lars

As most of you know, I've been warning people about the problems with the commercially-rebuilt Q-Jet carbs from various sources. The commercial rebuilders do some really odd things with Q-Jets, and set them up so that many of them simply don't run. Many of the modifications are such that a rookie Corvette handy-guy would not be able to fix these issues with a "carb kit" rebuild, resulting in a myriad of bizzarre drivability problems and a bad rap for the Q-Jet carbs.

A couple of weeks ago, I posted some results from the teardown and inspection of "Bobs77" carb. This week, I have a Q-Jet belonging to Jim from MA. This carb has been "remanufactured" by Holley, so let's take a look at how Holley set this carb up...

Carb: 7044202 (1974 350 Chevy)
Should be set up and jetted with 75 primary jets, 46 primary rods, and the "CH" secondary rods (.057"). Pretty standard jetting.

Here are the issues I found:

1. Stripped fuel inlet. The inlet threads were stripped out and a self-tapping inlet fitting had been installed. Bad way to repair a bad inlet, since these self-tapping fittings can easily tear the rest of the threads out. Repair: Machine and tap the inlet for a stainless steel helical insert.
2. Sticking secondary airvalve. The airhorn casting was distorted at the rear edge of the airvalve from over-torquing of the rear airhorn screws, resulting in jamming and sticking of the airvalve. Repair: File the displaced metal from the airhorn to obtain correct clearance to the airvalve.
3. Bent accel pump lever. The lever had been bent to better align with the accel pump lever rod (see next item). The bend looked "factory", but Q-Jet levers are straight... Repair: Straighten the lever.
4. Accel pump lever rod cut off. The "jog" on the top of the rod had been cut off and an odd retainer had been used to keep the rod in the lever. This slightly changed the geometry of things, requiring the lever to be bent. Repair: Replace the rod.
5. Bent secondary airvalve linkage & lever. The airvalve linkage system had some very odd, creative bends in it. It might look factory if you hadn't seen one before, but this just wasn't right. Repair: Bend it all back to match a "virgin" carb I had sitting here in the shop.
6. Incorrect vacuum nipples. Some very odd configuration vacuum nipples had been installed in the carb. Repair: Replace vacuum nipples with correct, stock nipples.
7. Warped airhorn. Holley had installed a big, fat airhorn gasket to compensate for a severely bent airhorn. Even with the fat gasket, the fuel transfer passages were not sealing - the idle circuit was not getting fuel on this carb. Repair: Straighten and machine the airhorn flat.
8. Incorrect secondary rods. Holley had installed a set of "generic" .040" diameter rods. Repair: Install the correct .057" Corvette rods.
9. Power piston installed without primary rods engaged in jets, bending and destroying both primary rods. Power piston was inoperative. No kidding: When the power piston was installed, they missed installation of the rods and simply jammed the airhorn down on top of the thing. This folded the rods over in the bottom of the float bowl, breaking one rod and destroying the other. The rods were not installed in the jets, resulting in an extreme rich condition. Repair: Replace the rods with OEM rods of the right size.
10. Accel pump jammed in bore - inoperative. When the accel pump was installed, it was installed "dry" with no lube. This resulted in the accel pump cup turning itself inside-out against the accel pump wall and jamming the pump into the bottom of the bore. The accel pump was completely inoperative. Repair: Replace accel pump and install correctly.
11. Very high float level. Float level on this carb should be about .375". Float level was set at .200" - too high for a street driven application. Repair: Replace the float and set to .375".
12. Float clip installed backwards. The clip attaching the needle to the float must be installed around the rear edge of the float arm in order to avoid binding the needle in the seat. The clip was installed around the forward edge. There was a GM Service Bulletin on this issue in the late 60's. Repair: Install the new needle clip the right way.
13. Idle air bleed holes plugged with lead plugs. The holes in the float bowl that transfer air to the idle air bleed holes below the throttle blades had been intentionally plugged with lead plugs, making the idle air bleed system inoperative. When this is inoperative, the idle speed screw must be cranked in a lot further, resulting in excessive exposure of the transition slots. This, again, will result in stumbles and off-idle hesitations. Repair: Knock the lead pluigs out of the holes with a pin punch and hammer.
14. Incorrect power piston spring. I don't know where Holley got this thing from, but it looked like the front spring off of a 3/4 ton truck. In order to spot incorrect springs, you have to know the correct part number for the right spring, and have some of the "correct" springs on hand for comparision purposes. Repair: Replace the spring with the correct OEM spring.
15. Idle mixture screws fully closed. This was likely not done by Holley. With all the problems with this carb, it would have run incredibly rich. The installer tried to lean it out by closing the idle mixture screws. I doubt this fixed the problem... Repair: Correctly set idle mixture screws during setup and test running to 2 turns out.
16. Incorrect springs on mixture screws. Long springs had been installed on the screws, resulting in the springs being in coil bind upon closing the screws. Repair: Install the correct short springs from a donor carb.
17. Fuel filter spring missing. The self-tapping inlet did not allow for a spring to be installed. The filter was installed with no backing spring, which means that the fuel was simply by-passing the filter. Repair: Install a spring.
18. Power piston stop pin cut off - piston destroyed. This is a favorite "trick" by many of the builders, and Holley always does this: The lower stop pin on the power piston, which determines how deep the power piston rides at cruise (and how far the rods engage in the jets to lean the carb out at cruise), had been cut off. This makes the rods ride way too deep in the jets, running the carb very lean. Repair: Find a virgin piston out of a donor carb and replace the piston.
19. Extreme lean jetted. The carb was jetted with #71 jets. If the rods had been installed in the jets, the carb would have run extremely lean, especially with the pin cut off the power piston. Without the rods in the jets, it ran incredible rich. Repair: Jet the carb correctly per spec.
20. One secondary discharge tube missing. On the secondary side, fuel is discharged out of two 1/4" diameter tubes sticking out of the airhorn. One of these tubes was never installed in the carb, resulting in little or no fuel discharge on the secondary side. Repair: Pull the tube out of a donor carb and install it in the carb.

That's it for this week's Commercial Rebuilt Carb! The carb is now back to its correct configuration, and it's set up and running as it should. So watch out for those commercial carbs, and look critically at every aspect of them if you ever find yourself rebuilding one.

Keep on tuning those Vettes!

The_Dude

Holley put a lot of effort into screwing up that carb. They should do them right, or just not do them.

74L82

I'm Jim and this is my carb. I'm embarrassed how bad this carb is.

I'm truly grateful there is a Lars in this world to fix such problems.

I'm really looking forward to getting this carb back and seeing the difference.

lars

Hi Jim !
Certainly didn't mean to embarrass you with the carb results...

The problem we're seeing is that there are a heck of a lot of guys out there with carbs just like yours, and they post things here on the Forum like, "My Q-Jet puffs black smoke and stumbles... what's the problem?"

It's important that people realize that these carbs are old enough (many Q-Jets are now 40 years old...) that many of them are seriously screwed up. Troubleshooting them via e-mail and Forum suggestions is virtually impossible, because there are so many really bizzarre things that can be wrong - none of them related to adjustments that can be done from the outside. In fact, most problems cannot be solved by simply doing a "rebuild" by cleaning the carb, installing a kit, and slapping it back together the way it came apart. Any more, with these old carbs, you have to be a detective to find all the little issues and defects that the previous 12 builders did to the carb. It's difficult to rebuild one of these without a stash of Q-Jet parts in the basement and a complete part number listing for the "correct" baseline. The commercial builders are doing a massive non-service to the hobby community by flooding the market with this bad workmanship and defective components.

Although "anything can be fixed," it's important that people are aware of the fact that there are a ton of bad Q-Jet carbs out there labeled as "rebuilt" and "remanufactured." Dont' trust anything.... (I tear my brand new Holley and BG carbs apart before I ever put them on an engine... and I find tons of problems even with the new carbs out of the box..!)

Entropy

I'm glad lars has already laid hands on my carb.
At least I know it's right if I ever need to regasket it.

mrvette

PAY ATTENTION, RE READ IT 6X OVER IF NEEDED....THE MAN IS RIGHT....DON'T ASK.....

Bob Onit

Lars, you say this is a "trick" that many builders use.
What is the purpose of cutting the pin?
I don't see any advantage by doing this, only destruction of a good piston.

By the way, I have those pistons ready to ship to you but didn't get a reply from my email regarding other parts you may need.
Please let me know so I can get the parts out to you.

74L82

Lars,

I'm just proud my carb was worthy of a post documenting how much effort it took to get it back to square one.

I'm mostly embarrassed I ran with this carb for way too many years in as bad of shape it was in!

Jim

lars

Bob -
I thought I had sent you a note, but I just sent you another to make sure...

I've never quite understood the cutting of the pin. The only thing I can think of is that it leans the carb out dramatically at idle and at light cruise. To compensate, they plug the idle air bleed bypass curcuit on the carb, thereby richening it back up by limiting air. Doing this would require a larger throttle opening in order to get the carb to idle, and would require a richer setting on the idle mixture screws, thus possibly giving more control to the idle mixture circuit. So much for theory: Every carb I have seen modified like this has had idle problems and lean-run problems that are solved by repairing the damage and returning the carb to its correct, stock configuration.

jpatrick636

Leo001

Hi Lars
Are you interested in rebuilding another carb? About 2 years ago you rebuilt my tach drive distributor (still runs great) as well as sent me a loaner carb. When I switched my carb eith the loaner you sent the car ran MUCH better, smoother, less backfiring through the exhaust and far more drivable. SO I had a local "carb shop" rebuild my carb. When I put it back on the car, it ran slightly less lousy, (loaner carb already sent back at this point) in any event I'm in need of a real "expert". Please email me back if you are interested in doing the rebuild, and if so what's the cost. Or do you sell rebuikt carbs and keep the old core (less down time for me) The reason I ask is I just got the car back after some water damage and would like to get as much use out of it as possible (it was gone for 9 months)

Thanks

Leo

Leo001@epix.net

RATT7

Great report Lars, I know that you have spoken about these rebuilt carbs and the problems with them in the past. I cant understand how the rebuilders are still in business.

markdtn

10-15 years ago I had good luck with Holley Q-jet rebuilds, but not so much any more I guess. I would have thought they would want to do it right. As fewer and fewer people need new carburetors with all the fuel injection out there, Holley must be suffering. One would think that they would want to keep a good name in carburetors by at least rebuilding them right. Wonder if they do a better job with their own stuff? I had them build the Holley carb on my boat, but that was 10 years ago. It still works great.

Maymyvetteliveforevr

Thanks for the detailed write up Lars. I agree with 72 stingray owner, I'm glad you rebuilt mine. Although I haven't installed it on the car yet, I know I won't have any problems with it.

adam

What a laundry list of problems, bubba's, etc.

No wonder nobody can figure out how to make a Qjet run properly
after possibly having as many problems as this.

Maybe Holley recognizes that screwing up Qjets will give them more business

jrotten

i was wondering has anyone ever had luck with mid america motorworks rebuilt q jets. i inherited a sick holley spreadbore on my vette and had no core.Placed order in the begining of dec and i'm still waiting. i guess i should hang on to the box so i can ship it to lars if it junk.

Bud2

This isn't the first time Lars has lifted the lid on sub-standard rebuilds. But this isn't with some little back alley carb shop, this is Holley. What's going on with the standards these days?

Bud.

shoptek

Unbelievable!!! No wonder so many folks bad mouth the QJ.


I had my QJet Larsbureator done by the man, the myth, the legend about 4 years ago----still idles SMOOTH, ZERO hesitation, and WOT is a thing of beauty

Thanks Lars !

speedreed8

maybe theyre shipping them to china for rebuild and sent back here.

R1234

Same here and still running great. Thank you Lars!
Bryan