't do anything) that you can set after you get the stops right. The calibration process is, you adjust the rich (upward) stop for best power, such as on a chassis dyno. Then after that is set, you adjust the lean stop downwards (lean) until the car begins to hesitate and stumble on a throttle tip-in, then adjust it back up until that goes away, then adjust it back up a scosche more. It may take a couple of iterations of those until it's perfect. Then once those are set right, you adjust the idle air bleed control for about a 50% duty cycle (in gear if auto, out of gear if stick) using an old skool dwell meter, and tune in small increments from there for perfect driveability. Stick cars tend to want the dwell reading to be a bit on the higher side, like 65% duty cycle, otherwise they tend to jerk and not be smooth letting the clutch out with light throttle.
The jets that come in one of these carbs are fairly large. I had my original L69 carb on a 400 with a Comp 282S cam for some years, and while I never did get it really quite "perfect", it was pretty damn good: wasn't near as far off as most people's old poorly maintained stock 305 peanut-cam cars. I'm not thinking you're likely to exceed the limits of adjustability of what's there by doing what you're proposing.
The things keeping the LG4 slow are, in order of importance:
Exhaust - EVERY SINGLE PIECE from the heads to the street
Gears - your 3.08s are meant for grocery carts, not hot-rods
Converter - although if you really have 3.08 you probably have a 5-speed? (unless it was a swap)
THEN AND ONLY THEN the cam - The Comp XE256 is well and widely known to work well in the LG4, and the XE262 in the L69
Heads - install 1.94" intake valves and port the bowls (NOT "gasket match")
Air cleaner - you have a Firebird so that's a bit of a challenge
Notice, the carb is not on the list AT ALL.
What cams are you looking at? What ELSE are you willing to do to the car FIRST to get rid of the things that are choking the motor?