General Category > Quadrajet Carb Talk and Tips
weak part throttle acceleration
bob69:
Looking for some advice to solve a weak part throttle acceleration. Light throttle acceleration is strong and heavy acceleration is strong. But part throttle acceleration (around half throttle opening) is weak. Looking at jets, power piston spring, or vacuum advance as possible causes.
Engine is a 1969 396 with a mild performance cam. 700r4 with 3.31 rear gear.
Qjet is 7043215, jet is 74, rod is 38, stock power piston spring. Cruz afrs are in low 14s to low 15s. Idles very smooth at 600 rpm in gear. Only change to carb are idle screws at .093, idle bypass at .106. All other measurements are stock. Vacuum at idle is 16 hg and stable.
Distributor is a 1111955 (date code 8m17). Stock mechanical advance is 22°, initial is 12°. Total comes in around 3000 rpm. Has a B1 vacuum advance can.
At light acceleration, like pulling from stop in traffic, the car pulls very strong. Assume due to ported vacuum opening and additional timing coming in.
The light throttle acceleration is noticeably stronger than what seat of the pants feels like for half throttle acceleration. Moderate acceleration power is low when ever the throttle is opened quickly. Accelerating from idle or from Cruz.
So, are the jets a little small. Is the power piston closing to soon. Is the drop in vacuum on acceleration causing the advance to drop too much. Any suggestions?
Cliff Ruggles:
The B1 advance can is WAY too late and slow for optimum performance in the "normal" driving range for what you are doing.
I've tested them and they start really late and too much vacuum to get them all-in.
I'd start there, and consider larger jets, different primary metering rods, and one of my external APT screws for those carburetors.....
bob69:
Thanks for the info. Any suggestions on which vacume can to pick up.
Shiny:
I hope Cliff can guide you on a preferred "curve" for your advance.
Lars Grimsrud published a useful list of advance unit specs for points distributors ("Distributor Vacuum Advance Control Units Specs and Facts for GM Point-Style Distributors"). You may find it on the net, but best to email him and request a copy. I just went through this and he responded quickly. Send me a PM and I'll give you his email address.
His table compares starting vacuum levels, vac for "max" advance, and the amount of advance at max. It includes the "B" labels.
For reference, B1 in Lars' summary is spec'd to start at 8-11" with max of 8 distributor degrees (16 at crank) @ 16-18"
I just bought a Standard Products VC24 that is labeled B1 and it measured 7" start, 9.5 max @ 21". On the advice of many, I limited the max to 6 (12 degrees at crank). I attached what I measured.
There are several "stock" options that start at lower vacuum levels, like B28 that starts at 3-5" but not sure if it is easy to buy. NAPA seems to carry a good selection... Crane also sells an adjustable unit but I don't know the range of adjustment.
Whatever you buy, measure it and good luck!
Cliff Ruggles:
Many of the VA cans listed are NLA, so be aware of that during your research.
I can supply the correct one which is one that I purchase and modify to supply less timing. I have to do that these days because very few of the short-slot VA cans are still available that have lighter springs in them.
You can also search and locate the correct VA for start/stop and purchase one of Lars "limiters", size of one half dozen of the other, same end result.
In any case the B1 cans I've tested that are currently being sold do NOT make the grade and WAY too late to be "all-in".
I'd also add here that if things are find at very light throttle but not so great heavy throttle but not enough to get into the secondaries it simply may want more main jet. With few exceptions the 1970-up big block carbs came with 77-78 main jets and 49B rods.
I'm not recommending that jet/rod relationship but larger main jets would be in order here in addition to a different VA.......
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