lars

Forum member "Bobs77vet" (Bob K) recently sent me a carb he procured via the internet (correct me if I'm wrong, Bob, but I think it was an eBay purchase or something similar). I want to use Bob's carb as an example of all the things that can be wrong with a carb, and why you simply can't just disassemble a Q-Jet, slap a kit in it, put it back together, and expect it to run right.

In all my papers and postings, I always warn people against buying "commercially rebuilt carbs," and I keep getting the question from people, "What's wrong with a commercially rebuilt carb?"

Bob's carb was a commercially rebuilt carb that had been recently rebuilt by a "commercial carb builder." Whoever bought it put it right back up for sale...

Here are the issues I found with this "near-new" 1975 Q-Jet rebuild and some of the things that had to be done to fix it:

1. Bent APT needle. Also incorrectly called the "altitude compensator," this device controls part-throttle mixture using an adjustable needle in a 3rd "jet" in the float bowl. The needle had been bent during rebuilt, causing the needle to not be engaged in the jet. This resulted in a "full rich" condition that somebody tried to compensate for with other "tuning" methods. Read on...
2. Lead plugs installed in idle air bleed holes. This is a common practice by most commercial builders. The Q-Jet uses an idle air bleed system that allows idle air to be pulled into the intake just below the throttle blades. By allowing a controlled amount of air to bypass the throttle blades, the throttle blade opening can be decreased, allowing effective use of the transition slots in the throttle plate. Plugging the holes for the idle air bleed results in the idle speed screw needing to be cranked up to the point that the transition slots are excessively exposed, causing off-idle stumble and hesitation.
3. Extreme rich jetted - damaged #78 jets. To compensate for other setup issues, the builder had installed #78 jets in a carb that should have #72 jets. In addition, they had slipped a bit with the screwdriver during assembly, damaging the jets and the fuel flow.
4. Throttle plate loose. The screws attaching the throttle plate to the fuel bowl were loose due to the factory-installed lock washers being deleted. This caused vacuum leaks at the throttle plate and a lean condition.
5. Severely bent primary throttle shaft & lever. The primary throttle shaft was bent, causing binding and sticking of the throttle.
6. Stripped inlet threads. Common problem on a Q-Jet. But in this case, the builder had installed a self-threading oversized inlet fitting, deleted the fuel filter and filter spring, and poluted the carb with metal debris from the self-tapping fitting.
7. Bent & destroyed secondary lockout lever. In an attempt to "adjust" the secondary lockout lever, the builder had bent the lever to the point of breaking the tang off the lower portion of the lever. The secondary engagement tang was broken, and the rest of the lever was severely bent and unusable.
8. Sandblasted finish. Common prcatice with the comercial builders: The entire carb had been sandblasted, removing and destroying all finishes and corrosion protection on aluminum and steel surfaces. Result is that aluminum surfaces are covered in white oxidation, and all steel parts are badly rusted. The carb had to be acid dipped and chemical conversion coated to bring back some of its correct finish.
9. Secondary airvalve loose & sticking. The builder had ground the staked feature of the retaining screws off, resulting in the screws loosening and jamming the airvalve.
10. Incorrect float installed. Brass floats don't cut it on a Q-Jet. Always use a NitroFill float.
11. Extreme low float level (.620"). To compensate for other issues, the float level had been dropped to an extreme .620" level. Correct level is .420. This low float level assured that the carb would not operate on the idle circuit, and the carb was idling on the main discharge circuit once the idle speed screw was turned in far enough.
12. Incorrect power piston spring. Common commercial rebuild problem: They install a "generic" power piston spring. This may work okay on a truck, but it sure doesn't work on a Vette. GM just discontinued the "Corvette" power piston springs 2 weeks ago...
13. Stripped threads in bowl for throttle body attach screws. Another common problem with these carbs is that they tighten the screws so far that the threads strip out. You have to have a handful of 10-32 HeliCoils any time you try to rebuild a commercial carb.
14. Bent primary rods. The tips of the primary rods were bent from incorrect attempt at installation into the jets. This eliminated any fuel metering control.
15. Accel pump rod broken. The commercial builders use junk parts from junk carbs. The rod attached to the accel pump lever was broken off at the top "jog," and the builder just installed a snap retainer rather than replacing the rod.
16. Bent primary throttle blades. In an attempt to get the throttle geometry right, the builder actually bent the primary throttle blades at an angle. This resulted in the primary throttles not fully closing. With the way the rest of the carb was set up, this was probably irrelevant, since the carb needed all the throttle opening it could get to idle at all..
17. Secondary throttles set to open over-center. Another common "speed trick." The secondary throttle on a Q-Jet should not open past vertical. In fact, they need to stop just short of vertical in order to produce most effective airflow through the secondary side.
18. Broken throttle plate. A common issue on the commercial carbs is that the builders install throttle shaft bushings in these carbs, whether they're needed or not. Very few Q-Jets really need throttle shaft bushings, and most people who install them screw up the install. In this case, the builder bored the throttle plate for oversized bushings and broke the throttle plate in the process. No problem - they just smacked the bushings into the broken throttle plate, left the massive vacuum leak unresolved, and let 'er go. The plate had to be repaired with correctly installed K-Liner type bushings.
19. Plugged idle fuel tubes. The smallest metering orifaces in a Q-Jet are the Idle Fuel Transfer Tubes. These tubes are pressed into the carb, and only the top oriface of the tube is visible in the top of the float bowl adjacent to the venturies. Commercial builders often pull these tubes out and install "generic" tubes with incorrect orifaces in the top and bottom of the tubes to compensate for the other "tricks" they do to the carb. In this case, the builder had installed tubes with lower orifaces too small, and these had been plugged up with the metal debris from the oversized inlet fitting. There was no idle fuel flow possible.

These commercial carbs obviously create some unique tuning and setup problems, and they are contributing to the notion that "Q-Jets are junk," since they never run right. If you have one of these carb, be sure you look critically at every aspect of the carb during any setup and rebuild process - you cannnot simply "install a carb kit" in one of these carbs and have it run right...

Have fun tuning!

Lars

427-390

Lars,

Great write up. When you did the TFB in Seattle last fall, you found about 5 of those things listed on my "Commercially Rebuilt Q-Jet" which you were able to correct. Aside from minor adjustments (Cold idle set & idle adjustment) the car runs like a dream. I'll add this article to my Lar's archive for future reference. Thanks again for contributing your knowledge to the CF and us enthusiasts.

Tony

Tim H

Im confused!
I thought q-jets only went up to #77s in the jets????

mrvette

Lars, sounds like dejavu all over again....daze of yore come back to haunt.....You probable the only guy left who knows how/what to do for these things....

77BuffaloVet

wow...thanks for posting more good information. I know that with my new LM-1 it is going to led me to all sorts of questions about the Q-jet, how it operates and what issues I am going to have to work out.



Chris

lars

You can get GM Q-Jet jets up through size 79. Part numbers as follows:
#78 jets: 7031978
#79 jets: 7031979

I've never had to use a set of jets bigger than 77's in any application I've worked on. I have several sets of 78's that I have removed from various carbs, so those are common. I've never seen a set of 79's, but they're still available from GM.

supercalvin

Sounds like Bubba's new job is doing commercial rebuilds.

Lars I love your write ups. When I am reading them I actually understand carbs. But when I go out to pop the hood somehow I forget everything you write and am afraid to touch it. Fortunately you rebuilt mine so I don't have to touch it.

I need a "Qjet for Dummies" book written by Lars with lots of Picts..I'd buy that anytime..probably two of them.

Bob Onit

Did you stock up Lars?
Looks like Ill have to contact the local dealers and see what they have
I have a few dozen 4MV's and would like to have a better assortment of springs.

Matt Gruber

lars
have u ever had to perform
mouth to mouth resuscitation?

lars

Yes, I did: I bought the last 2 packs available. I also got a print-out of all the dealers who have stock left. You can do the same by visiting your local Chevy dealer and having them do a "search" on the part number for dealer inventory. It's then just a matter of contacting that dealer directly and arranging shipping.

Yes, I have - I've had to suck and blow on quite a few carbs for various reasons... only problem is that you tend to get a mounthful of stale gas in your mouth every time... but that's why I keep a nice bottle of cold beer or a frssh-shaken Martini close at hand at all times.

Little Mouse

I wonder if they had a monkey or a human working on that carb,
great write up.

tjhuyser

so how much to straighten out my 75 L-46 carb? it dies under full throttle and comes back if you back off some...I think the yahoo before me figured bigger jets= bigger power..I also have an edelbrock proformer intake and an msd module...if that makes a difference...will you be at Carlisle?

lars

tjhuyser -
A few common issues come to mind that will cause that problem:

1. Insufficient fuel pressure/volume at elevated rpm. "T" a fuel pressure gauge into the line at the carb inlet and run a long hose to the gauge. Tape the gauge to the outside of your windshield and observe pressure when the problem occurs. You need at least 3 psi fuel pressure at all times.
2. Restricted fuel filter. First, make darned sure you do not have any fuel filter installed on the suction side of the mechanical fuel pump (like backl at the tank or on the frame rail on the suction side of the pump). Then, install a fresh filter in the carb.
3. Kinked fuel line. Very common problem with kinked rubber lines back at the fuel tank and between the fuel pump and the frame.
4. Secondary airvalve too loose. Make sure the secondary airvalve spring windup is set to at least 3/4 turn. If it still falls flat, try 7/8 turn.
5. Ruptured choke pulloff or secondary airvalve rod to pulloff missing. Make sure the pulloff retracts fully when the engine is running, and make sure the rod is holding the airvalve firmly closed.

Send me a plane ticket and I'll be there!

Good luck with the tuning!
Lars

Bob Onit

Lars, Do you know if GM discontinued any other 4 MV parts?
Do you have a part # for the discontinued springs?
Thanks

bobs77vet

hi Lars thanks for all the good work in making it right. i actually bought it from another forum member for $50 who thought it was running ok and only needed a quick rebuild. the only disclosed issue was the missing accelerator pump lever


i am looking forward to putting it on the 77 and this will be one of my winter projects! thanks again for all the work, bob

tjhuyser

ok...I'll check that first..I know the filter is new

lars

GM has discontinued almost all Q-Jet parts. Only a few early primary rods (1966-1974), a few jet sizes, and some post-75 truck rods are still available. These will soon be obsolete, so the only source for Q-Jet parts will be junkyard carbs...

Part number for the recently discontinued power piston spring is 7036019. This is the common spring used in most performance Q-Jet applications.

Bob Onit

Thank you Sir!
Glad I stocked up on rods, jets and tons of other used parts and cores

7T1vette

Not to steal a thread, but...

Just wanted to let everyone know what a great job Lars did on my Q-Jet (Edelbrock #1901 but still a Q-Jet). I stopped by his "pad" on our way to see family and friends in parts east. He is a wizard! Had both the carb and distributor torn down and parts cleaned in no time. Then he did his "magic" [which is really just a term to capture his vast knowledge and skills] to bring them back up to spec. I had him discard an old Pertronix module and put it back to points. After re-assembling, he put it on the old Sun spin tester; the diffy had a nearly perfect straight-line mecanical advance which finished at 36 degrees all-in at 2600 rpm. Then he ran it up to 6500 rpm and it never missed a beat and was stable as a rock. THEN, he got out the test bed engine and installed both on it to give it a running test and final set up. Wow! It was something to see the Master at work! We are now back from our trip east and have "goodies" installed on the old C3. What a difference! THANKS, LARS. YOU ARE THE MAN!!!!
7T1vette

Tim H

So my 77 carb has # 77 jets and .052 primary rods.
If I change to .048 or .040 primary rod it will richen up the idle and take off?
I have both sets of primary rod sets.