Adjusting the vacuum secondary goes like this. Start by putting in the lights spring in it. If it bogs when you feel the secondary’s open, put the next heavies spring in it until the bog goes away. Of course it has to be jetted right before you start with the secondary rate of opening adjustment.
Rochester Q-jet carb vs Holley 770 cfm
- Thread starter Francesco
- Start date
Go back and find some of my posts on this subject as I am pretty sure I posted how to do the tube with some pictures.
Mine wasn't. After the tube repair the 770 ran right on par with my good old 3310-1, the 82750-1 750HP worked as well along with another 3310-4 I repaired. Most of the 750 vs like 72 front and 76 rear jetting but I have seen as much as 84's in the sec side. Also without the tube on the newer 750's, 770's you can put the lightest (white spring) in the pod and it still won't open.
Yep, strange indeed.....keep in mind back in the day when GM wanted improved "pick up speed" on the higher performance optioned models, they actually took off the Qjet and replaced it with a quality calibrated Holley carb. Same goes for Ford and Chrysler as well for the most part.
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ez2cdave
New Member
By adding the tube into the main body where the hole is in the one primary barrel. I was looking for the video that showed how to do this but it looks like youtube or the video poster took it down. I will see if I can find the pictures again.
Mike Sinkoski
New Member
Your original Rochester also has a APT adjustment that can help dial in your cruise mixture. When you rebuild it I suggest to drill into the acces plug just enough to tap 10-32 threads into it so you will have a easy way to remove it. This regulates how far down the rods can go into the jets whith light throttle cruise vacuum. Early Q jets an no Holley has this feature
Constant issue, and almost like the old arguments... Chevy V. Ford V. Dodge...
To effectively work on a carb and maximize performance, you have to know how to do it. One of the best shops I worked with was in Vincennes Indiana, Tech actually put the new rebuild on a specialized flow bench.
Q-Jet and Holley or AFB Carter, all good carbs... Each had their limits, basic faults, quirks...
To effectively work on a carb and maximize performance, you have to know how to do it. One of the best shops I worked with was in Vincennes Indiana, Tech actually put the new rebuild on a specialized flow bench.
Q-Jet and Holley or AFB Carter, all good carbs... Each had their limits, basic faults, quirks...
Phone had to go back on charger so didn't get to finish this out more. Hope I didn't rub anyone the wrong way.
By my last year of HS I was the carb guy. After HS got ASE certified and still for local area was known as the carb guy. Basic thought back then was ... Running bad? ... Get a new carb. Shops did this constantly and not needed. Many times was bad wires, bad coil, plugged/stuck EGR, vacuum leak, EVAP canister open at idle, a whole bunch of things.
There was no OBDI, OBDII, etc. These are just basic runnability, drivability issues. Yet the typical customer wants an answer... What is it???. ... Well sir.... Bad Carb.... Or Bad Distributor... Etc...
When you are on the clock on flat rate, ... Parts R&R = $$$ I.e.: Turn and Burn, button it up, get it out the door...
Shops are not paying to dig deep and find the oddball stuff like a vacuum temperature switch keeping the EVAP open at idle, or dirt/debris that plugged an airbleed, or a throttle plate that went egg shape because the return spring was set up wrong or the metallurgy of the throttle plate was poor quality.
This starts all the Q-Jets are junk, autolite/motorcraft is junk, Dodges are junk...etc...
Each one has their good points and bad. So you have to look at the item, any item, as being part of a system.
The Q-Jet is dependable once setup. The stock unit that comes with the car is actually designed for the car that it is mounted on. Yet, it's design is based on the EPA and MPG requirements at the time of the build. To increase its performance, is when it gets difficult to mod it.
For the Q-Jet take it off and clean it every 6 months to a year. It's sore spot is the throttle plate for the primary shaft. It gets egg shaped and starts to leak air. This leak is below the level of the fuel mix screws and during a period of high vacuum.
This means it causes idle charge to run lean. The solution is to open mix screws 1/2 to 1 turn to richen mix, and increase idle speed. This puts the plates over the transition ports and distorts its mix. Problems cascade from there as the problem gets worse as the throttle plate continues to wear.
The fix is to repair the throttle plate with brass insert or replace carb.
Holley is more of a universal carb, so at that point you have to know what your doing to select the best option. Mainly spread bore for daily driver street car, square bore for off-road and race. The Holley is almost infinitely adjustable, with jets and power valves, but is still limited by its set size and layout. Vac secondary for street car, mechanical secondary double pumper for race.
Carter AFB (Edelbrock) very adjustable for basic street car, does not have a Spreadbore style, so needs an adapter or a square bore manifold. Main advantage to the AFB is the ability to change the primary metering rods and springs with out having to remove the carb and it is dependable once set up. You still have to remove and clean once a year. Big advantage was it had electric choke, when earlier on, many Holleys did not. The AFB will typically run right out of the box.
By my last year of HS I was the carb guy. After HS got ASE certified and still for local area was known as the carb guy. Basic thought back then was ... Running bad? ... Get a new carb. Shops did this constantly and not needed. Many times was bad wires, bad coil, plugged/stuck EGR, vacuum leak, EVAP canister open at idle, a whole bunch of things.
There was no OBDI, OBDII, etc. These are just basic runnability, drivability issues. Yet the typical customer wants an answer... What is it???. ... Well sir.... Bad Carb.... Or Bad Distributor... Etc...
When you are on the clock on flat rate, ... Parts R&R = $$$ I.e.: Turn and Burn, button it up, get it out the door...
Shops are not paying to dig deep and find the oddball stuff like a vacuum temperature switch keeping the EVAP open at idle, or dirt/debris that plugged an airbleed, or a throttle plate that went egg shape because the return spring was set up wrong or the metallurgy of the throttle plate was poor quality.
This starts all the Q-Jets are junk, autolite/motorcraft is junk, Dodges are junk...etc...
Each one has their good points and bad. So you have to look at the item, any item, as being part of a system.
The Q-Jet is dependable once setup. The stock unit that comes with the car is actually designed for the car that it is mounted on. Yet, it's design is based on the EPA and MPG requirements at the time of the build. To increase its performance, is when it gets difficult to mod it.
For the Q-Jet take it off and clean it every 6 months to a year. It's sore spot is the throttle plate for the primary shaft. It gets egg shaped and starts to leak air. This leak is below the level of the fuel mix screws and during a period of high vacuum.
This means it causes idle charge to run lean. The solution is to open mix screws 1/2 to 1 turn to richen mix, and increase idle speed. This puts the plates over the transition ports and distorts its mix. Problems cascade from there as the problem gets worse as the throttle plate continues to wear.
The fix is to repair the throttle plate with brass insert or replace carb.
Holley is more of a universal carb, so at that point you have to know what your doing to select the best option. Mainly spread bore for daily driver street car, square bore for off-road and race. The Holley is almost infinitely adjustable, with jets and power valves, but is still limited by its set size and layout. Vac secondary for street car, mechanical secondary double pumper for race.
Carter AFB (Edelbrock) very adjustable for basic street car, does not have a Spreadbore style, so needs an adapter or a square bore manifold. Main advantage to the AFB is the ability to change the primary metering rods and springs with out having to remove the carb and it is dependable once set up. You still have to remove and clean once a year. Big advantage was it had electric choke, when earlier on, many Holleys did not. The AFB will typically run right out of the box.
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