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Accessing & Adjustment, Q-Jet Idle Mixture

fine69

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Dec 12, 2004
Messages
975
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Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
Hi Folks,

Can I please get some advice on adjusting the idle mixture on a Q-Jet? ;help

Keep in mind, I installed the Q-Jet last March, but have never tuned it properly - or even driven with it installed.

As I understand it, the idle mixture screws are located in the front base of the carb, and that you're supposed to adjust by 1/4 turns.

It may be just me, but I'm having a heck of a time accessing and adjusting these screws. Is there a special screwdriver or something? I spent a considerable amount of time trying to adjust yesterday. It was really difficult to see and turn the screws accurately, and I injured my hand by accidentally touching the fan! Talk about feeling foolish.

I was in the midst of tuning and mapping the adavance cruve per John Z's article. So now, I have a cut hand (yes, it could have been much worse), and the adjustment is totally off... the car would hardly turn off! It just kept dieseling. It finally coughed and died. :cry

Ralph
 
B

bmotojoe

Guest
Here is how I adjust my q-jet. I first set my timing I run it at 11 DBTDC. Set the idle to 700 rpm after a good warm up. I pull my modulator valve vacuum line from the tee on my intake manifold. I hook up a vacuum gauge there and hookup my timing light and note both rpm of timing light and the vacuum gauge reading; mine runs a smooth 23 inches of vacuum.
I guess it doesn’t matter where you start, left side or right but I start on the right side mixture screw. I adjust the right side mixture screw so I get maximum rpm and highest gauge reading. Then I readjust the rpm back to 700 and move to the left side and do the same. After adjusting left side max rpm and max vacuum I reset my rpm to 700. I then go back to the right side and do it again, then left side. It will even out and maybe if your lucky on the first or second time through but I have played with this more then 3 trips through on other cars before, so just take your time.
It sounds like you have recessed mixture screws and there is a special tool for them. The one I use is made by K-D tools and the number on the tool is 2776 and it is 7” long and the center between knurled knob and the tool end is made out of a spring so it is flexible.
I put the orange electrical tape on the knurled end so I could put a black sharpie mark on it I use as a reference so I can count my turns if I have to.

Brian
 

fine69

Well-known member
Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
"... It sounds like you have recessed mixture screws and there is a special tool for them. The one I use is made by K-D tools and the number on the tool is 2776 and it is 7” long and the center between knurled knob and the tool end is made out of a spring so it is flexible.
I put the orange electrical tape on the knurled end so I could put a black sharpie mark on it I use as a reference so I can count my turns if I have to...."

Brian,

This is incredibly helpful information. Yes, I have the recessed mixture screws. I'm going to order the GM Carburetor Adjusting Tool right away. Also, excellent idea on using tape and a Sharpie mark as a reference point.

I removed one of the plugs and it is completely soot covered, probably fouled. So it looks like I'm running too rich.

Probably would be a good idea to get new plugs, points, etc. as well.

Thank you.

Ralph
 
B

bmotojoe

Guest
These mixture adjusting tools are quite common and can be found at any major auto parts house in there tool section. The K-D tool I have is made in the USA like all of the tools I have bought over the years, it could save you a few bucks compared to dealer price for the same tool.
I have run only AC delco plugs over the years and found them as good as any other of the more expensive brands. By the way you’re describing your plugs I am sure one or more are fouled out. Like I said earlier, just take your time and you can really dial in the q-jet using the above method.
I hope your injuries heal quickly!
Brian
P.S. When your really close and your fine tuning the mixture screws only turn them a maximum of 1/8 of a turn at a time. Let the engine even out by letting it run for 20 or 30 seconds in between adjustments. Like I said before, just take your time.
 

Vette79

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Jan 23, 2007
Messages
1,392
Location
Millersville, MD
Corvette
1979 L-48 Black Coupe
Brian, what are the symptoms of needing to adjust idle mixture screws? I don't want to adjust unless I need to.
 
B

bmotojoe

Guest
IMO, the stock or the replacement edelbrock 1903 q-jet carburetors are way too large for a stock 180hp engine so they need to be tuned correctly for the best performance and fuel economy. My original date coded carburetor is boxed up a long time ago and I am currently running the 1903. My original EPA fuel window sticker states I should have a City/Highway combined 15 miles per gallon average, which of course at .65 cents average per gallon back then you probably didn’t think about it like you would at today’s prices. I try to maintain as close as I can to the 15 miles per gallon or better average by keeping the engine as tuned as I can doing it myself, this includes ignition tuning and carburetor tuning.
It never hurts to hook up a vacuum gauge and have a timing light that displays the RPM and just see where you’re at. There are some people out there that are really good at carburetor tuning doing it just by ear, but I have not had to good of success doing it that way, (Too many hours in front of a 100 watt Marshall stack and my Les Paul,,:))
Brian
 

fine69

Well-known member
Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
Update

These mixture adjusting tools are quite common and can be found at any major auto parts house in there tool section. The K-D tool I have is made in the USA like all of the tools I have bought over the years, it could save you a few bucks compared to dealer price for the same tool...

I ordered the tool last week. Hopefully, it will be here this week.

As I wrote earlier my car was running terrible (I was going to change my handle to "POS69"!) -- so I replaced the following:
  • Spark plug wires (applied a small dab of dielectric grease on the connectors), and routed them carefully to avoid any contact with the headers - I also loomed them;
  • Spark plugs with AC R44T, gapped 'em at .35; and,
  • Points and condenser;
Since the plugs were a black sooty mess, I figured I was running rich. As a base setting for the Q-Jet, I just closed the mixture screws all the way, and then opened each at 1 1/2 turns. After setting the dwell to 30 degrees, I was surprised to see find that everything kinda fell into place - the timimg was right at 8 degrees BTDC @ 750 RPMs, and the spitting and sputtering stopped. I did not hook up the vacuum gauge, but the vacuum it's good enough to open and close the headlights! (first time since I rebuit the engine!).

While this is not the full "JohnZ" tuneup, this enabled me to get on the road while I await for the carburetor tool to be delivered.

Ralph
 

LT4man

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Joined
Jun 25, 2003
Messages
2,214
Location
Chicago
Corvette
96 Collectors Edition LT4
The following procedure works with any type and kind and size of carburetor.

I have used this procedure for the past thirty-seven (37) years.

This is how I adjust carburetors:

With the engine off, turn each idle mixture screw in till it seats lightly. Then turn out two (2) full turns.

Hook up a vacuum gauge to manifold vacuum.

Start engine.

While looking at the vacuum gauge, start turning idle mixture screw in slowly until you see the vacuum start to drop. Turn screw out slowly until vacuum reaches highest point. Turn screw out one (1) full turn from that point.

Move to other screw. Do same procedure.

With a stock camshaft you should have between 17 and 21 inches of vacuum when you are finished.

Set idle speed to specification.

SAVE THE :w




 

wishuwerehere82

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Joined
Sep 6, 2003
Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
Another instrument you can use if you don't have a vaccuum gauge is a handheld dwell/tach. Instead of going for the highest vaccuum reading, turn the idle screws alternately until you get the highest RPM reading on the tach.
Both vaccuum and RPM are an indication of your combustion efficiency at idle. And you can see the change as you tweek the fuel/air mixture with the idle mixture screws with either vaccuum gauge or tach.
If you notice a lag coming off idle when you test drive it, you can open up the mixture just a touch richer to remove the lag.
 

wishuwerehere82

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Joined
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Messages
2,314
Location
Rochester, NY
Corvette
Red '82 Coupe,Sebring Silver '98 Coupe
Brian, what are the symptoms of needing to adjust idle mixture screws? I don't want to adjust unless I need to.

The most common one is a rough idle.(balance not equal between the two sides)
Others are:
Excessive smoke out the exhaust, or sooty plugs. (Too rich)
Pinging on acceleration from idle. ( Too lean)
Lag, or jump in power from idle to open throttle. (Too lean)
Stalling at idle. (Too rich or too lean)
 
B

bmotojoe

Guest
If replacing your original Carburetor with the Edelbrock Q-Jet

Respectfully, if you own a C3 a vacuum gauge is one of the must have tools to have in your tool box.
Here is a tip I learned when I replaced my original Q-Jet with the Edelbrock 1903. On the original Q-Jet the EGR vacuum port was machined higher in the carburetors venturi then the Edelbrock. This may have an effect on the off-idle performance resulting in a flat spot. If using the Edelbrock Q-Jet you may have to install a vacuum delay valve between the carburetor and the EGR valve. A simple test will let you know if a delay valve is needed. Temporarily remove and plug the vacuum line to the EGR valve. If the flat spot disappears you will need to install the vacuum delay valve between the carburetor and EGR valve (GM # 14020691). This valve will delay the opening of the EGR valve long enough for the carburetor to transition through the off-idle phase without a stumble.
Brian
 

fine69

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Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
Respectfully, if you own a C3 a vacuum gauge is one of the must have tools to have in your tool box....

Yep, I agree. The vacuum gauge, dwell/tach meter, and advance timing light were some of the very first items I purchased after getting the old girl.

Not sure what prompted the vacuum gauge alternatives, I didn't hook it up yesterday, but I generally pull in approx 12 in Hg (varies bewteen 10-14).

I picked up a NAPA #VC-1810 advance can that I plan to install on the original tach drive distributor (currently in pieces...). Hopefully I'll be able to get a better engine response by getting the advance to fully deploy at a lower vacuum.
 

fine69

Well-known member
Joined
Dec 12, 2004
Messages
975
Location
Maryland / D.C.
Corvette
'69 Convertible Vette; '72 Z28 Camaro Rally Sport
Respectfully, if you own a C3 a vacuum gauge is one of the must have tools to have in your tool box.
Here is a tip I learned when I replaced my original Q-Jet with the Edelbrock 1903. On the original Q-Jet the EGR vacuum port was machined higher in the carburetors venturi then the Edelbrock. This may have an effect on the off-idle performance resulting in a flat spot. If using the Edelbrock Q-Jet you may have to install a vacuum delay valve between the carburetor and the EGR valve. A simple test will let you know if a delay valve is needed. Temporarily remove and plug the vacuum line to the EGR valve. If the flat spot disappears you will need to install the vacuum delay valve between the carburetor and EGR valve (GM # 14020691). This valve will delay the opening of the EGR valve long enough for the carburetor to transition through the off-idle phase without a stumble.
Brian

Brian,

I believe I am experiencing the "flat spots" you referred to in your post. What's an EGR? Is is something that's on all Q-Jets?

I spoke too soon... I do need to map my advance and do the total timing. It's not running very well.

Ralph
 

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