MakoJoe

I had rebuilt my carburetor not long ago in my 1977 Corvette and notice I was getting crappier fuel mileage. It went down from 16 to 18 MPG way down to between 10 and 11 MPG. I had a feeling it was running way too rich so I pulled 2 of the spark plugs and they were Pitch Black with carbon deposits. My Suspicion was correct and I needed to readjust the 2 Fuel/Air Mixture Screws on the front of the Rochester Carb. I have not done this in about 25 years and had to remember how to perform the adjustment.

Make sure your engine is up to full operating temps then remove the Air Cleaner from the carb and plug that one vacuum line that plugs into the Air Cleaner. Some older cars may not have one.

When you adjust a carburetor with a Vacuum Gauge what you are looking for is the highest possible vacuum from the Intake Manifold that is a steady vacuum pressure.

Mine was sitting about 12 PSI when I first started to check it. I plugged my vacuum gauge into the Positive Crank Case Vacuum Line because you need to have the Vacuum straight from the Intake Manifold to adjust it properly

I turned the Mixture screws in until I was getting a consistent 16 PSI on the Intake manifold. What you want to do is turn the mixture screws in while listening to the Idle the idle will go down and start to sputter a bit because it is too lean. Than back it out until you get the highest steady Vacuum on the Gauge. Perform this on each of the Mixture Screws one at a time.

This is the easiest way to set the mixture screws on the Rochester Quadra Jet carburetors.

68Thunder427

Yep in / out go in between. Vacuum gauge is not mandatory if you have a good ear. Now if you do all the above as stated and it is still to rich or lean once you drive it, then you need to re-jet the carb and start all over. Lars will guide you in setting up the carb right and you will most likely only have to do it once.

MakoJoe

I have to agree with this because I live at over 5000 Feet above Sea Level and think my car is a High Altitude engine. Back in the days of the 1977 or any model year they had 3 standards. California, High Altitude and Low Altitude carburetors. I believe mine to be a High Altitude Carb since it got better gas mileage before my carb rebuild then after then needed necessary adjustments.

I know people in my car club that have moved older cars up here at over 5000 Feet Above Sea level then had install new Primary and Secondary Jets into the carb because no matter how they adjusted it they just would not run right at this Altitude. In their case it was running way too rich and they had to install new primary and secondary jets to lean it out more. The lower oxygen levels at high altitude in reality will need to run a leaner fuel mixture then at lower altitudes which is also why we can get away with lower octane fuels then at lower elevations. The cooler air and less dense air help to add to lower octane fuels. our low grade fuel is 85 Octane where in most states it is 87 octane.

Because my 1977 L48 engine is lower compression then the L82 engines of this era I can get away with running the 85 Octane fuel that also has 10% or more Ethanol mixed in with it. Most of us hate Ethanol these days in an older engine because it will dry rot seals not meant for this fuel over time in the fuel system and it also attracts more moisture into the fuel system and causes rust in the old gas tanks.

If I had a L82 I would need to step up to the 87 Octane Fuels since these are still lower compression the 1971 and below engines that would have 10:1 or more compression. My L48 is 8.5:1 compression at lower altitudes and more like about 8:1 at this altitude with the less dense air.

68Thunder427

Send Lars an email with your specific carb, year model, etc.

V8FastCars@msn.com