I am running a 750CFM Q-Jet on my mild cammed 464 and it is running rich at part throttle cruise after altering the idle circuit because it was idleing lean (16.4). Idle AFR is now 12.8-13.0 and cruise AFR is now 11.9-12.2. The carb has 76 jets, 46b primary rods, 7036019 spring, BG secondary rods, F hanger. Can one of you Q-Jet guys make a suggestion on what to change to get the part throttle cruise up around 13.8-14.0 or whatever would be best. I would make the change and see where it ends up and take it from there if more tuning is needed. Thanks in advance for any help. Mike D.
I'm not a pro carb guy like some of the gang here, but those guys have often suggested getting the carb back to stock tuning and work from there. I went with that approach after two decades of thinking I knew what I was doing by increasing fuel on a cammed and headered car. When I put stock jets and rods back in, the car made WAY more power, pings less, has much better throttle response, and gets better mileage. The change was instantaneous. Just sayin...
Like Ken said, more information would be helpful. Specifics on engine and the carb number. What are the idle tube and down channel sizes? Any other modifications to carb? Most quadrajets have an adjustable part throttle, some in baseplate and some in main body.
Engine and drive train details. Carb# 7044240 KA 3313 Cam Grind # 288/296-12H Duration @.050 Intake 231* Exhaust 239* Lobe Seperation 112* Lobe Lift Intake .323 Exhaust .338 Degreed Intake Lobe to 108* ATDC 9.6/1 Compression Bulldog ported aluminum heads SP1 Intake Poston full length headers 700R4 with lockup converter 3.73 gears Sorry I don't have any more info. on channel sizes or feed restrictions. Carb was originally done years ago by John Osborne and was always lean @ idle but ran fine every where else. I had a local guy recalibrate the idle circuit by removing a brass air bleed and redrilling an aluminum slug and pressing it in where the brass air bleed was originally. Let me know what you think. If I have to send the carb. out I can do that too. Thanks, Mike D.
That is good info. Thanks. Would help to know internal circuits, but I'll wing it from here. The quadrajet you have is the 800+cfm. It has the APT in the baseplate that is set at factory. They can be removed and modified to use an external adjustment. You should dry dropping jets to 75 for a start and see if that moves the numbers for you.
Where can I get jets for the carb.? Are they specific to Q-Jets or can a holley type jet work? If I can find jets I'll try them thanks for the information. Mike D.
I sell used jets. They are specific to the quadrajet. If you want new jets, check with www.cliffshighperformance.com
Thats what I thought. I'll be at a show at Lane Automotive today. I'll see if I can get some there. Thanks, Mike D.
I too use 76 jets but 52m rods. I would suggest a larger diameter / higher number rod , the largest rod u can find. Your spring may be too stiff as well . That would be the easiest place to start
76 jets are pretty big for that carb number and what you are doing, but part throttle A/F shouldn't be that far off teamed up with a .046 primary metering rod. 52M metering rods are WAY too short or that carburetor number and the tips are WAY too large at .036". Not sure about the installation and drilling of an "airbleed" that was mentioned. Airbleeds are main contributors to fuel delivery, so if the carburetor has been modified internally standard tuning practice may not apply or be successful.....Cliff
If you are using a dual plane intake, you may try a 75 on driver side and 74 on passenger side. Ditch the "m" rods for sure. 76 jets are really big. I use the 75-74 combo to help make sure #1 cylinder does not run lean on the driver side (with a dual plane intake) and I use the 74 to compensate for being near sea level and burning gas that is not as good as when our cars were new. I do know that lots of guys get great performance on 73 jets but beware of detonation and sugest you have a TA super water pump to run cool in hot weather. Just swap the rear metering rods when rejetting for weather conditions at the drag strip.
Mr. GS. Mike, For ur 700r4 which adapter plate did u use? Did it come with a hub spacer ? Mine wore itself to nothing . It may have been cause my flexplate was cracked for quite a while before I noticed it.
I think I bought one from Summit. Can't remember it's been too long. I spaced the torque converter back on the flexplate the same distance as the thickness of the adapter. I remember doing that. Don't remember anything else. Mike D.
I got mind from Vic Hubbard, but they closed down. It's a pretty hefty piece, 2 thick washers and a skinny one to space the converter . I wonder how much does it act as a engine plate giving support to the block.
Update. I was able to get a handle on the carb. calibration. I downsized the idle air bleeds and dropped the main jets to #75. I can now set the idle mixture about any where I want with the mixture screws and part throttle A/F is about 14.2. I'm going to run it for a while and see how it goes. Making progress. Thanks for the help everyone. Mike D.
