Rochester Carb tune ; Which is best method?

[Eighty5ov]

Hey everyone,

I’ve got access (finally) to my mixture screws on my Rochester quadrajet and I’m wondering what’s the best method of tuning the old gal.

On YouTube, I can see the vacuum method but I’ve been advised to do an rpm method? I have the supplies to do both.

Truck is an 85 350. Slight sputter when gas is applied. Did the plugs and wires and it’s still not all there. Any help is appreciated thanks.

 

[Snoots]

ALWAYS get your timing and ignition spot-on before you start dinking with the carb.
Get Cliff Ruggles book on Q-Jets and go from there.

 

[Eighty5ov]

ALWAYS get your timing and ignition spot-on before you start dinking with the carb.
Get Cliff Ruggles book on Q-Jets and go from there.

Both have been double checked.

 

[Snoots]

Has the carb ever been rebuilt? An off-idle stumble may be a starvation for fuel due to the pump seal. Those carbs were made befor the ethanol disaster. A new kit could fix it right up. Send Cliff the specs on the engine and the number on the carb and he'll set you up a lot better than the clowns at auto stores.

 

[Eighty5ov]

Has the carb ever been rebuilt? An off-idle stumble may be a starvation for fuel due to the pump seal. Those carbs were made befor the ethanol disaster. A new kit could fix it right up. Send Cliff the specs on the engine and the number on the carb and he'll set you up a lot better than the clowns at auto stores.

Before I bought the truck. The previous owner said it was, but never tuned it after getting it back.

 

[Eighty5ov]

I just need to know the best method of tuning. Lean drop or vacuum

 

[RaisedK5]

I've never had issues with the lean drop method. I have two small screwdrivers tucked away in the toolbox just for the mixture screws

Lean drop is what the factory manuals specify.

 

[Eighty5ov]

I've never had issues with the lean drop method. I have two small screwdrivers tucked away in the toolbox just for the mixture screws

Lean drop is what the factory manuals specify.

Ok. My screws are the D shaped ones. Which I had to get a tool for. Hard to get at with all the vacuum hoses

 

[Bextreme04]

I just need to know the best method of tuning. Lean drop or vacuum

I use lean drop.

Make sure the engine is FULLY warmed up first and the choke is fully off.
Adjust each screw out in 1/2 turn increments side to side until rpm peaks and starts going down again.
Adjust the idle screw as you go to keep the rpm around 750 or so.
Once you have backed the idle screws out far enough for the rpm to have peaked and started going down again, you need to screw them in again to peak RPM. Once you have peak RPM, turn the screws in(in 1/4 turn increments alternating side-to-side) until you have 650-700 rpm. Now you have a slight lean mixture at idle.

 

[Eighty5ov]

I use lean drop.

Make sure the engine is FULLY warmed up first and the choke is fully off.
Adjust each screw out in 1/2 turn increments side to side until rpm peaks and starts going down again.
Adjust the idle screw as you go to keep the rpm around 750 or so.
Once you have backed the idle screws out far enough for the rpm to have peaked and started going down again, you need to screw them in again to peak RPM. Once you have peak RPM, turn the screws in(in 1/4 turn increments alternating side-to-side) until you have 650-700 rpm. Now you have a slight lean mixture at idle.

Ok so this is best way or how does it oppose to vacuum

 

[Bextreme04]

Ok. My screws are the D shaped ones. Which I had to get a tool for. Hard to get at with all the vacuum hoses

I use this tool. I have newer design hex head idle mixture screws on my quadrajet that I put in when I rebuilt it using a kit for my model number from Cliff.

https://www.amazon.com/dp/B072K31TJJ/
This is the tool I use, it also has the double D socket. It makes it much easier to adjust the mixture screws on a runnning and very hot engine.

 

[Bextreme04]

Ok so this is best way or how does it oppose to vacuum

You should end up in generally the same area with both methods. Lean drop is easier and more precise if you have access to a tach. If you don't have access to a tach, a vacuum gauge is generally easier to find and cheaper.

 

[Bextreme04]

I have a timing light with built in tach, so I have always used lean drop... even with a holley.

 

[Eighty5ov]

I have a timing light with built in tach, so I have always used lean drop... even with a holley.

Yeah that’s a good idea I believe mine has the same. I’ll try the lean drop. All the vacuum lines are going to make it tough.