On my 76 #17056206 quadrajet was just rebuilt and not working very well, it back fires when applying throttle and gets worse as it warms up. I just took the carb apart and was reading your reply to Project 76 about the power piston height adjustment and you advised him to adjust the stop screw so .020 of the brass sleeve extends above the lip of the retainer. I took mine apart this morning and my power piston brass sleeve is below the top of the retainer. I guess thats why I am having this problem. plus the plastic retainer is not very snug in the port. Should I lightly stake the port to hold the retainer in place? Or just replace the power piston assembly?

Thanks

 

Until Lars gets back to you maybe you could find something usefull in his paper...
http://www.corvettefaq.com/c3/QJetTuningPaper.doc

 

Tom -

If the power piston stop is set too low, the primary rods will ride too deep into the main jets, and the carb will run very lean. A lean condition will cause a backfire up through the carb upon throttle opening - just as you are experiencing. Your diagnosis is correct.

With the "top" (the Airhorn) off the carb, first verify that you are, in fact, jetted with #77 main jets and #48 primary rods. Verify that your float level is at .420".

If your power piston plastic retainer fits very loose in the power piston hole, don't stake it. This can crack the casting. Rather, "shrink" the hole a little by lightly tapping all around the power piston hole wth a regular hammer. This will form the metal without cracking it, and you will get a very nice fit on the retainer depending on how much you tap around the hole with the hammer. It works very well with no damage to the carb.

Finally, set the piston stop as follows: Drop the power piston into the hole and seat the plastic retainer. Note that the plastic retainer has a lower outer diameter and a raised inner diameter "collar." Note on the power piston itself that there is a brass sleeve that the formed hanger is pressed into. The top edge of this brass sleeve should be about .020" - .030" above the upper lip of the plastic retainer when the piston is fully depressed. Adjust the stop to achieve this spec. If your piston sleeve is currently riding below the collar, raising it to this spec will richen you up significantly.

Note that this spec is an approximate adjustment that will get you very close to the ballpark. While you have the top off the carb, knock the aluminum adjustment plug out of the top of the carb before you assemble it. This will allow you to adjust the power piston stop without removing the airhorn.

Take the car for a ride. You can play with the adjustment stop by screwing it down 1/2 turn and up 1/2 turn from this initial setting with the stop plug removed. Use a pair of long-nosed thin-needle pliers to turn the screw.

Good luck with the tuning. Let me know if you need assistance.
Lars

 

Lars, thanks for the info, I took the carb apart earlier, and set the float at .420. And the power piston at .020 off the retainer. I could not read the number on the primary jets as they were very dark. I have had the car since new and the jets have never been changed whatever jet number (I assume #77) came as stock in the carb #17056206 is still there. The primary metering rods are #48. The rebuild kit that was used was a echlin kit and upon taking the carb off the car today I found the echlin accelerator pump that came with the kit had swelled and would hang up in the bore. That was a surprise. I did keep the original pump though and it worked fine so I will put that in tommorrow, put the carb back on the car and try it out. Thanks for the help
Tom

 

Tom good job on the setup. Although Echlin usually makes some pretty good carb kits, they have been doing an exceptioally crappy job on their accel pumps over the past few years: Their accel pumps are not compatible with oxygenated fuels, and they tend to swell up and seize in the pump bore. They are aware of this issue, and refuse to remove the bads parts off the shelf - I'm about ready to slap somebody..! I have 2 bad pumps just like yours on my workbench right now.

Tag me back if you need further assistance.