Setting Carburetor air / fuel mixture (Quadrajet)

[Chief Brody]

I intended to do this nearly 2 years and lost focus when my brother died. I fired up the Blazer this past weekend and it smells like it is running rich.
It spit out some black carbon colored water and was sputtering a little....exhaust smells like gasoline a bit.
I believe I was going to adjust the air / fuel mixture screws and even bought a Rochester repair manual.

Any of you have an tips or suggestions on the best way to do this? Where should the vacuum gauge attach and how much should I see?
Should I set the timing based on vacuum?

Haven't done this in years.

 

[6872xtc]

Is this a small block? I usually have 12* BTDC initial, with the vacuum advance unhooked and plugged on the carb. Then listen for detonation while driving. I will go higher if I can check for detonation while driving. I don't know what factory specs are, but I can't follow them at 4600ft anyway.
Then hook the vacuum advance back up.
The idle mixture screws are just adjusted for the best vacuum reading at idle with a warm engine. You have to find a port that is manifold vacuum, not timed, or ported. I set them on first start up, then again after driving it.
The vacuum reading will depend on lots of factors, the engine condition, cam, elevation...

 

[Babaganoosh]

One thing to look for that changed the way my old quadrajet ran was i put stock jets and metering rods in. Then opened up the air horn to adjust the apt along with the secondary air valve.


It dmped gas down like it was cool but it ran awesone and made old guys rubber neck from the sound.

 

[diesel4me]

When using a vacuum gauge to adjust the idle mixture,you should adjust the screws until you reach the highest and steadiest reading on the gauge,then turn the screws out about 1/3 of a turn till the gauge drops an inch or so,that allows for cold starts and prevents stalling until the engine and carb warms up..

You want the gauge hooked to a manifold vacuum source (below the throttle plates on the carb--a good port to use is the one the vacuum modulator hooks to if you have an automatic...using any ports on the carb might not be 100% manifold vacuum,and if you remove the PCV valve and hook the gauge there you wont get an accurate reading due to the fact the PCV valve is a "controlled" vacuum leak and affects the fuel mixture at idle..

Most of the V8's I've had ran best when I set the timing "by ear",I'd advance it a little at a time until you get a slight ping under a hard pull uphill ,then if it binds up while cranking it after its fully warmed up you can back it off a bit more..
A 400 small block I had with a stock Q-jet ran best with the timing advanced beyond the factory specs,but then would sometimes not want to crank over after it was warmed up and shut off for a few minutes..I rigged a toggle switch to the ignition coil so I could get the engine spinning over full speed,then flick the switch to fire it up..racers use that trick,that is where I learned about it..

 

[Chief Brody]

Is this too rich?

 

[diesel4me]

UM..maybe just a tad....

Looks like someone's float is stuck and they put a spark plug in the O2 port!..

 

[Chief Brody]

I've been watching videos and reading posts and I saw something about "Total Timing". I was not familiar with that, so how do I find how many degrees I need to set mine to for total timing. One corvette video says 35* BTDC and another says 12* BTDC. How do I know what it should be for my specific engine. Mine is a 1975 small block 350 with a correct Quadrajet. Stock everything, it has HEI.

Should I just do everything by vacuum reading? Set the timing for the highest vacuum reading? Set the Carb mix for highest vacuum?

I dont have tachometer

 

[Babaganoosh]

I've been watching videos and reading posts and I saw something about "Total Timing". I was not familiar with that, so how do I find how many degrees I need to set mine to for total timing. One corvette video says 35* BTDC and another says 12* BTDC. How do I know what it should be for my specific engine. Mine is a 1975 small block 350 with a correct Quadrajet. Stock everything, it has HEI.

Should I just do everything by vacuum reading? Set the timing for the highest vacuum reading? Set the Carb mix for highest vacuum?

I dont have tachometer

That's what I would do, every engine is different. Do you happen to know what size jets and rods you have?

 

[Chief Brody]

That's what I would do, every engine is different. Do you happen to know what size jets and rods you have?

No, I don't know the carburetor specifics. I don't think I kept the box it came in but I assumed it was whatever stock was.

 

[Chief Brody]

Is this a small block? I usually have 12* BTDC initial, with the vacuum advance unhooked and plugged on the carb. Then listen for detonation while driving. I will go higher if I can check for detonation while driving. I don't know what factory specs are, but I can't follow them at 4600ft anyway.
Then hook the vacuum advance back up.
The idle mixture screws are just adjusted for the best vacuum reading at idle with a warm engine. You have to find a port that is manifold vacuum, not timed, or ported. I set them on first start up, then again after driving it.
The vacuum reading will depend on lots of factors, the engine condition, cam, elevation...

Yep small block.