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Topic: Q-Jet Cough

in Forum: C3 Fuel, Emission Control, and Exhaust Systems

Q-Jet Cough

Posted: 9/29/08 6:45pm Message 1 of 24
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Dutchess County, NY - USA
Joined: 8/27/2007
Posts: 2484
Vette(s): White '79 Corvette. It's a driver.
My Corvette is a '79 L-48 with the TH-350 trans. It has the original Q-Jet carb that was rebuilt last winter.
 
Recently I have noticed that it hesitates for just a second when I first hit the gas.
This only happens when the car is at a complete stop and in gear.
There are no other issues with this carb. Otherwise it runs great.
 
I'm not a good mechanic and I don't want to screw it up.
Any suggestions and descriptions of where to start are appreciated.
 
Kevin 
 



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Q-Jet Cough

Posted: 9/29/08 7:17pm Message 2 of 24
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Duncanville, TX - USA
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Vette(s): #1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas
Sounds like the accelerator pump is takin a doo-doo, or....you may have a vacuum leak.
The accelerator pump may also just need to be adjusted a little.

Take a look at Ken's carb article for a really good course on Quadrapuke carb building/adjusting. Click on the link, then "Ken's Projects"(on the left side of the page), then "Quadrajet"(top of page)....


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Q-Jet Cough

Posted: 9/29/08 7:49pm Message 3 of 24
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Dutchess County, NY - USA
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Posts: 2484
Vette(s): White '79 Corvette. It's a driver.

Good info there!

I may have a few questions but I'll wait til the hood is up tomorrow.

Thanks.




Q-Jet Cough

Posted: 9/30/08 8:37am Message 4 of 24
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Mounds View, MN - USA
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Vette(s): 70 LT1 coupe, 69 350 HP coupe, 69 390HP 427 coupe, 71 LS5 convert, 85 coupe, 93 coupe
That is all good advice.  You might want to check the ignition tune-up first, so you know you are starting from a solid base.  Just look at the wires, cap and rotor, and check the timing, since you have HEI.

Larry



Q-Jet Cough

Posted: 9/30/08 11:33am Message 5 of 24
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Dutchess County, NY - USA
Joined: 8/27/2007
Posts: 2484
Vette(s): White '79 Corvette. It's a driver.
Well,
I might be on to something here.
(Due to poor health my Corvette didn't get much use this summer except for the occasional short ride close to home.)
This morning I filled up the gas tank all the way. It had sat with an eighth of a tank. Then I took it for a 20 minute ride around town and noticed a small but steady improvement from a dead stop.
My son and I took it for an hour ride of mostly highway driving after lunch.
Now it seems fine with no issues from the carb.
Bad, stale gas from sitting?
 
I'll keep my fingers crossed.
 
P.S.   Plugs, wires & dist. were installed new in the springtime.
KDADDY792008-09-30 11:36:40


Q-Jet Cough

Posted: 10/7/08 6:43pm Message 6 of 24
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Dutchess County, NY - USA
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Posts: 2484
Vette(s): White '79 Corvette. It's a driver.
"Bad, stale gas from sitting?"
I wish it were that simple but it still seems to "cough" a bit when taking off from a dead stop. But only sometimes. I noticed when I start off easy the "cough" seems worse. If I get right on it hard it seems to come to life with no hesitation.
 
I noticed that the rod that travels up to connect with the accelarator pump seems to have worn the hole where it connects. Its not a tight fit and could be causing a delayed reaction for the accelarator pump. 
 
Does this make sense?
 



Q-Jet Cough

Posted: 10/7/08 9:03pm Message 7 of 24
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CUYAHOGA FALLS, OH - USA
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Vette(s): 1975 C3 Red, T-Tops, Black Interior. All I need is time and money! Getting there!
It does, but that is fairly common and not necessary a problem.  Depends on how bad it is.   And yes, old stale gas can also cause the problem.  Check the basic ignition system and vacuum leaks, then perhaps put a good carb cleaner like Justice Brothers or Techron and run it a bit more.  Might cure the problem without any headache.  Other wise check the accellerator pump and adjustments.  Dont overlook the EGR valve.  If it is poening too soon, it can cause the same problem.  Opening too soon is cured by replacing the valve.  IF you disconnect and plug the vacuum line to the EGR and the problem goes away, that's your problem.


Q-Jet Cough

Posted: 10/8/08 6:31pm Message 8 of 24
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Vette(s): White '79 Corvette. It's a driver.

Kyster,

Thanks for the reply!
Today I contacted the fella who rebuilt it last winter. He's going to send me the new part to the pump where the rod connects and the hole is worn.
Addtionally, I did away with the EGR when I went with the Edelbrock intake manifold.
I'll refer to the carb. cleaners if this don't fix it.
 
Kevin
 



Q-Jet Cough

Posted: 10/8/08 8:05pm Message 9 of 24
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Vette(s): 1972, 1980
Before you change parts...try to adjust the idle mix screws at the base of the carb, you may find that the carb is drooling a bit, check your plugs and see if they are black, white, or brownish. Setting the idle mix can be tricky, turning the screws clockwise will take your plugs from black to brown then white...you want the plus to be brown...if they are white then it is to lean you will need to turn the screws counter clockwise...a little each time and make sure that both screws are set about the same turns. For more info...you should first start with turning the screws inward until they lightly bottom out. When you are doing this make sure that you count the amount that you turn each screw so that you can return them to their orginal spot. This will let you know if they where set the same before you start. The base set of the carb orginally is three turns out...you can start your fine tuning from there. Each time you turn the screws a little you can take it out for a run and then check the plugs again for colour. If you have a vacumm guage you can T hook this up to the vacumm port on the back of the intake manifold, while adjusting one screw at a time reach the highest vacumm reading, but I would still check the plugs for colour...you don't want the plugs to be white...this can cause possible cause damage to the motor down the road.
Hope this helps
John



Q-Jet Cough

Posted: 10/9/08 5:51pm Message 10 of 24
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Dutchess County, NY - USA
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Posts: 2484
Vette(s): White '79 Corvette. It's a driver.
John,
That's a great description of the "how to's" on the Q-Jet!Thumbs%20Up
Thanks for taking the time to spell it all out!
I'll be back with some results when I take a day off of work.
Kevin  



in Forum: C3 Fuel, Emission Control, and Exhaust Systems


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