CARB ADJUST?

[mike82]

i recently installed true duals on my '78. since then the car hasn't run as good as usual. the motor hesitates under aggressive acceleration but not on WOT. since the exhaust is more free flowing and less back pressure, does the carb need to be readjusted? if so would only adjusting the mixture screws do the trick? thanks,,,,,mike.

 

[JohnZ]

The mixture screws only affect the engine at idle; from there up, it's on the main metering system and accelerator pump.

 

[SIXFOOTER]

Thats not entirely true, the Idle mix screws also feed into the off idle circuit.
Most likely you need to reset them, odds are it wasn't right to begin with. Timing has to be correct also.
What year, motor, mods are you running?

 

[mike82]

Thats not entirely true, the Idle mix screws also feed into the off idle circuit.
Most likely you need to reset them, odds are it wasn't right to begin with. Timing has to be correct also.
What year, motor, mods are you running?

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'78 L-82 auto
new wires, new plugs(AC-Delco rapid fire-gapped per spec), new cap&rotor(brass terminals), a hotter coil and the dual exhaust.

you mentioned correct timing,,,,, factory spec is 12 deg. i have it @14 deg.(thought i could get a little extra oomph)

as far as mixture screw setting,my Chilton's manual says to seat the screws and back them out so motor barely runs(2 1/4 turns),unplug vacuum advance hose and plug end of hose. then back out screws 1/8 of a turn 'til i reach highest rpm and reset idle speed(700rpm in drive). then lean in the srews 1/8 turn until i reach 750 rpm, reset idle speed (700rpm in drive). reconnect hoses.

 

[SIXFOOTER]

OK, whats the total timing and at what RPM?

 

[Vette79]

I thought you back out the idle mixture screws until you reach highest vacuum and smooth idle??? I am at four turns out and it runs very well. At just two turns out, my engine wants to stall. When I first did this several weeks ago I had a symptom of the Quad puking up fuel occassionally. I checked the idle fuel mixture screws and they were turned out almost eight turns. After my adjustment, no more problems

 

[bmotojoe]

Here is the way I have been setting the Idle Mixture Screws (IMS) on a Q-jet for ever using a vacuum gauge and my digital timing light with good results.

• Fully warm engine and ensure choke is fully open.
• Install air cleaner assembly.
• Set desired speed with the idle speed screw.
• Adjust the IMS on one side to get maximum possible rpm and highest manifold vacuum. Do not go rich beyond the maximum speed point.
• If the procedure above changed the idle speed more then 40 rpm, then readjust the idle speed.
• Adjust the opposite side of that in step 4 to get maximum rpm and vacuum.
• Reset idle speed.
• Carefully trim each IMS to again get maximum idle rpm and manifold vacuum.
• Go leaner just enough to get a 20 rpm drop in speed.
• Reset the idle speed screw to desired rpm.

This is a Lean-Best idle setup. Setting richer than this will not improve idle quality or performance, but could cause higher hydrocarbon emissions and tend to foul spark plugs.
Brian

 

[minifridge1138]

That is a good explanation of how to adjust the mix screws.

1) I always set my timing first (assuming the carb is set well enough to run).
2) Then get the engine warmed up.
3) I hook a vacuum gauge up to the line that goes to the vacuum advance (this keeps the vacuum advance from kicking in and plugs the line).
4) Set the primary and secondary mix screws to the same amount (4 turns out or whatever is approriate for your carb).
4) I start to adjust the mix screws for the primary side. As I do this the idle will lower a little and the vacuum gauge will show more and more vacuum. Eventually you will hit a peak vacuum and the number will start to go down. Richen until you hit the peak vacuum.
5) Keep track of how many turns (and in what direction) you adjusted the primary, and do the same to the secondaries.

-Good Luck!

 

[mike82]

it seems i have found the problem. i removed the vacuum hose to the EGR valve, went for a drive and didn't have the hesitation problem. can't find a new one so i'm gonna block it off.

 

[petes74ttop]

That is a good explanation of how to adjust the mix screws.

1) I always set my timing first (assuming the carb is set well enough to run).
2) Then get the engine warmed up.
3) I hook a vacuum gauge up to the line that goes to the vacuum advance (this keeps the vacuum advance from kicking in and plugs the line).
4) Set the primary and secondary mix screws to the same amount (4 turns out or whatever is approriate for your carb).
4) I start to adjust the mix screws for the primary side. As I do this the idle will lower a little and the vacuum gauge will show more and more vacuum. Eventually you will hit a peak vacuum and the number will start to go down. Richen until you hit the peak vacuum.
5) Keep track of how many turns (and in what direction) you adjusted the primary, and do the same to the secondaries.

-Good Luck!

That sounds like a good easy way to do it, I have an auto, should the car be in park or drive? Thanks, PG.

 

[Vette79]

Make sure the vehicle is warm and in park when performing this procedure.

 

[big2bird]

it seems i have found the problem. i removed the vacuum hose to the EGR valve, went for a drive and didn't have the hesitation problem. can't find a new one so i'm gonna block it off.

Rock Auto

 

[glen242]

That is a good explanation of how to adjust the mix screws.

1) I always set my timing first (assuming the carb is set well enough to run).
2) Then get the engine warmed up.
3) I hook a vacuum gauge up to the line that goes to the vacuum advance (this keeps the vacuum advance from kicking in and plugs the line).
4) Set the primary and secondary mix screws to the same amount (4 turns out or whatever is approriate for your carb).
4) I start to adjust the mix screws for the primary side. As I do this the idle will lower a little and the vacuum gauge will show more and more vacuum. Eventually you will hit a peak vacuum and the number will start to go down. Richen until you hit the peak vacuum.
5) Keep track of how many turns (and in what direction) you adjusted the primary, and do the same to the secondaries.

-Good Luck!

Do Q-Jets have secondary idle mixture screws? Mine sure doesn't.

If I missed them, let me know where they are.

 

[big2bird]

Do Q-Jets have secondary idle mixture screws?

Nope.

 

[Vette79]

I think minifridge meant drivers side (primary) and passenger side (secondary).

 

[minifridge1138]

That is what i meant.
Naturally, each carburetor different.
I suggest getting a book for your particular type (Rochester, Edelbrock, Holley, etc).

 

[mike82]

today is the first time since my last post to be able to use my computer(received a virus). i appreciate the advice and for being schooled on how to properly adjust my carb. i'm going to purchase a vacuum gauge but could use advice on a good one.....thanks,,,,mike

 

[big2bird]

today is the first time since my last post to be able to use my computer(received a virus). i appreciate the advice and for being schooled on how to properly adjust my carb. i'm going to purchase a vacuum gauge but could use advice on a good one.....thanks,,,,mike

Best. ........Snap-on, craftsman...........good.................Auto-zone, Kragen,etc.

 

[GerryLP]

A non-dampened vacuum gage is best, for you'll see the idle mixture changes you make almost instantly reflected on the gage. A dampened gage will take longer to stabilize, so it takes longer for your adjustments (i.e. you make an adjustments, and you have to wait, but also, the dampening gets you in the ballpark, but won't help you fine tune for small increments).

If you have one of those brake-bleeding pumps, then it would be most likely un-dampened. The gage will move rapidly between the peaks and the valleys of the vacuum signal.

 

[mike82]

i bought a vacuum gauge and a tach meter(i can understand why they so handy to have). after dialing in the carb or should i say ,what seems to be dialed in,here's what i got,,,,,,

highest vacuum; was 13,is that in the ballpark or does
that seem low?


4 turns out on the mixture screws gave me highest
vacuum and rpm, doesn't that seem too rich?

the worst part is i still can't get rid of the hesitation. the motor idles great,it doesn't hesitate at all while idling,only under acceleration(moderate to heavy). while accelerating i can tell the motor is wanting to go balls to the wall but somethin just won't let it. i enjoy tinkerin with ths car, but i have just about had it.( sorry about that ,had to vent).
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