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#1
01-25-2021, 01:18 PM
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Measuring Q-Jet Primary Jets
I'm going through my stash of jets and doing a little measuring....where I can.
One thing I've found is that very few of the jets are actually what they are stamped. Even what appear to be new ones. Some are 3 sizes larger than the stamped number. One thing I've run into is lack of the correct numbered bits in my measured bit box. The box jumps from .073 straight to .076 and none are missing. I've got an .075 stamped jet that I think is actually .075 but I can't measure it. Is there a work around to measuring the .074's and .075's that are actually what they are stamped?
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 frittering and wasting the hours in an off hand way.... 1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. 
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#2
01-25-2021, 01:33 PM
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Well it’s kinda the same thing with old Style Holley jets where stamped numbers could wander up or down from the stamp.
I would wait for Cliffs response to your post,
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#3
01-25-2021, 01:53 PM
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Common problem, and why I toss them in the round file (trash can).
Those carbs have been out there for at least half a century at this point. Many are worn from use (metering rods sliding up and down in them) or just engine vibration. There will be a percentage that have been hand drilled to larger sizes by the owner or his well meaning beer drinking buddy or local "guru" trying to fatten things up to help make more power, etc. I don't even look at them and haven't for many years, we just replace them. The metering rods fair a little better than the jets as they are most likely a little harder, but they still need to be measured before reusing them. I replace those as well about 95 percent of the time, and why I've stepped up to have custom ones made. On the same subject we recently added another metering rod to the line-up, it's a full tapered "M" style rod starting at .050" and tapering down to .036", instead of being "stepped" like the factory "M" series rods.......Cliff PS: I need to add here that currently there are 4 different jets being sold that screw right into the casting. 3 of them are incorrect as the Quadrajet metering jets were very specific for depth of the orifice and leading entrance/exit angles, etc. I get carbs sent here all the time with the WRONG ones in them....FWIW.....
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If you can read this, thank a teacher. If you can read this in English, thank a Veteran! https://cliffshighperformance.com/ 73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile), |
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#4
01-25-2021, 04:19 PM
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Decades ago i found that stamped number on jets does not reflect drill size.
By measuring factory new jets i found drill size of a #78 Quadrajet jet may very well measure .081", and in the other end a #66 jet measures .064". The reason for this is that Rochester flow tested jets and then stamped the appropriate number, just like Holley, Carter and other carb manufacturers did. As for checking a #74 jet a 1.9 mm drill bit is suited. HTH |
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#5
01-26-2021, 07:25 PM
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If you need to measure the inbetween's, fractional metric drills fill in many of the gaps.
Jon.
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"Good carburetion is fuelish hot air".  "The most expensive carburetor is the wrong one given to you by your neighbor". If you truly believe that "one size fits all" try walking a mile in your spouse's shoes! Owner of The Carburetor Shop, LLC (of Missouri). Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings. |
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#6
01-26-2021, 07:51 PM
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For those oddball jets and other orifices where a drill bit doesn’t match, I’ve I used cheap 1/8 brass punch’s(or appropriate closest size) , chucked it up in my hand drill and simply filed and sanded to the size I want.... marked it and threw it in the kit
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71 GTO, 463, KRE 295 cfm heads ported by SD Performance, RPM intake, Qjet, Dougs Headers, Comp cams HR 246/252 ...11 to 1 , 3.55 cogs, 3985lbs.....day three- 11.04 at 120mph ....1.53 60', 6.98 1/8 mile |
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