Friday, September 5, 2014

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.

Thursday, September 4, 2014

Carb tuning #5: The High Speed Circuit (HSC) Explained

Having gone through our first 4 posts discussing carb tuning you should now:

1. Have a pretty clear understanding as to the basic operation of your carbs

2. Know the exact procedure required to setup the carbs on your Dino

3. Have an understanding as to how the Low Speed Circuit (LSC) operates and controls the flow of fuel to the engine from just past idle all the way up to about 3000 rpm

The High Speed Circuit (HSC)

With this knowledge we can now proceed to look at the Main or High Speed Circuit (HSC) which governs the flow of fuel to the engine from about 3000 rpm all the way to redline. While there are many parts to the system we will concentrate on the pieces that are most likely to be changed as part of tuning through carburetion. These parts are:

1. Main Jet
2. Air Corrector
3. Emulsion tube

Below are some images illustrating the location of these parts, what they look like assembled, and what they look like apart. We will add that on a stock engine the Emulsion Tube does not really enter into the tuning equation so it is really the Main Jet and Air Corrector that we will focus on.

The image below will illustrate how these pieces work in the carburettor at engine speeds over about 3000 rpm.

Note that the vacuum created at higher engine speeds directs fuel away from the LSC to eventually reach a point when only the HSC is supplying fuel. 3000 rpm is not a definitive point where the HSC takes over the LSC. A transition takes place at about 3000 rpm so we use that as a guideline to determine the source of a carburetion problem. Below 3000 rpm look to the idle jet, above 3000 rpm look to the air corrector and main jet.

To explain what is happening in the above image, as the engine speed rises to about 3000 rpm and above, the increased flow of air through the throat of the carburettor causes the Venturi #3 to create enough vacuum that it draws fuel from the float chamber #8 and through the Main Jet #7. This fuel is mixed with air whose volume is metered by the Air Corrector #1. The mixing takes place in the Emulsion Tube #5 and the mixed fuel travels to the Venturi #3 where it atomized by the incoming air. The atomized fuel travels down the throat of the carburetor, past the throttle plate, and into the engine for combustion.

As such if you want to affect the amount of fuel going to the engine in the HSC you can achieve this in one of 2 ways

1. Increase the Main Jet size to let more fuel in or decrease the size the supply less fuel.

2. Decrease the size of the Air Corrector to let more fuel in or increase its size to supply less fuel. Remember the Air Corrector introduces air to the system so less air (ie a smaller Air Corrector) results in an increase of fuel delivery.

The explanation of the HSC often leads to a basic question:

Why is an idle jet enough to run the engine up to about 3000 rpm while above that speed you need the Air Corrector / Emulsion Tube / Main Jet combination? Why is a Main Jet not enough?

The answer is simple and lies in a basic weakness that makes up a basic carburetor. Because the engine in a car is expected to perform across a wide range of both speed and load its carburetor needs to be able to supply fuel as the engine accelerates and decelerates at a rate consistent with a optimal air/fuel ratio of  about 12.5-16:1. At lower engine speeds the combination of the idle circuit and progression holes that make up the LSC are adequate to keep the air/fuel ratio within the required range and therefore a simple idle jet is all that is needed to meter the fuel.

As the engine increases in speed, the vacuum created in the Venturi starts to draw fuel at a rate that is faster than the increase of air that is coming in. The faster the engine turns the worse the problem becomes as the mixture gets richer and richer with any increase in engine speed. This is called The Mixture Enrichening Phenomenon.

To correct this, the Air Corrector / Emulsion Tube / Main Jet assembly was created. Fuel is metered by the Main Jet (just like the Idle Jet in the LSC) but additional air to correct for the Mixture Enrichening Phenomenon is introduced via the Air Corrector while the Emulsion Tube takes care of mixing the two. This assembly ensures that fuel is supplied to the engine in the HSC at the correct air/fuel ratio regardless of load or engine speed. Super simple yet very clever.

So there it is. The HSC explained and the parts identified to pay attention to when tuning. In our final and perhaps most important installment we will discuss modern fuels and proper jet selection for maximum performance.

Wednesday, September 3, 2014

Carb Tuning #4: Setting the idle mixture & understanding the low speed circuit

The final step in adjusting the carburetors is setting the idle mixture. This is regulated by the idle mixture adjustment screws located at the base of the carbs.

At this stage I will re-iterate that the idle mixture screw is for setting the mixture at idle ONLY and not for correcting low speed running problems. We will discuss this in detail later in this post when we cover the basics of the low speed circuit that governs the flow of fuel to the engine from idle to about 3000 rpm

The most accurate way to regulate the idle mixture is to use a CO meter however this is a rather obscure & costly tool that does not make up the tool box of almost any mechanic. There is however a simple procedure that can yield near perfect results using nothing more than your tachometer and some basic powers of observation.

The idle mixture screw regulates the fuel that goes to the engine at idle. Close it too much and that particular cylinder will starve for fuel and it will cease to fire. Conversely open it too much and that particular cylinder will get too much fuel and will cease to fire.

One simple technique to see the effects of a non firing or 'dead' cylinder is to disconnect a spark plug wire while the engine is running. With the engine idling, pull one of the spark plug leads off of the plug and note the change in the behaviour of the engine as well as the drop in RPM registered on your tachometer. Doing this for a few seconds does no damage to anything but will clearly show what the result is of being down a cylinder.


Procedure for setting the idle mixture

1. We will assume the carbs are synchronized, the idle speed is set, that each carb is set to a baseline of 2 turns open on each idle mixture screw, & that the engine is up to temperature and running.

2. Using a screwdriver (preferably one with an indicating marking as shown in our previous posts) close the idle mixture screw 1/2 turn at a time allowing a few seconds to observe the result of the change. You will note that at one point the cylinder will cease to fire therefore making the idle speed drop much like what was observed when the spark plug wire was removed.

3. Having established the point at which the engine starves for fuel start opening the idle mixture screw again 1/2 turn at a time again giving a few seconds between adjustments to see the result of the change. You will note that the engine will progressively increase in RPM until a point at which the RPM's will start to drop (the point at which the mixture in that cylinder is too rich causing it to stop firing properly).

4. The goal is to set the idle mixture screw to the point where the engine runs fastest just before it begins to slow from being too rich.  Because you were making your adjustments 1/2 turn at a time you are now in a range where the increase of 1/2 turn (the final observation from Step 3) caused the engine to slow.  As such close the idle mixture screw 1/4-1/2 a turn from that point to achieve the optimal mixture.

5. Repeat this process for the other 5 cylinders.

TEST YOUR WORK When you are all done to test your work you can pull one spark plug lead at a time (never running with less than 5 cylinders) and note the number of RPM's the engine looses. You will know things are operating correctly if the engine drops a near equal amount of RPM every time a spark plug is disconnected.

Having varied the mixture at idle, it is likely that the idle speed will have changed. If this happens re-adjust the idle speed as previously outlined checking with your synchrometer that the carbs continue to flow equally. Dino's seem to like an idle speed between 700-900 rpm.

Your carburetors are now set and you are ready to start an on road evaluation of your carburetion.

The Weber low speed circuit (LPC) explained (idle to 3000 rpm)

Before explaining the low speed circuit on the Weber carburetor we need to review some basic points outlined in our previous posts:

1.  The idle mixture screw is for setting the idle mixture only and not for correcting low speed running problems

2. The idle jet despite its name controls the flow of fuel up to about 3000 rpm in what is known as the 'progression' or 'low speed circuit' (LPC for short)

3. The idle speed screw should not be turned in too much (more than about 1.5 turns) to avoid introducing the LPC during idle.

To best picture what is happening reference the photo below:

At idle air is flowing through the main throat of the carburettor and its flow to the engine is regulated by the throttle valve #14 (or plate as it is often known). The movement of the throttle plate is controlled by the position of the accelerator pedal and its closed position is controlled by the idle speed screw. When properly set, at idle, the throttle plate is opened just enough to allow air to mix with the fuel metered by the idle mixture screw #15 but not open so much that the progression holes #13 are exposed.

The progression holes #13 by their name allow for a progressively increased amount of fuel to the engine as the throttle plate opens beyond the idle position therefore increasing the fuel/air mixture to the engine and consequentially increasing the engine speed.

The operation of the engine from idle to approx. 3000rpm is controlled by the LSC whose only source of fuel is the IDLE JET. As such the idle jet controls the flow of fuel to the engine up to about 3000rpm (when the main circuit kicks in; to be discussed later).

With this understanding of how the LSC works and where the engine gets its fuel from in the range of idle to 3000rpm, you can now see how very important the idle jet is and why it is often overlooked as the source of problems when trying to get the low speed running properly calibrated.

Dino owners will often talk about having a low speed stumble, or backfire when pulling away or when the engine is operating at or below 3000 rpm and these issues are almost always approached with corrective measures involving the idle speed screws and the idle mixture screws. This is WRONG. Idle mixture screws and idle speed screws are for idle only. If you have a low speed running problem at engine speeds under 3000 rpm,  your problem is the idle jets.

We'll discuss this and jet selection in greater detail later once the high speed circuit and the potency of modern fuels is explained in our next two posts.