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Anyone have a good way to measure "springiness" of qjet power piston springs?
I'm tuning up a rebuilt qjet and have quite a collection of power piston springs. The one delivered with the carb was too rich, but I have 15 or so to test. They're various lengths and springiness (or stiffness?). I suspect some are from the later 170 series qjets (75 & up) - they're shorter, I suspect some are from the earlier 70 series carbs (66-74 or thereabouts).
Have any of you built any kind of contraption to measure how much vacuum or weight force it would take to hold a power piston spring compressed and allow it to expand? Holley used to color their vacuum secondary springs, but GM did no such thing.
Since gas metering is a delicate thing and controlled by these springs, I'd love to be able to at least put my springs in order from stiff (i.e. rich) to spongy (i.e. lean) so I could play around with them in some kind of organized way.
I can think of cobbling up a vacuum chamber, but that's pretty involved. I have also tried to create a stand (really just a piece of bent wire) and put a varying amount of washers on it to measure spring resistance. I didn't get far on that because it took more washers than I had on hand to compress the springs.
Anyway, is there such a tool or how have you all figured this out? I'm hoping there's a better way than trial and error.
I found this on the 'Web some time ago, in an old Q-Jet Tuning paper by Lars Grimsrud. I haven't tried it, but it makes sense to me.
Using your carb, or a junk float bowl from another carb, as a testbed, remove the carb air horn (the “top” of the carb) and remove the power piston and its spring. Remove the primary metering rods from the piston. Now, drop a spring into the power piston bore and install the piston. Find a Phillips screwdriver, and place the handle of the screwdriver on top of the power piston with the shank of the screwdriver pointing straight up. Use a screwdriver that is light enough to NOT compress the power piston and its spring, but close. Now, drop flat washers onto the shank of the screwdriver and keep stacking them up until the piston compresses the spring and seats in the bore. Count the number of washers it took to compress the spring and label the spring as a “6-washer spring,” for instance. Do the same with the other springs you want to test. You’ll end up with a comparative rating of springs, like “4-washer,” “6-washer,” or “10-washer” springs. You now know exactly how to arrange them from softest to stiffest.
Last edited by BackInTheGame; June 23rd, 2023 at 04:33 PM.
That's actually a really clever way to do it. The next level would be to use a dial indicator to measure the deflection for each washer and then weigh the washers. Now you have a way to plot the spring force (lbs per inch).
I found that Lars Grimsrud article this evening & am considering sacrificing a carb body to make a car-connected spring simulator. I have a couple good spare
carbs but would hate to donate a good one for testing, but I’m wondering if I have a junk body under the house…
I’ve been following Lars and Cliff Ruggles qjet carb notes for years. They are really experienced in qjets. If I needed to point at an oracle fo qjets, I’d point at both of them.
I measured my collection of springs tonight and found (digital caliper measured) lengths from .090” to 1.50”. I was surprised at the number of lengths: .90”, .95”, 1.00”, 1.15”, 1.25”, even 1.5”. At least I have a starting point for organizing them by length.
What’s interesting is that within a given length, there is variety with stiffness from spring to spring. It feels like “as designed” not “after use” - meaning GM was well aware of spring rates to control gasoline metering and had tons of options when tuning a given carb for a given car.
I suspect I’ll cobble up the washer method as recommended.
Sounds like fun, if I can find a junk carb body in my hoard. If I get there, I’ll try to photograph and document my experiences.
I have followed Cliff and Lars for a while also and have wondered if you could apply a spring rate equation to these.
This is a coil spring equation that I think is the same as in my Machinist handbook(not with me right now)
I know I have seen some specs floating around on Cliffs site of known specs and would make a good comparison.
Basically with the formula you could at least calculate a known spring rate and caculate what you have in your pile and get a range to compare.
Not sure how this would relate to how the spring reacts to vacuum force.
I do know the two series of Quads use different springs, so atleast with some searching on Cliffs site you could come up with 2 piles to 'test'
Sounds like fun, if I can find a junk carb body in my hoard. If I get there, I’ll try to photograph and document my experiences.
Cheers
cf
Actually, if you plan to build a fixture from a junk carb body, the better way to do this is to use a variable vacuum source plumbed to the bottom of the power piston well. What really matters is the plot of spring (power piston, actually) height vs. manifold vacuum. Yeah, you can calculate that from spring rate, but it's probably easier to measure it directly. A dial indicator gives you height and a needle valve in the vacuum line probably gives you the variability you need. Lacking a vacuum pump, you could even use a running engine to provide the vacuum source. The metric you really care about is the location of the tapered primary metering rod in the jet vs. manifold vacuum.
I was thinking along the same lines as you were Joe. I watch a Youtube channel called Farm something or other where the guy tests all sorts of things using scientific methods, he is probably an engineer. I bet he would do it that way as well.
Yes, any measuring instrument will contaminate the results. How does your force gauge measure distance without a dial indicator? I'd argue that once set up, vacuum is both easier to use and more representative of how the system actually works on the car. As I said above, the important metric is power piston height vs manifold vacuum.
Yes, any measuring instrument will contaminate the results. How does your force gauge measure distance without a dial indicator? I'd argue that once set up, vacuum is both easier to use and more representative of how the system actually works on the car. As I said above, the important metric is power piston height vs manifold vacuum.
Doesn't everyone have a shadowgraph?
The other problem with vacuum is that there is clearance between the power piston OD and the ID of the hole and that's going to have an effect how much vacuum it takes to generate a given spring deflection. But as long as the same hardware is used, it can certainly be used to compare different springs.
Yes, any measuring instrument will contaminate the results. How does your force gauge measure distance without a dial indicator? I'd argue that once set up, vacuum is both easier to use and more representative of how the system actually works on the car. As I said above, the important metric is power piston height vs manifold vacuum.
But aren’t the springs different lengths as well? I think I’ve seen them that way from some of the Qjet parts suppliers.
But aren’t the springs different lengths as well? I think I’ve seen them that way from some of the Qjet parts suppliers.
Yes, but you know that all the springs are seeing the exact same load, and you have an easy way to measure uncompressed and compressed length. This won't give you actual spring rate values, but it's an easy way to compare different springs.
But aren’t the springs different lengths as well? I think I’ve seen them that way from some of the Qjet parts suppliers.
There will be variability in length, but that is only one factor on spring rate. In any case, a vacuum rig will allow you to measure power piston height (including the effects of initial spring height) relative to some reference point on the carb, so one can make an informed decision of how to combine a given spring with metering rods to achieve the desired fuel curve.
I like Joe's idea with a vacuum pump, but I want to see how far I can get with this much cruder method. I'll let you know if I get a wild hair and make a proper test rig out of that carb body.
Also I think the carb I'm working on does not have a smooth power piston bore, so I want to clean and polish it before moving on to the next spring or rod.
Actually, if you plan to build a fixture from a junk carb body, the better way to do this is to use a variable vacuum source plumbed to the bottom of the power piston well. What really matters is the plot of spring (power piston, actually) height vs. manifold vacuum. Yeah, you can calculate that from spring rate, but it's probably easier to measure it directly. A dial indicator gives you height and a needle valve in the vacuum line probably gives you the variability you need. Lacking a vacuum pump, you could even use a running engine to provide the vacuum source. The metric you really care about is the location of the tapered primary metering rod in the jet vs. manifold vacuum.
I made a fixture, https://www.youtube.com/watch?v=Uq-p...ature=youtu.bea few years ago, and used an A/C vacuum pump and old gauge manifold. Then it was just a matter of adjusting the vacuum with the hand wheel and watching the piston and vacuum gauge. I checked all my springs and labeled them for future use.
Joe
The only concern I would have is that the OD of the power piston and ID of the hole are going to vary from carburetor to carburetor, giving you a varying gap, affecting your deflection at a given vacuum level. Otherwise I really like it.
The only concern I would have is that the OD of the power piston and ID of the hole are going to vary from carburetor to carburetor, giving you a varying gap, affecting your deflection at a given vacuum level. Otherwise I really like it.
This won't matter, since he only wants to know the relative strength of each spring vs the others. He will select the actual spring by trial and error, or a wideband. So he just needs to know what's the next spring I am going to use if I want it richer (or leaner) than it is with the current spring.
I made a fixture, https://www.youtube.com/watch?v=Uq-p...ature=youtu.bea few years ago, and used an A/C vacuum pump and old gauge manifold. Then it was just a matter of adjusting the vacuum with the hand wheel and watching the piston and vacuum gauge. I checked all my springs and labeled them for future use.
Joe
Nice job!!! I have had something similar in my mind for a while but never have been able to find the time to build the test fixture.
June 23rd, 2023, 04:16 PM
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