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Carb Adjustment

Vette79

Vette79

Well-known member
Joined
Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
Quadrajet Carb was just rebuilt on a 79. Installed headers and exhaust. Prior to it being rebuilt the choke seemed to of operated fine. After starting the car, it would eventually run up to 2 on the tach and stay at 2 to warm up. Push on the gas pedal and then it would come down at a lower RPM. Now, I continually have to feed it fuel for a few minutes for the car to stay running and then it eventually takes over. The RPM's never do increase like before and it stays at 1 on the tach. Correct me if I am wrong, but all I have to do is adjust the choke to resolve this right? Or is it an internal carb problem? Need some direction...Thanks...Jeff.
 
I've seen this article before. Very informative. I spoke with the mechanic who did the rebuild and install for me and now know why I have the problem. My choke has been disengaged due to a intake manifold issue. Needs to be cleaned or replaced for the choke to function properly. Damn carbon buildup! Might be a good oportunity to remove the emissions and replace the intake manifold with an edlebrock performer.
 
I don't understand your mechanics explanation. The choke worked before he touched it, he rebuilt the carb and now it can't work because of an intake manifold is dirty with carbon. That doesn't make sense to me.

If you rebuilt the carb what good would it be if your intake manifold is clogged with carbon. Shouldn't you clean that up before you put the recently rebuilt carb back on? If your intake manifold is clogged with carbon what good would disconnecting the choke do? This is very confusing. Anyone with any ideas?
 
The way it was explained to me was the Carb's Choke was never working properly to begin with when I brought it in. They had already rebuilt the Carb and installed it and then found out that the port on the intake manifold that is used to heat the tube that supplies warm air to the Choke is built up with carbon hence fine tuning the carb but disengaging the choke since it won't work properly anyway. This port apparently does not supply enough warm air to work the choke correctly. It is an exhaust port that heats up this tube which in turn is used to provide warm air to the choke to function. They are extremely confident that this is the problem and tell me that if they are wrong then the work is free. Does this make any sense?
 
It does make sense, it just seems odd that it started to happen after it was rebuilt, and seemed to work properly before it went to the shop.

You could try disconnecting the tubes and running some baling wire and compressed air down the tubes. Or have the shop do it.

red70Vette-they probably disconnected the choke because it would never come off high idle as it wouldn't get warm.

outwest-I don't know if the Carter has a hot air tube going to the choke coil to assist with warming it up. If it does, it probably is a possibility.

Rick
:gap
 
RodsnRides, if you clean the intake that way won't the chuncks of carbon fall down into places like the cylinder, etc.? Not sure if the carbon is easy to burn or create further problems. Either way I guess it didn't make sense with my initial post. Think I may attempt to remove and clean the intake myself. Step ? on my list of wants with the Vette.
 
No, they won't fall into the cylinder. Those tubes meet with the heat crossover passage under the base of the intake and to the exhaust ports. The carbon will get blown into the exhaust manifolds and out the exhaust upon fire up.

But, going to your other post-you could always yank the junk and put on a Performer. Piece o' cake, although you need to know your emissions laws. You could pass visual and install the Performer EGR and "BB" all the vacuum hoses for a "cheater" look. Did I say that?:ugh :L

Rick
:gap
 
Hey Rick. Thanks. Good to know. I was thinking that when I get to this probably next winter I will replace the intake manifold and also remove the rest of the emissions. Fortunately, I can do that here. Besides I actually think I will gain some HP with the new intake and removal of the emissions....Jeff.
 
From what you've said in your first post- installed headers- it could be that the incorrect operation of the choke is linked to this and not the carb rebuild.

The stock system has a heat riser valve built into the passenger side exhaust to assist with cold engine operation. This is the widget that forces hot exhaust air up to the intake manifold to help warm the carb and bring it off the choke. When you install headers, this piece gets tossed out.

Maybe your mechanic disabled the choke knowing that this would happen?
 
Keep your Q-Jet!

A properly set up Q-Jet is the very best street carb you can have on your plastic hot-rod. GM had it engineered for economy and street performance by a group of high-dollar and talented engineers. At cruising speed the air-fuel mix is very efficiently atomized by the presure drop and airspeed increase thru the small primaries, then at WOT the gigantic secondaries open up for TONS of flow. The standard Q-Jets are 750 CFM, and opened up to 800 CFM on the L-82s (and 500 CI Caddies). You can differentiate them by sliding a 3/16" rod between the primary air horn and the bore, if it fits, it's an 800 CFM carb.

One of the main flaws in these dinosaurs is that there are no bushings in the bare aluminum body at the throttle plate rods. If there is any slop between the rods and the worn body bores, it needs to get some bushings on a proper rebuild. I bit the bullet and had mine rebuilt by Jim McGowan at the Quad Shop in Rockford, IL. The guy taught me more than I'll ever need to know aout Q-Jets. He ain't cheap and mine took forever to get back, but it was perfect when I did get it back. He even installed an electric choke on it knowing I was running headers.

Dave
 
VetteHead Mikey I wish that were true. My mechanic's primary goal with the carb was to dial it in without the choke. Once the intake is cleaned the choke will be fully functional. He tells me the choke does work but not properly with the dirty intake. My carbs number is 17058477. Can't really find much information on it. Can anyone help?
 
Vette79 said:
VetteHead Mikey I wish that were true. My mechanic's primary goal with the carb was to dial it in without the choke. Once the intake is cleaned the choke will be fully functional. He tells me the choke does work but not properly with the dirty intake. My carbs number is 17058477. Can't really find much information on it. Can anyone help?
Click to expand...

Jeff,

I believe I was able to decode your carb, except for the emissions code. Check it out....

17058477

170= 1976 or later

58 = 1978

4 = Denotes emissions code. Supposed to be 2 or 5 ? (2 is supposed to denote 49-state emissions, 5 is supposed to denote California & high altitude geographical locations.)

7 = Pontiac

7 = Usually odd number denotes manual tranny, but Rochester supposedly did not always follow this rule.​

I'm sure someone could refine findings with more details.

Ralph
 
Ralph,

Good to hear from you. Thanks for the info. It was a big help. Doesn't appear that this is the correct carb for my Vette. Well, it does run well except for the choke. Hope someone else can chime it in with more detail...
 
Vette79 said:
Ralph,

"...Doesn't appear that this is the correct carb for my Vette. Well, it does run well except for the choke..."
Click to expand...

It should be fine.

From what I understand is that it doesn't matter if the numbers are for a Chevy, Cadillac, Buick, Oldsmobile, or Pontiac - the carb can likely be identical in every way, down to the metering rods and jets.

I personally don't really care about numbers since it's not my goal to restore my car back to original condition.

In fact, the numbers for my "new" Quadra-jet are 17080213, which decodes to: 1980, non-California emissions, Chevy, manual transmission. Who cares? I just wanted a Quadra-jet with an electric choke as I was not very happy with the Edelbrock carburetor.
:lou
 
Good Point, Ralph. The Vette does run well. Very pleased. Next time you are down this way give me a yell...
 
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