Weber tuning questions and experience

kent olsen · kent olsen's Garage

First the lessons learned.

1. Idle jets and main jets should be measured with a wire gauge to confirm they are what the number stamped on them says. Even if you buy them new not a bad idea but used ones are often reamed out to a larger size which makes your tuning really frustrating.

2. Make sure you have a good fuel filter and change it often.

3. Fuel pressure should be around 3-4 psi. Holley makes a pressure regulator that is simple to install and set.

4. Bruce Andersons fix to the bottom of the float bowls to prevent starving in high G turns at lower RPM's like during an Autox is a must.

5. Float level must be right or all the above is a waste of time.

Now a question. This is for a 2.7L application.

1. I have 140 mains with 160 airs and F3 emulsion tubes. I believe the standard for a 2.7L is 145 mains and 180 airs. I had this engine on a dyno once and we had to terminate after one pull ( another story). If I remember they did say I was lean and should richen the mixture.

Now educate me on the application. If I install larger airs wouldn't that lean the mixture even more. I'm running pretty good now with the above but I want to try the 145/180 set to see what will happen. My question is my going up one size main jet and two sizes on the air will I lean it out even more??

TVRLOTUSTR3 · 05:22 PM

From a Weber book:
The air correction jets meters the air from the emulsion tube well. The higher the RPM ranges are effected more than the lower ranges by alterations.
The relationship between air and main jet calibration is approx 3 to 1 eg. A decrease in main jet from 105 to 100 is approx equal to an increase in air corrector 175 to 160 (the larger the air correction jet the leaner the mixture).

RoninLB · 12-21-2008, 07:41 PM

PMO delivered my carbs w/140 mains and 190 ac for a street engine w/E-cams.

and that was rich for my act with NGK BPR5ES. I have hard info to back that up.

seems track guys tend to run richer than street.

too rich is ng imo. I little rich is doable without crashing power.

unless you can find a definitive hard answer I'd find a way to borrow or buy a digital air fuel meter. A more expensive gizmo is the LM-1 that pelican sells.

When you get into fine tuning carbs it ain't easy.

JohnJL · 12-21-2008, 08:35 PM

Make one change at a time. Start by going to 145 mains and leaving the airs alone. Ronin is riht about getting yourself some way to monitor mixtures in a real-world environment. Dynos are good to get started and for track work but the street has a lot of variables involved.

kent olsen · kent olsen's Garage

One change at a time is what I'm doing today. I'm trying the 145 mains and leave the 160 airs. Then I'll go to the 180 airs and see what that does.

Your right I wish I had access to some exhaust gas analyser.

I did measure the venturis. They are 36's and should be 34 for the 2.7L but that's the right size for the 3.0L so they'll stay.