Hi, I am new to the forum, and just bought a really nice 81 Vette a couple of months ago. Runs good, all gauges read good, runs cool with great oil pressure. My problem is that I don't think the secondaries on the carb are opening. I can cruise fine, but when I kick it down, I get a lot of noise (fan?) but no 4bbl induction sound. The linkages don't seem to bind at all. Is the operation for the secondaries vacuum, or does the computer control it? Love the car, it is a blast to drive, but I would like to get this fixed. Any ideas are appreciated.

 

I could be wrong here (it happens often), but here I go anyway. The only thing the computer adjusts is idle mixture. The secondaries are mechanical, ie, the throttle plates open mechanically. There are also air valves on the top of the secondaries, which open according to the amount of airflow required.

There is a secondary lockout lever going from the choke mechanism to the secondary throttle rod on the passenger side. Make sure it's not preventing the secondaries from opening when the engine is warm.

Also, consider buying a copy of the book Rochester Carburetors by Doug Roe. It covers all the QJets, including the computer controlled models like yours. You can find it on Amazon.

 

Qjets, if not adjusted right will not open the secondaries. Try it manually with the breather off and see if they are dumping. Or, hold the throttle open and see if you can open the back barrels (use a small screw driver in the throttle shaft slot). If they won't open there is a small "tang" that needs adjusting/filing (look on web for Qjet adjustments) or you may have other problems. If Holley carb - disregard.

 

HI,

 

I to have an 81' also. I was having some problems with my carb also.The computer also controls the distributor advance. The September 2004 issue of VETTE MAGAZINE in the Q&A column had a great fix. Replaces the Qjet with an Edelbrock 1904 and a Mallory distributor. Cost me about $650.00. the carb bolts rightup and uses all the factory vacumn line and pipeing. this setup eliminates the computer altogeather. since I live in a non inspection state I was able to get rid of all the smog junk. The vette runs like a neww car now. I wish I had replaced the intake with an Edelbrock Performer also, will be doing that this winter.

 

Page (Hi) pretty much nailed it except that the computer controls the primary's mixture. The secondaries may have the AV spring tension set too high? You CAN get an '81 to run well with the stock carb, especially if it's stock or mildly tuned (mine runs great with the computer still in place). To check if the secondaries are opening the engine has to be under load, so you can't do it while booting the throttle open while looking under the hood. I can't remember the name of the part/tang, but manually open the air valves & on the side of the carb you'll see a little tang that moves with the AV's & contacts a stop (it's been ages since I've done this, so the description is probably very vague, but have a fiddle & you'll see what I mean). Once you've found it, stick some bluetak/plasicene/tippex/etc on it & go for a drive. Let it warm up & then floor the throttle. Come to a halt & look at the tang. There should be a mark in the plasticene/whatever from the air valves opening. If there is a mark then they are opening & you've just discovered the smogability of an '81!!! That's improved by sticking true duals on If there isn't a mark then they're not opening & you need to find out why. Doug Roe's book is good, but rather than shelling out on a new carb & dist, spend out on the shop manual for an '81. It's not cheap, but there's very good sections in it on both the carb & the computer system, which are worth the cost of the manual on their own.
I never noticed any extra induction roar on mine, even though the secondaries are opening. Maybe the stock air filter housing does a good job of silencing it (noise regulations)? Or maybe it hasn't got any chance of being heard over the din coming out the back?
Good luck.

edit: typed the above in a hurry (it's not very good!). Best bet is to check out the archives. The '81 secondaries are just like earlier Q-jets from the point of view of what you're looking at & there's been a few posts on secondaries not opening in the past. They generally get better replies than what I typed above. If you can't find anything, just create a new post asking how to check for the secondaries opening, but don't mention '81 (a lot of people skip posts regarding '81's & their "complicated" computer control, even though they probably know the answer & probably frequently sort out problems with their PC's - which are far more complicated than the basic computer in the '81)!!!!

 

Paul makes a good point about the induction roar You can't really hear the secondaries opening with the stock air cleaner system. It was easy to hear on the mid-70's Vettes because of the cowl induction hood, which by all reports makes a goodly amount of noise.

By the late-70's, however, the system was changed to a cold air intake over the radiator...which pretty well muffles any secondary "whoosh" you might have otherwise heard.

 

I had the same problem with the tang. I adjusted it and under heavy acceleration I can hear the secondaries opening. I have the computer with a super chip in it and true duals with no cats. It runs great and fuel mileage is good. After spending some time studying the basic computer system and asking questions here on the forum, I'll keep the computer system.