Hello, The QJ I have in my '66 Olds does not have a vacuum break for the choke. Choke closes when cold, but looking at it, I get the impression that engine vacuum overcomes it too easily when first started, which defeats the purpose of the choke. My question is - Does the vacuum break open the choke under high vacuum, or prevents it from vacuum overpowering the choke? Thanks.
The primary vacuum break opens the choke against thermostatic spring tension. If it did not, the engine would load up and stall. You can watch it work in this video, There was also the secondary choke break at the rear of the carburetor which has a delayed action and opens the choke blade a bit more. The primary vacuum break also slows down the secondary air valve opening. Without it, most engines will bog when transitioning to the secondaries.
Here is the 66 Buick Chassis Manual page. The primary choke break is built into the choke housing. It's function is the same.
I got a later (80s) QJ to rebuild, since I wanted to experiment with the APT. At light throttle everything is good, but at heavier throttle (before secondaries open), engine falls on its face and A/F ratio goes super lean. I retained the original primary rods/jets, as well as the original power piston spring. Power piston moved freely when I assembled it. Besides a sticky power piston, what else could it be? I blew compressed air through all the passages and everything seemed clear. Thanks.
Every QJ built was calibrated for the specific engine and year it was built for. You can't do a basic rebuild on any QJ and expect it to run right. It might take more than jets and rods to calibrate it correctly. I would think any 80's QJ is calibrated too lean to begin with. What is the part number? What was the original application?
The stamping is not very clear, but I think it's this one, from an 80s chevy truck. https://www.carburetion.com/CarbNumber.asp?Number=17085225 It's now mounted on a mild Olds 455. Thanks.
Probably for a small block. It will be all wrong for your 66 Olds engine. Send it to Ken Gies. He can re calibrate it for you.
Yeah, follow one of the 3 recipes in the back of the book. https://cliffshighperformance.com/simplemachinesforum/index.php?topic=4537.15 https://cliffshighperformance.com/simplemachinesforum/index.php?topic=3961.0 https://cliffshighperformance.com/simplemachinesforum/index.php?topic=4189.0 https://cliffshighperformance.com/simplemachinesforum/index.php?topic=2054.0
