General Category > Quadrajet Carb Talk and Tips
Quadrajet choices
davel:
The disk on the right - the undamaged one - measured 0.136 which is the stock size. I replaced the damaged disk with the same size and then the carb performed fine with smooth transition to the secondary circuit. I'm pretty sure that, for street performance use, you need to have the stock size discs installed. Otherwise the carb will go way too rich during the transition when the secondary throttle blades are open and the air flaps are still closed. The fattest part of the secondary metering rods should only be a couple thousands of an inch smaller than the opening in the disk. This way fuel flow is almost fully restricted through the disks while the air flaps are closed. I'm sure Cliff can elaborate more on this.
davel:
Now you're getting too much interest in your post :)
Don't mean to hi jack the thread, but another thing to consider is that you likely have a thick style airhorn gasket on that carb. The thick gaskets can be 0.020" - 0.030" thicker than the OEM gasket. That's great for sealing the airhorn. BUT...It also positions the secondary needles that much higher in the secondary disks. This may require running a leaner hanger - Meaning a hanger with a higher letter. If you have an M hanger now, you might need to run a "S" or "T" hanger. The first post I made to this forum addressed exactly that issue with the first Jet carb I purchased - which was the only Jet carb I bought new. Here's the post:
https://cliffshighperformance.com/simplemachinesforum/index.php/topic,3294.msg14372.html#msg14372
Your secondary discs need to be replaced with the 0.136 stock size discs first. But if you still have loading up when the carb transitions to the secondary circuit, I would try a leaner hanger. Or take an old hanger and file the letter off of it and bend it to see if it makes a difference.
Also, don't get discouraged. I'm still running my original Jet carb on this truck and I've got it dialed in perfectly now. It's my daily driver farm truck and has been running great for the past 2 years. Took it to the track this summer and put down a 14.7 at 96mph in the 1/4. Not bad for a 4500lb K5 Blazer with a mild 383 and SM465 manual tranny. Went through the traps redlined in second gear :)
hiy_po:
Hi Jack away. when it comes to info more is better and only crickets arent wanted.
Funny fact when I started this thread I was asking about what was the better casting of the two types I have, but it seems everyone is onto that it came from Jet and all the associated problems. So I guess it is now pretty much a doorstop.
I have had so much success getting my carb to work with my engine, since I just started making everything smaller I am wondering what the smallest secondary disc fitted OEM is and how hard are they to change?
I have just purchased a 7045281 2674. It appears to have the 0.135.
davel:
The carbs that start with 1705 and 1708 are generally better to work with since they have an Adjustable Part Throttle (APT) system.
I believe ALL quadrajets come stock with 0.136 secondary discs. They are easy to change out. They're just held in by those bronze bushings which are pressed into the casting.
hiy_po:
Yes am having thoughts APT may be just adding a level of complex that I am not able to use as intended. I generally sit on about 2Hg Vac at cruise. Between 2Hg and 2psi is normal for me. Even with the softest of springs under the Power piston I still cruise at pretty much full rich. Being able to block and delete APT/power piston saves a boost referencing modification to the carb as well.
If I can find the combination of main air bleed area and primary main jet that is just big enough to supply my needs at cruise then have the secondaries for on boost enrichment, I think I have brought down my Idle system to a minimal and is lean of stoich.
Main Air-Bleed Airhorn 0.064" #52
Main Air-Bleed body 0.040"
Primary Jet 0.072
Primary Rod 0.048
works well for me but want to find the bottom so I can sit just above it.
Main Air-Bleed Airhorn 0.064" #52 reduce to 0.040" and a 70 primary jet will be next test.
Once happy with this 1705 will transfer settings to the 704 and start the delete process
If I find some standard sec discs for the 1708 will jump on them and see if I can bring that back as well.
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