Hi, I am fairly new to this forum, and have an 81 Vette that I bought last summer. The car runs and drives fine in normal conditions. However, when I really step on it, I am not convinced the secondaries in the carburetor are opening. I get lots of fan noise, but no "induction moan" like on most other cars when they are in 4bbl. The transmission does downshift, and the car revs fine, but just no real push in the back from acceleration. I know it is only rated at 190hp, but I was expecting a nice V-8 howl. I have good pickup from standing start, and it feels pretty quick there, just at highway speed when I want to pass that I have the problem. All the gauges read normal, good oil pressure, temps are fine (oil and water), good volts, etc. The car has 81K original miles and is in great condition. Maybe I am expecting too much, but when I kicked in my 68 Charger R/T, there was no question about all 4 venturies being open. Can anyone help?

Rich

 

With the cold air intake system used on the late-model C3s, you don't hear much of an induction roar. The secondaries are likely working, but I'm sorry to say a stock '81 isn't even as quick as my V6 Mazda Tribute

Stick around, and learn about some mods you can do on the '81...smog legal or not (depending on where you live).

 

First thing is go check that the choke linkage is properly adjusted and that the blocker lever (it's on the choke side and is a little lever near the secondary that has a notch that the secondary pin on the shaft fits into) is not stopping your secondary from opening. You can easily check this with the choke off by opening the air valve, looking down into the secondary and pushing the throttle lever wide open. You should see the secondary open when you do this. If that's not the problem, then there is a screw under the lever on the top cover that you can adjust. This controls the tension on the air valve.

 

The Q-Jet is a mechanical secondary carb, so there is nothing to prevent the secondaries from opening unless there is an actual mechanical blockage of the linkage. You can verify that the secondaries are working by simply doing a manual actuation of the linkage: With the engine hot and off, locate and observe the secondary throttle shaft on the passenger side of the carb (it has a steel pin going through it). While observing the shaft with the pin, simply push your throttle arm lever to the wide open position. The secondary throttle shaft should rotate. If it does, your secondaries are working. If not, or if you want to check things out further, there are 3 things to check for:

1. With the engine at operating temperature, check and verify that the secondary lockout lever is fully retracted. This lever prevents the secondary throttle from opening when the engine is cold, and normally retracts when the choke is wide open. The lever is located right at the secondary throttle shaft on the passenger side of the carb: It interfaces with a steel pin that goes throuhg the shaft. When the engine is hot, the lever should not touch the pin. If it is engaged with the pin when the engine is hot, you have a choke adjustment problem.

2. With engine off, have someone sit in the car and push the gas pedal to the floor while you observe the throttle linkage on the driver's side. When the pedal is to the floor, grab the linkage and see if it is wide open or if it has more travel than the gas pedal allows. If you're short on pedal travel, bend the throttle cable attach bracket back towards the firewall to get more travel to allow full secondary opening.

3. Stick your finger onto the airvalves (with the engine off) and verify that the airvalves are not stuck due to distortion of the casting at the rear edge of the airvalve. The airvalves should operate smoothly with no catching or stickiness. If they stick, loosen the 4 attach screws 1/2 turn, slide the airvalves as far forward as they will go, and snug the screws back up.

If these item check out, your secondaries are working. There is nothing that can prevent them from operating.

 

Easy to determine. Next time you try it out, and it doesn't seem to work, check your choke to be sure it is fully open. If it isn't, re-set your choke to allow the choke plate to fully open.

Then verify this, by holding the secondary air valve open, while fully opening the throttle, (wth the engine stopped, of course) to see if the secondary throttle plates open. If they still don't then you have a linkage problem with the lockout tang. The lockout tang is located on the choke mechanism, and contacts the lockout pin on the end of the secondary throttle plate shaft. When the choke is not fully open, the tang keeps the pin from permitting the secondaries to open.

If none of the above works, you then need to move up the carburetor to the air valve adjustment. To do a quick and dirty test of that system, tap the butterfly open to see if it moves freely with not alot of resistance. If it doesn't move easily, you'll need to find out why. That might mean adjusting the spring load on the shaft.

Try all of the other stuff first and report back.

 

Thanks for the information everyone! I will check it out this evening and let you know. Is this a great forum or what!!
PS: I doubt it makes any difference with this problem, but I did convert to the true duals from Mid-America. Other than that, the car is totally stock. I would like to get a little extra grunt out of it, but want to keep it fairly original.

 

Check this thread

http://forums.corvetteforum.com/show....php?t=1027084

 

All,

Thanks for the advice and help. I had a chance to check it this last weekend, and it appears they are in fact working fine. I guess I was expecting too much from 190HP. Still, I love the car, looks great and drives nice. I did end up replacing the upper radiator hose while I was under the hood. It was seeping around the thermostat housing and may well have been the original. (81K miles) I also added a new 180 thermostat while in there, though I think the old one was fine. The other hoses were all good and had been replaced fairly recently by a PO. I was wondering how much I might gain if I replaced the carb with a newer one and dumped the computer controls? Would I need to completely disconnet the computer and would this cause me any problems?

Thanks in advance!

 

The stock Q-Jet carb is 750 cfm, so the carb is not the limiting factor in your performance. Dumping the carb and replacing it will gain you little to nothing. If you go into the engine and do other mods to warrant a carb change, you can then swap the carb to take advantage of other modifications. But by itself, a carb change will have little impact other than lowering your gas mileage.

 

They say that changing the belt driven clutch fan for an electric one will gain you 10hp or so. Its a fairly simple non invasive mod to do.