My 70 GS455 is mostly stock with just as mild stage 1 blueprint cam and quicker timing curve. It has plenty of full throttle power but not great throttle response until the pedal gets into back barrels. So, thought I’d check the power valve today and noticed two things. The tube is missing above the air horn vent and I get no movement of the power piston when I push my “calibrated plastic straw” down into the hole. So what’s going on? Carb is original 1970 QJ 7040240.
Your power piston should go up/down when operating your calibrated plastic straw.... One of mine was stuck also ( it was a fresh rebuild from a putz called Qjetpower.. I got it out of the carb.. it was like pulling tooth....Then saw a pos Filthy PP,,,polished it with some 00 000 Or 0000 steel wool it did operate after that... But to this day the Qjetpower carb is nothing but a parts carb.....
So with the engine off, you should be able to put your straw down the vent, and push the power piston down, and feel it spring up. With the engine running, and the straw down the vent, you should be able to wing the throttle and watch the straw move. If the power piston is stuck down, the carburetor will be lean on the primaries.
The engine was off. I know the piston should spring up and down but no movement at all. Can I tell if the spring is stuck up or down or missing even?
Well think about it, if the piston was up, the engine would run rich. It must be stuck down if you can't push it down.
If it was stuck in either position it wouldn't be driveable. If it was stuck closed you'd have no primary enrichment. It would rev in park/neutral but it wouldn't move the car unless you were in the secondarys. If it was stuck open it would be so rich at part throttle it would stumble and buck and barely run. Plus you'd be getting 5 mpg. I would put a cold beer on it that the spring is tired after all these years or has been replaced with the incorrect spring. I would put a slightly stronger spring in and see how it runs. You can always stretch it out some and see what happens. Oddly enough that spring is such an essential part of the carb's operation and it's not included in the rebuild kit and there are only a few replacement springs available. You can't expect a 50 year old spring too not wear out..
Greg Gessler has a power piston spring kit, https://quadrajetparts.com/quadrajet-dualjet-power-piston-spring-assortment-color-code-p-210.html
Yes ..... Yes I rode his name hard.. IIRC I sent that carb to him back 5 times. Had to rub that in his face on this site.. He shut up and crawled away like beatin &^%#@..
My advice Dont... I got a used one from a California member here that bought it from Ken at Everyday Performance.. NICE... http://www.everyday-performance.com
What part number is it? I’ve fixed a few that people thought were scrap. I can help. Ray, should be an easy fix but probably best to look at the entire carb. I can help talk you through it or fix it or send it to your place of choice.
Remove the top and check the power piston to see if it's stuck. Very common problem and often the cause is that someone staked the chit out of the top of the power piston well to hold in a plastic retainer than should have been replaced instead. The staking operation often pushes the collar in and seizes up the piston. I put new retainers in ALL my rebuild kits for this reason and put a new retainer on every carb that we work on here. Good idea to replace the power piston spring while you are in there with the correct one for what you are doing. You are also going to find the power piston hanger arms bent or uneven 99 times out of 100 if anyone was in there for any reason. At least that's what I see here with just about every single one that comes in here having issues of one type or another. I stock the slashed vent tubes if you need a new one.......Cliff
