[cobalt327]

A while back, I had shown some Rochester Quadrajet carb parts. Among them were some power pistons and the springs that are used w/them.

These springs allow the power piston to behave like a Holley's power valve, that is, the enrichening system is vacuum controlled.

At high vacuum, the vacuum exceeds the PP spring rate and the needles are pulled down into the main jets, leaning the fuel/air mixture. Conversely, when vacuum drops, like when under a load or the accelerator pedal is whacked WFO, the PP spring rate exceeds the vacuum, which lifts the needles up to their smaller diameter "rich" portion.

These PP springs are seldom mentioned when Q-Jet tuning is being discussed, but need to be addressed just like if it were a Holley. The same principals apply, AFA tuning for low vacuum cams, etc.

If a “medium” PP spring = a Holley 6.5 in/Hg, a "soft" PP spring (allows enrichening to come in at a lower vacuum) would be like a 3.5 Holley PV; a “stiff” PP spring = a Holley PV of, say, 8.5 in/Hg. Longer duration cams will “like” a softer PP spring.

Below is an image showing a variety of different springs, another shot shows three different types of power piston. The springs vary in length, coil diameter and wire diameter (0.012"- 0.020" in my collection).

Edelbrock has about the only Q-Jet tuning parts, AFAIK they show only 4 different PP springs- but they only come in a "Race Calibration Kit" that contains a selection of main jets and rods, 2'ndary rods, hangars, PP springs, high flow needle and seat and an accelerator pump.

GM part # 7029922 is a rather weak spring that was OEM for HO Pontiac engines from the early ‘70’s. It delays enrichment until vacuum drops to 3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050”. Another GM p/n is 7037305 for a PP spring that is set for 6 in/Hg, and would work well w/a high vacuum, smooth idling and/or wide LSA-type cam. Thing is, unless the parts counter guy is cool w/you, they come in packs of 10 springs.

Another seldom-mentioned adjustment that can be made to a Q-Jet, is the travel limiter for the PP. It can be adjusted (screwed) up or down to allow the primary needles to sit higher or lower at max vacuum, and is another tuning aid when dealing w/long duration or overlap cams.

It is shown w/a (poorly drawn) arrow pointing to it, below.

[DoubleVision]

Thanks Cobalt, great Tech. I`ve known about the PP system and part throttle adjustment stuff, Problem is it seems nobody carries just the springs. It seems like if someone came along that offered a variety pack of springs to tune to the cam and given vacuum then we`d be home free. So when it`s came down to it I`ve had to get creative on springs doing it myself then hoping it was right but I`d rather eliminate the guessing and drawing board routine. The sound of performance should be the sound of a Quadrajet at wide open throttle packaged in a box. Open the box and get the classic "WO-WHOOOOOOSSHHHHHHH" like the hoods being sucked in. Gotta love it.

[Greg Gessler]

Originally Posted by cobalt327

Edelbrock has about the only Q-Jet tuning parts, AFAIK they show only 4 different PP springs- but they only come in a "Race Calibration Kit" that contains a selection of main jets and rods, 2'ndary rods, hangars, PP springs, high flow needle and seat and an accelerator pump.

I keep in stock 100's of parts for the Rochester Quadrajet Carbs. Check out our website for more information.

www.gesslerheadporting.com

[cobalt327]

Originally Posted by Greg Gessler

I keep in stock 100's of parts for the Rochester Quadrajet Carbs. Check out our website for more information.

www.gesslerheadporting.com

This is the first place that I've seen replacement secondary air valve cams. I've had to make them by hand in the past, not a fun way to spend an hour or so!

Power piston springs in sets of 4. Cool. Button head throttle blade screws, ditto.

Not seen secondary "jets" before, either. I've never really needed them, but still. On the Primary Throttle Shaft Bushings, you should consider packaging 5 of the 1/2" and 5 of the 5/8" bushings together instead of 10 each of just one size or the other.

All in all, a damn good selection of parts for the Q-Jet.

You might be asked to do any further "advertising" in the free classified section, I hope I didn't bring too much extra heat on you- but I felt compelled to give a positive comment, all things considered.

[techinspector1]

Cobalt, if you haven't made a wiki article of this yet, please do.

[cobalt327]

Originally Posted by techinspector1

Cobalt, if you haven't made a wiki article of this yet, please do.

I'll definately see what I can do, TI.

[cobalt327]

Originally Posted by techinspector1

Cobalt, if you haven't made a wiki article of this yet, please do.

Just a heads up for the Q-jet gurus out there- LT1Silverhawk has started a page on the Q-jet, located here. It is a compilation of rebuilding and tuning info and is not complete. So if there's anything you'd like to add, please do.

[TommyK]

Originally Posted by cobalt327

Just a heads up for the Q-jet gurus out there- LT1Silverhawk has started a page on the Q-jet, located here. It is a compilation of rebuilding and tuning info and is not complete. So if there's anything you'd like to add, please do.

That is a great resource.

One comment. JB Weld is not recommended for well plug sealing as the gas will eventually soften it. Cliff Ruggles recommends Marine-Tex Epoxy for well plug sealing.

[John long]

I hope I am not hijacking this thread. Along the same lines, there is also a spring in a carter WCFB that counter acts the vacuum plunger. Does any one have any stats or maybe a source for WCFB parts. I have a WTB add in the classified add now for it. If I knew the correct spring stats I might be able to get "creative". Mine has been stretched and rendered useless.

John L

[F-BIRD'88]

Good thread.. Thanks for the link to the Qjet carb parts.

http://www.quadrajetparts.com/

[cobalt327]

Gessler does have a good selection of hard to find parts. What he doesn't have Ruggles does, mostly. And vice versa.

I don't know who sells step up springs for the WCFB, you might give Mike's a shout- http://www.carburetor-parts.com/.

If they turn out to be the same as AFB/Edelbrock springs, here are AFB step up springs. Edelbrock also sells step up springs for their Performer series carbs (they're the same as AFB).

[75gmck25]

I have finished with most of my Quadrajet rebuild and am close to getting it back together. I have the complete kit and the book from Cliff Ruggles, and I tried to follow the process completely. However, even with Cliff's book as a reference its a challenge to look through all the quadrajet info in the book and online and figure out what applies to your situation. Every "expert" seems to have a slightly different list of do's and don'ts.

I got the power piston spring set with my kit, but it included no information on which spring is which. I finally emailed Cliff and he recommended one as a starting point. I've also modified the carb so I can adjust the APT after its installed, so that should give me some flexibility.

I'm installing my rebuilt 17080213 Qjet (one of the last with no electrical controls) on my very mild '75 350. I rebuilt it to mostly stock settings for now, and will adjust after I get it running and see how it works. However, I will probably pull the carb to make some of the changes (like swapping a piston spring), since there are too many small parts that could fall into the intake if I leave it installed. On my Edelbrock 1406 those type of changes were very easy to make without pulling the carb.

Bruce