Carb Tuning #6: Modern Fuels & Jet Selection When Tuning

Understanding Modern Fuels

Todays post will have no pictures but we cannot stress enough the importance of reading and understanding the following:

Modern fuels sold worldwide are WEAKER than those sold 40+ years ago when the Dino was a new car!

If you choose not to take this at face value then below is a link to a more technical explanation of why modern fuels differ from those of a few decades ago:

Impact of Today's Fuels on Carbureted Engines



Returning to carb tuning this is important because understanding that modern fuels do not carry as much energy per volume means that the jet sizes listed in the factory shop manual are incorrect. This is not to say that the factory is wrong and that we know best. Remember that the shop manual was written over 40 years ago and the jet sizes quoted would have been right for the fuels available back then. Since the manual has not been updated for the times we hope that this blog post helps to determine the correct jet sizes for the fuel that is available in your area.

Regular blog followers will also know the meaning of the term 'pixie dust' and it is from one of these experiences that we were compelled to make the series of posts on carb tuning. We were misled into believing that a particular and well regarded mechanic had a special knowledge as to how to set up the carbs on our Dino. In the end we wasted a bunch of time and money on a guy who only would fiddle (and who knows how knowledgably) with the idle speed screws and idle mixture screws. He never once came close to replacing a jet nor did he know that modern fuels differed from what is available now. For him the 'Ferrari jet sizes' were carved in stone tablets brought down from the mountains so it was blasphemy to question them. Needless to say we got the heck away from him, learned what was right, and chose to share it here on the hopes of saving someone else the problems and unnecessary expense we went through.


Selecting the best jets

In 2014 if you are using the stock jet sizes in your carbs then your engine is not running at its best and likely far from it regardless of the condition of your engine or ignition system.

If you have read our previous posts on carburetor tuning you will now know 3 simple things:

1. Fuel today is weaker than fuel from 40 years ago when Ferrari specified jet sizes for the carburetors. The weaker fuel requires an increase in fuel delivery to achieve the same levels of performance and for your engine to run properly.

2. Idle Jets control the flow of fuel from idle to about 3000 rpm

3. Air Correctors and Main Jets control the flow of fuel above approx. 3000 rpm.

So the good news is that tuning the carbs requires only 3 jets per cylinder. Because the carbs are matched to the cylinders then any change is equal across all the carbs and there is no need for different jetting in different carbs. When you think of the number of parts that make up the carb, narrowing it down to only 3 pieces is welcome news indeed.

Jet selection guide:

1. If you are running the stock jets then you will need to increase (not decrease fuel flow to the engine) for proper performance so only buy jets that will take you in the right direction.

2. If your engine is struggling, mis-firing, or is just plain not smooth between idle and 3000 rpm you need to go to a LARGER idle jet.

3. If your engine struggles, mis-fires, or lacks pull over 3000 rpm you need a combination to either DECREASE the size of the Air Corrector or INCREASE the size of the Main Jet. As a guide each change in size of Air Corrector normally results in a difference that is somewhere in the middle of a change in size of Main Jet. In other words think of a change in Main Jet as a larger change and a change in Air Corrector as a finer change.


The question then is 'what is the correct jet size for my car'?

Always assuming an engine and ignition in good condition there is no one answer however it does depend on 2 main factors:

1. The general altitude you drive at. Higher altitudes = less oxygen requiring smaller changes from stock therefore the same car in La Paz Bolivia will have different jetting to one running in Southern California next to the ocean.

2. The composition of the fuel you use. Fuel sold in Germany is different to that found in Toronto making tuning very much a regional thing.


2 Ways to Tune:

1. On a rolling road

The best way to tune an engine is on a rolling road (also known as a 'dyno') with a Lambda probe in the exhaust. The rolling road simulates load while measuring horsepower & torque, and the Lambda sensor measures the quality of the combustion of the fuel by sampling the exhaust and feeding its data into a computer for immediate analysis. Jet changes can be done right on the rolling road, an immediate test can be done, and changes can continue until you make the most power while having the best air fuel mixture as measured by the Lambda sensor.

Realistically if you have a car with properly set up carbs as per our tutorial, with only the jetting requiring adjusting, & you have a basic selection of jets at your disposal, then 1 hour on the dyno is plenty to make all of the needed adjustments. Remember there are only 3 parts to play with and the stock sizes supplied by Ferrari get you fairly close to start.



2. On the open road

The other way to tune involves doing real on road testing relying on the seat of the pants feel to determine how your changes affect the way the car drives. Doing this can be a lot of fun and a great learning experience. Here are some guidelines to help you:

1. On road testing should ALWAYS be done with the air cleaner installed to avoid the risk of ingesting something that could damage the engine.

2. Because factory jetting is too lean with modern fuels rest assured that your tests towards adding more fuel is SAFER for the engine than running the factory settings. Running lean is much more risky to the engine than running rich so if anything your attempts to tune will protect your engine.

3. Break down your testing in 2 parts: 1-idle to 3000 rpm , 3000 rpm and up. Doing so will allow you to concentrate on Idle Jets as one task and then Air Correctors and Main Jets separately.

4. Keep notes of your changes and how the car feels before and after. Note the weather, temperature, approximate altitude, and fuel being used.

5. Be sure to continue adding fuel until you feel that performance drops off. This way you will be able to know when you have gone too far and scale things back accordingly. Remember there is little to risk in going too rich with your jetting.

Knowing that starting from stock that the goal with modern fuel is to increase fuel delivery means that there are not a lot of combination of changes that can be made before you start giving the engine too much fuel and performance decreases. On a Dino buying the next 2 sizes up of Idle Jets and Main Jets as well as the next 2 sizes down of Air Correctors should be all you need to get your car dialed in regardless of fuel and where in the world you are tuning. We bought all of these parts for a total of less than $200 and strongly recommend Pierce Manifolds for all the Weber components you need.


A note on our jetting experiences at time of writing

We are currently in the experimenting phase and with only about 500 km on the engine since finishing the car we are still in the running in period so have done little running over 5000 rpm. When the engine is fully run in we will go to the rolling road and give it a full tune. That said we eliminated a low speed stumble and off throttle popping by increasing idle jets from the stock 0.050 to 0.055 and will soon experiment with 0.060 idle jets to see if there is a little more power on the table. After this we will go down one Air Corrector size to give a little more fuel over 3000 rpm because we are for sure a little lean given the stock jets that are currently fitted.

A future blog post (not likely until next year) will document our dyno testing and we will share all of our jet selections then.

Until then we thank you for following our carburetion posts and hope that they were both informative and entertaining.