I think you hit it right on the nose there.

Does the carb have to be on the actual car to be properly set up or is that something that can be set up on your test engine? I'm just trying to figure out if the fine tuning of the fuel/air mixture needs to be dialed in when it gets installed..

I'm just spoiled with the Holley which was install and then the next thing to worry about was to hold on... No adjustments needed...

 

Interesting, mine does exactly the same thing, an LS5 as well. I had Cliff go over my carb about a year ago, assumed the apparent sticking was an issue with the spring or linkage. I ended up switching back to a carb you had taken care of for me, my backup carb. Figured I'd address the throttle issue another time.

 

I probably just got lucky with the Holley...

There is certainly no substitute for tuning a carb on the actual engine. Short of that, a carb can be set up and verified on any engine of similar displacement and vacuum signal, and you can get it pretty darned close: a carb doesn't "know" what engine it's on, and simply meters a certain amount of fuel to a certain mass of air passing through the venturies. Idle and transition fuel is a little different, in that the amount of fuel metered varies with actual vacuum signal below the throttle plates. But both Cliff and I have a pretty good "feel" for the setup that needs to be done based on the engine that the carb will be going on, and we do jetting based on our experience with that.

That said, there should not be much adjustment needed to the carb once you get a carb from either me or Cliff: At worst, you may have to give the idle speed screw a small tweak, and perhaps make a very slight adjustment to the idle mixture screws. My test engine does not have a "brake" on it, so I can't test a carb under load. I don't believe Cliff can, either - you have to have an actual engine brake/dyno to do that. But the no-load engine test will detect any gross metering issues with the carb, and you can get a very good "feel" for throttle response and carb setup by running the carb on the test engine.

Lars

Back in 1975, I went to the Rochester Training Center, and I later taught various subjects, including Q-Jets, at a GM Training Center. One of the things that Rochester pointed out, and that we always relayed to the students in the classes, was that the Q-Jet throttle shafts were assembled "loose" in the throttle plates: The carbs were "sloppy" when they were brand new. Mechanics at the time would actually complain about it, and a lot of carbs were returned under warranty for "loose throttle shafts." Now, 40 years later, enthusiasts and owners of these carbs are still complaining about "loose throttle shafts," and the object is now to rebuild them and re-bush the shafts to make them as tight as possible (like a Holley). The looseness in the shafts was intentional: It allowed the shafts and the throttle plates to move a little and "self-center" when things deflected under manifold vacuum, and it prevented the throttles from sticking when returning to idle. The slight vacuum leaks through the loose shaft ends was regarded as insignificant. If this looseness becomes grossly excessive (due to wear), the throttle shaft and plates can shift enough under vacuum to cause an inconsistent return-to-idle. But if it's too tight, extreme attention must be paid to throttle plate alignment and positioning to allow the throttle to return to idle under all manifold vacuum conditions. Engines with mild, stock cams are more prone to this condition than engines with bigger cams: a mild cam will require a very small throttle opening at idle, making throttle plate alignment more critical. A bigger-cammed engine, utilizing a little more "cracking" of the throttle, will not have this return-to-idle alignment problem.

If you are not concerned with "correctness" of the carb setup, you can often solve this problem by removing the throttle return spring from the lower part of the throttle lever (pulling backwards) and installing the spring at a point above the lever rotation point and have the spring pull the lever forward. This will offset some of the forces on the throttle, and will make the carb return to idle much more consistently.

Lars

 

Lars, tried sending you a message. Can you pm me?

 

Lars,
I have a '69 Corvette with '69 Carb on a '74 block and '73 heads. In short, the car looks and drives great--but nothing is original.
The Rochester runs good, but it is not set up for the motor. (Needs Rods and jets for a standard cam, new intake and exhaust.)
Don's Speed Shop wants to replace with a Holley. (I suspect they don't have enough Rochester parts for the adjustment.) My question: Is there anyone in central Connecticut you would recommend for properly tuning my Rochester Carb?
-a grateful soldier-

 

The only guys I know who set up and test the Q-Jets are Cliff, Henry and me. I'm not aware of anyone in CT doing that type of work.
Lars

 

If it was me, I would send my Qjet to Lars in a heart beat. You will be happy that you did. It's well worth the shipping cost to send it to him. He also has a much better turn around time as well..

 

Another post suggested that I send you the carb. I read your directions--about isolating the primaries and such. I am confident that I could get it correct (eventually). I am more concerned with the need of finding and trying three (or more) sets of jets / rods to get the setup correct for optimal performance and mileage. I'm not sure the suggestion to send you the carb would work. What information would you need to tune the carb off the car? '74 block completely rebuilt to speck, edelbrock intake, 11:1 compression, 350/350 w/ ~380 torque. Non-header exhaust. Enough info?
If so, any idea about cost and turn-around time?
If not, do you have a suggestion for acquiring an assortment of jets / rods so I can tune it my self?
LT B, -ron-