primary air bleed size for large venturi

I have a 17056240 originally from a 1976 455 LeSabre. It is the 800 cfm carb with the larger primaries. Currently running 73 jets and .044 metering rods. Primary air bleeds are .042" lower in the body, and .047" upper in the air horn. It is just a bit rich and was considering opening up the bleeds a bit. Does the bigger venturi need smaller air bleeds? I think opening them all up to .050" would help. Would .070" be better? Could also just drop down a jet size. Home is at 3900 feet asl and nearly always drop to 3200 when out and about, and often go up to 5000 or more.

 

OK, well, I think I answered my own question. Left the air bleeds alone and installed 72 primary jets and also a lighter power piston spring. Power piston was just floating off seat at idle in gear and rpm was surging from 600 to 700. Just went for a test drive and idle is nice, but has that little lope in gear at 700 rpm, smooth out of gear at 850. Feels real snappy with fantastic throttle response and no surging at cruise speeds. Hot starts are much better. And I swear that gas gauge needle is not plummeting nearly as fast ;-)

 

72 jets are a good choice for that carb number.

Employ the APT system to fine tune part throttle A/F, it has the most effect on power, fuel economy etc. The jet size effects fuel delivery at heavy/full throttle, so tune for jet size first by doing a few heavy pulls up steep grades without the secondaries. Once you determine the best jet size (I'll bet that will be 71's or 72's), then fine tune with the APT in the "normal" driving range or all light throttle/light load operation.....Cliff

 

Thank you for the reply! Heavy throttle right up to before the secondaries did feel better with the 72's compared to 73's. Might just try 71's. Yes, I need to play with the APT now. This is my first APT carb and it's taking a bit to get my old head wrapped around it. Thanks for the book. Very clear and extremely helpful. Always liked these Q-Jets, but like them even more the more I work with them. Just Awesome!

 

The order of tuning is to find the ideal jet size first, then leave that alone. Then install metering rods that will provide full control via the APT system at light part throttle. I use full tapered .044 rods with .026" tips in most of those carburetors instead of the "K" rods most show up with. "K" rods only have a .005" tapered upper section, then "stepped" to the .026" tips, so limited control of A/F at part throttle.

You can also play around with PP springs at this point, but they have a minimal impact as there is ALWAYS plenty of vacuum for "normal" driving that the PP is down, so the height of the metering rods in the jets becomes the key player here.

It may take several tanks of fuel to find the ideal APT setting, you don't want to go too lean as it will actually consume more fuel than a slightly richer or ideal setting. The vacuum advance is a "player" here as well, so adding to or taking away some timing at part throttle will impact the results as well.

Once you have effectively dialed in the primary side of the carb, do NOT touch it! Do ALL full throttle tuning with the secondary side of the carb from this point on. A dyno or dragstrip will be the best place to be for this, but you can get close by the "seat of your pants".......Cliff

 

Ok, so order of things: Dial in primary jet size, then primary metering rods, then fine tune with APT and perhaps part throttle ignition (vacuum) advance, and finally adjust WOT with secondary side only. Got it. I believe the primary metering rods are the good full taper .044" I got from you, so I should be good there. Thanks for keeping me on track. I will give 71 primary jets a try to see if the motor likes those better, and then I think I am ready to play with the APT adjustment. Thanks for your help.

 

When you finalize tuning with the secondary metering rods, make sure you are using the same style. Better yet, have Cliff custom-make you 4 or 5 sets and go from there. Tuning with stock secondary rods is all but impossible if you are mixing short, medium, long ones.