Took the 81 in to the local Vette specialist for a flat spot in heavy acceleration...he said the secondary butterflies aren't opening, therefore it's too rich. Also, the drivers side is too lean...gotta be a bad rebuilt carb...
Any feedback ? thanks :banghead:

 

what kind of carb ?

 

It's a rebuilt q-jet from Ecklers...computer controlled, electric choke...They are very good and spent a lot of time te$ting it, so I believe what they say...
Now I am wondering whether to send it in for therapy or buy new...

 

I know I'll get a lot of flack from the Q-Jet contingent on the forum but I really think they suck (JMHO). The original that was on my car when I got it leaked more fuel than it metered. I bought a brand new Q-Jet from GM when they were still making them. It's been OK and I stress the word OK. The chokes are finicky and they have more adjustments to get it right than a chinese tailor. The idle mixture screws are plus or minus 2 turns to make any difference. I've about had it with mine and I'm thinking of going with a new Edelbrock or Demon when I change the intake manifold. I know about all the articles that say the Q-Jet is one of the best street carbs when set up properly but setting them up is a pain and their longevity in regards to leaks in the wells is laughable. I'm leaning toward the Edelbrock only because of the leak issue. There's no fuel above the gaskets. The Holley and Demon both have gaskets which can leak fuel if they go bad. Like I said this is just my opinion but I had to get it out. I feel much better now. :rant:

 

Stevo -
I don't work on the ECM-controlled carbs, but just a couple of opinions on what you have been told...

First, the driver's side of the carb can't be any richer or leaner than the passenger side, since both sides are jetted the same, the metering rods are the same, and the two sides do not operate independent of each other (the metering rods are tied together on a common hanger). The driver's side of the carb feeds half the cylinders on the driver's side of the engine, and half the cylinders on the passenger side, so there is no way that a guy can tell if one half of the carb is running lean (and I've never heard of such a thing) unless he pulled the 2 plugs from either side of the engine that are fed by the driver's side of the carb and compared them to the 2 plugs on either side that are fed by the passenger side and actually noticed a significant difference. There are some talented racers that are able to obtain a very slight advantage when racing by "cross jetting" a carb to compensate for miniscule differences in the right and left mixtures, but this is not something that is realistic for a street car. I have a real hard time believing that one side of your carb is running lean unless you actually have a blockage of a fuel metering passage on one side of the carb (pretty rare).

The butterflies on a Q-Jet cannot "not open" unless the secondary lockout lever is improperly adusted and is locking the secs out. You can easily see this yourself by just looking at the lever and verifying that it is in the retracted position when the engine is warm (it should lock out the secondaries until the choke is wide open - this is normal). The secondary airvalve (the "butterfly" that you can actually see on top of the carb) can only be jammed if the airvalve is interfering with the airhorn casting at the rear edge of the airvalves. You can test for this by simply poking your finger on the airvalve when the engine is off. If the airvalve opens smoothly and does not "catch" on anything when you push it with your finger, the airvalve will operate correctly when the engine is running. You cannot test the airvalve operation by starting the engine in neutral and "flicking" the throttle - there is inadequate air mass requirement when the engine is in a no-load situation for the airvalve to open by flicking the throttel in neutral.
If the secondary lockout lever is preventing the secondary butterflies from opening, it will not cause a rich condition.
And a flat spot on heavy acceleration is usually an improperly adjusted secondary airvalve spring...


[Modified by lars, 7:29 PM 2/8/2003]

 

Once again, thanks ! That is exactly what I had hoped for.
Now if some computer whiz can tell me how to copy just Lar's post so I can take it to the shop?

 

Just drag through the text in the post, hit "copy" (Ctrl+C), open a word document, and hit "paste" (Ctrl+V).

 

No, no... you simply need to ctrl+c a Lars built qjet (if he would do an E4ME) and ctrl+v directly onto your manifold and life would be good... :lol: :cheers:

Problem with going to 'other' carbs is that us '81 folks (& some '80) have the dreaded emissions inspection problem, so we've got to keep these things working. :smash:

 

It worked! I printed it and will talk to the shop again tomorrow. Thanks.
Where can I find the secondary airvalve spring? Visible on the outside?
My Haynes says that unless you have a degree in Q-jet, don't even LOOK at an E4ME.
I see the spring that is in the vacuum-operated dashpot assembly on the passenger side... hooks up to the sec butterflies... thanks in advance :banghead:

 

Stevo,
There is a way in which you can have your mixtures wrong on these E4ME carbs (I had it). The solenoid bounces the "dancing needle" up & down to control the mixture on the primaries. At the top of this "dancing needle" (solenoid plunger) is a type of wing which contacts the primary needles. On mine the wing was badly bent so that one needle was being held at a completely different posiion to the other. It should be OK on a rebuilt carb.... but you never know :(
Here's a bit of more info on the AV that Lars mentioned (it's part of a document on mods to computer controlled Q-jets that I found on the Net):

The AV is the flap that covers the secondaries of your carb. It opens up when airflow through the secondaries demands it. Decreasing tension on it has 2 main benefits. It allows the AV to open faster, giving you full power sooner. It also allows the AV to open more easily with airflow, giving you the possibility to make more total power. It is held shut by 2 things.
First by the choke pull-off diaphragm (which is on the front pass. side of the carb.) When the engine is running , the choke pull off keeps the AV closed by pulling the linkage via the rod that connects them. When you go to/near WOT, the choke pull-off pulls out(due to the loss of manifold vacuum) and allows the AV to open. The pull-off generally releases slowly, slow enough that even with no AV spring tension you shouldn't get a bog. The rate at which it pulls out is not adjustable with the metal can pull offs our carbs came with.
The AV is also held shut by spring tension, and that is adjustable. To adjust it, you will need a small allen key (either 3/32 or 5/32, i forget) and a small flat screwdriver. (**note: it is possible that instead of an allen key you will need a small torx bit, but they are rare.) Now, here you go:
1. Look at the pass. side of the carb (with the air cleaner off)
2. Observe the linkage on the side of the AV. Through the slot you can see a small flat screw. That's the one you need to adjust.
3. To adjust it you need to loosen the setscrew, which is on the underside of the lip of the airhorn. The setscrew is where the allen/torx bit comes into play. Loosen it only enough that you can turn the adjusting screw.
4. Turn the adjusting screw counter-clockwise until the AV flops open on its own. Now slowly turn it clockwise until the AV just shuts. That is 0 tension. If you were setting it to factory specs you would then proceed to turn it down the specified turns (usually 1/2-1 turn). Instead, tighten the setscrew and go drive.(**note: You may have to hold the AV rod to the pull-off out of the way while you adjust the tension. You can also remove the pull-off, but that is really excessive. If you exercise some common sense, it isn't hard to figure out how to hold everything.)
5. With your car fully warmed up, come to a stop and then nail it. Did it bog? If it didn't, consider yourself done. If it did, then there are a few ways to proceed. First ask yourself if you have a fresh tune-up. If not, what are you doing making performance mods when your basics aren't even done. Shame on you. Also, be sure your choke pull-off is working properly. If it is bad, it could cause you problems too. Second, now is a good time to read the next section on changing secondary metering. Finally, if you don't want to change secondary metering (and why not?), you can increase the AV tension until the bog goes away. Go in 1/8 turn increments at a time, and stop when the bog goes away. Do not increase the tension to over 1 turn down or you will permanently distort the spring.

Couple of extra points worth mentioning which are, when you loosen off the lockscrew make sure that you are holding the adjusting screw in position with a flat blade screwdriver. If you don't it will just pop out from it's adjusted position, so you'll have no way of knowing what it was set to. The 2nd point is that the stock setting for mine is 7/8 turn in from zero tension (which is handy to know once you've discovered that the adjustment is lost when you slacken the lockscrew :D ).
Good luck
Paul
p.s. How did the mechanic determine that it's running rich on one side? A good indication of rich at idle is an irregular popping in the exhaust (which is what led me to find mine was wrong).

 

In addition to all the great advice from the others, there is one possibilty that hasn't come up, but does occur. On rare occassions, one or more of the pick-up tubes can come loose and fall out of the top, down into the carb. At that point, the carb no longer works properly on one half and can cause the problems you describe. Admittedly not as common as the other problems, but something to look for. Unlike some of the others, I have had very good luck with Q-jets and reccomend them if you are looking for decent fuel economy, especially the feed-back models. Most of the problems I run into have to do with bubba having a hand in the adjustment and/ or rebuild process.

Hans

 

>p.s. How did the mechanic determine that it's running rich on one side?
:iagree:

Determining its rich on the left side is easy - read the O2 sensor.
Determining its rich on the right side is harder...

Agree with the above, check secondary spring - mine was too tight too. When they
open, you'll know - duooouoouoo

Good luck, let us know.

 

So...as an experiment, I taped the secondary air valves open and went for a test drive. It used to have a sputtering flat spot upon heavy acceleration and now has a silent dead spot...hmmm But I did spot the spring adjusting screw, so will go there next... pray for me... :nopity