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Q-Jet Power Piston Spring Question

Old 07-14-2017, 02:53 PM
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Default Q-Jet Power Piston Spring Question

long story short I had to rebuild my carb/fuel system. When I opened my carb there was no power piston spring in the carb. I was going to add one back in as I'm have issues with it right now trying to get it to idle nicely

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in this wiki section
http://www.crankshaftcoalition.com/w...4MV_carburetor

they give several options to choose from

Power piston springs

A power piston spring allows the power piston (aka "PP") to behave like a Holley power valve (or "PV"), that is, the enrichening system is controlled by the amount of vacuum the engine is producing- which is also an indication of how much load the engine is under. More load = less vacuum. Less vacuum = more enrichment, all else being equal. PP springs aren't always mentioned when Q-jet tuning is being discussed, but need to be addressed just like if it were a Holley PV. The same principals apply, as far as tuning for low vacuum cams, etc.At high vacuum, the vacuum exceeds the PP spring rate and the primary metering rods 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 pull from the engine vacuum, which allows the spring to lift the primary metering rods up to their smaller diameter, or "rich" position.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 use a softer rate PP spring.Below, left is an image showing a variety of different springs. The springs vary in length, coils per inch, and wire diameter (0.012"- 0.020" in the collection below, as an example).The image to the right shows three different types of power piston. The power piston on the left with the rod extending from the bottom is used with the baseplate mounted APT adjustment.Edelbrock has four different PP springs: gold (4"/Hg), orange (5"/Hg), black (6"/Hg), and yellow (8"/Hg) in their p/n 1994 spring kit. These springs are also available in their "Race Calibration Kit" that contains a selection of main jets and rods, secondary rods, hangars, PP springs, high flow needle and seat and an accelerator pump. There are also other suppliers of PP springs and other various tune up and repair parts; see the links below this section.
  • GM power piston spring p/n 7037305 is set for a tip in point of ~6 in/Hg, and would work well with a high vacuum, smooth idling and/or wide LSA type cam.
  • GM PP spring p/n 7036019 has a tip in of about 8 in/Hg.
  • GM part #7029922 is a rather weak spring that was OE for HO Pontiac engines from the early ‘70s. It delays enrichment until vacuum drops to ~3 in/Hg. This spring will work with durations >/= 230 degrees @ 0.050” lift.
GM/Delco PP springs come in packs of 10 springs unless you can talk your way into buying less.
i'm guessing i should just run the ~6in/Hg as that what is "stock" or as close to 6in/Hg as I can find. seems pretty hard to find parts for carbs these day as the parts store. I can find them online but then its 3 days of shipping i cant take it back and swap it if its not right...story:
boat was not running right, motor was getting air and spark so fuel is the next missing piece.... fuel filter was full of sand, water and lots of rust. pulled the carb and the screen (#4 above) in the carb was full of a very fine sand, the pump was packed full of sand holding the 1 way valves open so there was suction and pressure on the discharge or the pump. boat got a new filter, a carb rebuild and i cleaned out the fuel pump. The hard line from the pump to the carb was trashed, the previous owner(s) couldn't get it off and rounded the brass fitting, I tried using a 1" wrench to take off the plug (#1 above) and started to twist the line in the process. so I pulled it with the carb and had to use a bench vise, several 4 letter words, and elbow grease to get them apart. I tried to bend new hard lines and after kinking the lines with the benders I gave up and went to a 8" long 5/16" compression fitting brass T, the T has a 3/8" bar fitting that goes to soft hose to the carb and then goes to a 8" long 5/16" compression to a 1/8" compression to a 1/8" compression to 1/8" NPT adapter to a fuel pressure gauge.


best I can tell I don't have a Big Bore Small Block
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Old 07-14-2017, 03:32 PM
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The missing spring would not affect idle, but i would expect a lean conition under heavy load before and during the opening of the secondaries as there is no spring to lift the rods out of the primary jets which increases fuel under low vacuum. 6.5 is a common number for a stockish v8
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Old 07-14-2017, 05:54 PM
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Originally Posted by Mr Maine
The missing spring would not affect idle, but i would expect a lean conition under heavy load before and during the opening of the secondaries as there is no spring to lift the rods out of the primary jets which increases fuel under low vacuum. 6.5 is a common number for a stockish v8
this is a bone stock V8. I did find a local supply for a spring pack that ranges from 4in/Hg to 11in/Hg it has a 6in/Hg. is it better to go high or lower for a stock v8?

i have some other issues going on that i also need to solve, i think the flow is not seating correctly as its dumping fuel out of on of the atomizer vents. I kept the only float seat and used the new needle. I also hooked the needle wrong.

I did exactly what the wiki is telling me not to do.... oops.
[img] http://www.crankshaftcoalition.com/w...loat_clip1.jpg [/img]

tomorrow i'm going to the junk yard to see if i can find a Chevy 350 TBI system, collect all the parts for an EFI conversion...
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Old 07-15-2017, 07:23 PM
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Just buy a new carburetor. Get an Edelbrock or a Quickfuel. The efi is a project to marinize.
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