I know the ultimate answer is going to be "Send your carb to Lars", but I thought I would throw this out there.

I have an '81 with the computer removed. The carb and distributor have been replaced with units from a '79, according to the previous owner.

When cruising along, I can give the car about half throttle, and it jumps forward; accelerating at a reasonably quick rate, considering what engine it has. Going to full throttle makes no difference. I could give it full throttle from a rolling start, and it feels no different than giving it part throttle from the same roll.

Nothing seems to be stuck.

I'm not a total QJet newb, as I used them on my old Pontiacs, and rebuilt one or two. I could do a total rebuild, if needed, but I thought I'd get a few ideas before tearing it down. If I take the carb off, I'm going to want to take the intake off. If I take the intake off, I am going to want to replace the heads, and the cam, and rockers, etc.

Combination of carb and timing?

Thanks in advance,
Tony

 

I would verify the secondary throttle blades go wide open first , have someone floor it ( engine off ) and look to make sure secondary blades go full open , adjust throttle cable or secondary linkage … maybe make sure the choke secondary lock-out is not stuck preventing the secondary throttle blades from opening when choke is full open

 

Some info from my tech paper that may assist you:Technical Explanation: Help! My Secondaries Don’t Work!

Here’s one of the most common “complaints” I hear. “My secondaries don’t work. I don’t get that jerk when they open, and the diaphragm is holding them closed!”

There is almost nothing that can prevent the secondaries from opening on a Q-Jet. As described in the previous section of this paper, the secondaries are purely mechanical. If the throttle cable pulls them open, they will open. The airvalve will open if there is mass flow demand to open the airvalve. The choke pulloff will relax and control the airvalve opening rate upon loss of manifold vacuum and allow the airvalve to open. Very few things can prevent this sequence of events.

When “testing” for secondary operation, many people make a couple of basic mistakes. First, many will flick the throttle momentarily wide open with the engine running in neutral. In this condition, the secondary airvalve will seldom open since manifold vacuum never drops enough, or long enough, to allow the choke pulloff to relax: The engine must be under load with a high air mass flow rate through the carb in order for the secondary airvalve to open. You cannot “wing the throttle” in neutral to get much, if any, indication of secondary operation.

The second presumption of secondary operation is a “kick in the butt” upon secondary opening. Actually, if the secondaries are operating correctly, the transition into the secondaries will be so smooth that there should be no “kick” feeling at all – only smooth acceleration. What many people regard as “that secondary kick” is an improperly set up secondary airvalve which actually causes a momentary hesitation before “catching” and pulling. This incorrect operation will first throw you forward, and then throw you back in the seat. Although you may be able to impress your teenage son with this, the car is not running as fast as it should with such an incorrect setup.

There are 3 issues that can actually prevent secondary opening:

1. Secondary lockout lever staying engaged. There is a lockout lever on the secondary throttle that will prevent the secondaries from opening before the choke is wide open. I.e., the secondaries are not allowed to open when the engine is cold, thus preventing engine damage from the engine being placed under excessive load before being properly warmed up. The secondary lockout lever can lock out the secondary throttle if the choke is not opening fully, or if the choke is incorrectly adjusted. It is very easy to observe if the lever is retracted and if the throttle is operating – simply look at the lever and verify that it is allowing movement of the secondary throttle shaft when the choke is open. Some carbs, such as Pontiacs and pre-’68 Chevys, use a lockout lever on the secondary airvalve rather than on the throttle shaft. Same principle applies – simply look at the lever and make sure it is retracted when the choke is open.

1. Mechanical jamming of the secondary airvalve. There are 2 things that can commonly cause this:

1.  
   1. Distortion of the airhorn casting at the rear airhorn attached screws. It’s common to see these two, long, rear attach screws grossly over-tightened, and this will actually bend the airhorn around the screws. When the airhorn bends in this area, it can jam the secondary airvalves and prevent them from opening. If you see that the airvalves are touching & “catching” the casting in this area, you can do 2 things:

i. Take a fine file and gently re-shape the distorted area. You need to remove the carb to do this to avoid getting filings down into the intake manifold.

ii. If the distortion is not really bad, you can loosen the secondary airvalve screws about ¼ turn and slide the airvalves forward as far as the clearance in the screw holes will allow. This will usually free up the jammed airvalves.

1.  
   1. Bent secondary airvalve shaft. If the airvalve is not operating smoothly, or is binding and jamming as the airvalve opens, it’s possible that you have a bent secondary shaft. Unhook the secondary airvalve windup spring from the lever and then operate the airvalves by hand – they should be perfectly smooth with no binding. If the airvalves need to be forced through their rotation without the spring hooked up, you have a bent shaft. You need to remove the airvalves by grinding off the staked back side of the attach screws and slide the shaft out of the airhorn to straighten it.

1. A more common and subtle problem is the issue of inadequate throttle cable travel. This is very common on Corvettes and Camaros. With a helper in the car and with the engine “off,” have the helper fully depress the gas pedal (with the engine hot and the choke wide open to assure that the lockout lever is disengaged). Observe the carb on the driver’s side and see if the secondary throttle moves to the wide open position: You can grab the throttle lever once the assistant has the pedal to the floor and see if you can move it further. If the gas pedal does not fully open the secondary throttle, you have one of several methods to fix this:

1.  
   1. Remove your floor mat. I call this the “25 horsepower floor mat tuning trick.”
   2. Install the throttle cable in the correct position on the throttle lever. The Q-Jet has an “upper” and a “lower” cable attach hole: The upper hole was used for trucks and station wagons. The lower hole was used for Corvettes, Camaros and Novas. If you install a Corvette throttle cable in the upper hole, you will never achieve wide open throttle since there is not enough cable travel to open the throttle from the top lever location.
   3. Straighten your throttle cable attach bracket. The bracket that supports the cable at the carb is often bent slightly forward. If the bracket is bent or moved forward, it will not allow enough cable travel to open the throttle fully. Simply bend the bracket back just a tad: I use a big hammer and a steel rod to give it a whack. I call this the “25 horsepower hammer tuning trick.”
   4. Fix your gas pedal linkage. Very often, the actual steel linkage coming off your gas pedal will be bent a little “flat.” This will cause the pedal to hit the floor before the carb is wide open. You can give the pedal more travel by simply grabbing the gas pedal and bending it up off the floor. Care should be taken not to damage any plastic pedal rod bushings when doing this: Support the plastic pivot points when bending the pedal rod so the plastic bushings do not break.
   5. Fix the slop in the gas pedal attach lever. Many C3 Corvettes have a bit of wear on the lever that attaches to the gas pedal rod: The rod is "D" shaped, and the attach lever has a "D" hole with a screw tightening feature. If this "D" hole is worn, you can remove the lever, grind some material off the locking feature surfaces so it snugs up tighter, and re-install it to gain some travel

Lars

 

If all the above doesn't pan out, I'd be suspicious of the power piston being stuck in the down position.

 

The movement of the power piston will change the mixture, in rough numbers, from the mid-14's to the mid-12's. You'd be surprised how much power you can actually make at 14.5:1 air/fuel ratio... I've done it on the dyno. The problem with the leaner mixture is that exhaust gas temperature goes through the roof, but you can still make a lot of power, even if it's not the best amount of power. In a car as heavy as a Vette, it takes about a 15-20 horsepower change in order to be able to "feel" a change in power level in the "seat of your pants." From my testing, most normally aspirated street engines will not see a 20-hp change between 12.5:1 to 14.5:1 mixture change. In other words, most people would never notice the difference in performance with or without a jammed power piston. But if you're running full lean while towing a horse trailer up a mountain pass at wide open throttle for 20 minutes, you'll probably notice the difference once you burn a hole through the tops of a few of the pistons...

Lars

 

The Quadrajet linkage causes the secondaries to fully open ONLY in the last bit of travel. Unfortunately, most old Q-jet cars have had 'changes' that prevent the throttle cable from adequate travel to get that job done. To test, you must use the accelerator pedal to evaluate WOT success; you cannot do this by hand at the carb.

LOTS of opportunities in the foot pedal and cable system to cause this problem. Lars' papers describe well all the ways Q-jet secondaries are limited in their travel or prevented from opening at all.

 

Plugged cat or fuel restriction

 

Other than the carb not getting the secondaries open I think its possibly just a tranny issue.If your pedal isnt bent and you get full throttle pull and you can visualize the secondairies openning to vertical then look else where.

is it an automatic or manual car. If its automatic then maybe your kick down cable isnt pulling it into a lower gear or it isnt connected. Or you arent down shifting a 4 speed when you jump on it. If your going 50 and just jump on it in fourth gear its going to accelerate nice and smoothly. If you drop a gear and jump on it you'll be happy.

 

Thanks for all the replies, guys. I have not had anyone help me with this yet, so tonight I'll get one of my boys out in the garage with me to check the cable travel. By hand, I can move the throttle and see down in there that the butterfly plates are moving. It's an automatic, and the tranny kicks down a gear right near half throttle. I have followed some tutorials on how to adjust it, and it "seems" ok.

Nothing but rain here for the past two days, so no chance to take it out and run it to warm it up. I'll do the checking that I can inside the garage. Not a lot of room to work in there.

I'll update when I get the opportunity. Thanks for the pointers and idea.

 

Well, maybe you just have to keep it simple sometimes.

I had my son check the throttle pedal, and it looked great. I stared at it for a while, then grabbed my small ratchet with my 1/2 socket. Rear driver side carb bolt was tight and didn't move at all. Passenger side turned about 1/2 a turn before it snugged up.

Front bolts? Driver side did a complete turn before starting to snug, and another half turn to get it to where I thought it should be. Passenger side almost a turn and a half to snug, with a little more to make it tight.

Took it for a drive, and it felt nice. But that is purely subjective. I think I'll get a new carb to manifold gasket in case this one is all wonky from age and not being tightened down properly.

If that doesn't fix it, time for a JEGS 355ci Crate Engine Kit (that'll happen...)