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.

Sunday, August 31, 2014

Carb Tuning #3: Setting idle, synching carbs, & air correction

Having established what the various adjustments on the carburetor do, amassed the tools required to do the job, and prepared your car for its tune we can finally get to the fun stuff which is adjusting the carbs.

Baseline Settings

We will assume that we are dealing with a car that is totally out of adjustment or carbs that have been freshly re-built. As such we will need some baseline settings that allow you a point from which to start your tune. That said the starting settings should be as follows:

Air bypass screws - Set totally closed an all carbs. Note that these are taper needle seats that only require a light tightening when they bottom out. Be careful not to over-tighten them. Once they are tight be sure to re-set their locking nuts.

Idle mixture adjustment screws - Two full turns open from the closed position. Again these have tapered seats so no need to close them tightly.

Idle Speed Screws - One full turn in from the moment they touch the throttle mechanism. As a tip you can use a piece of paper between the screw and throttle to feel the moment the screw touches the throttle mechanism. The moment it touches the paper will no longer slide between the screw and the throttle and it is from this point that you turn an additional one turn in (therefore opening the throttle butterflies a little bit). Do this on all three carbs.

If you have spent time reading the blog, getting your tools in order, or just doing your pre-tuning prep, bear in mind that your car will have cooled considerably. At this stage re-start the engine and let the car come up to operating temperature.

While the engine is warming up you can practice using your synchrometer to get a feel for how it works. Try placing it in the throat of one carburetor and mentally note what number it reads. Now move it to the other throat of the SAME carburetor. If you are lucky the numbers may read the same but more likely they will be a little different. We'll equalize these later.

Having played with one carb repeat the steps on the other carbs noting the differences from carb to carb and between throats of the same carb. Making all of these equal will be the task we are faced with and knowing what affects what will make it seem all very simple by the end. Now on to setting the idle speed.

Setting the idle speed

There are 2 schools of thought as to how to set the idle:

1. Remove the arm that connects the throttle linkages and set the idle using the individual speed screw on each carb. Once this is done fit the arm back to the carbs ensuring that it slides on without changing any of the settings.

2. Set the idle using only the speed screw on one carb and the linkage installed. This is the method outlined in the owners manual and the one we will go by in this tutorial. I prefer this method because it utilizes the throttle linkages which is how the final assembly is anyways.

In order to use the 2nd method you use the idle speed screw on the #3 carburetor (the one furthest from the distributor) and adjust the position of the mounting blocks on the connecting arm on carbs #1 & #2 in order to get all of the throttle plates opening the same amount at idle.

This may sound confusing but it is really easy when you wrap your head around the mechanics of what is happening. At idle we will measure the throttle plate opening is using your carb synchrometer referenced to the engine speed measured on your tachometer.

Here are the steps to follow:

1. With the engine up to temperature and idling back off the idle speed screws on the #1 & #2 carb so that they are no longer touching the throttle assemblies.

2. Using the speed screw on the #3 carb turn it so that the engine holds an idle of about 900 rpm

3. Using your synchrometer note the readings on both throats of each of the 3 carbs. The values will differ from carb to carb and between throats of the same carb. You are interested in the HIGHER value recorded per carb regardless of which of the two throats it comes from. In other words you will have three pairs of numbers (values of two throats per carb x 3 carbs) and you are interested in the higher number that each carb (NOT the outright 3 highest numbers) produces regardless of which throat generates that number. Hopefully that is clear.

The final goal is to achieve an idle speed of 900 rpm while matching the HIGHER throat to throat value from carb to carb. At this stage this is achieved ONLY via a combination of the speed screw on carb #3 & the blocks on the linkage of carbs #1 & #2.

You are not after a particular number on your synchrometer but to have a value where each carb has at least one throat that reads the same number as one throat on the other two carbs.

This will require some detailed adjusting of the #3 carb speed screw and linkage blocks of the #1 & #2 carbs. Achieving equal numbers on at least one throat of each of the 3 carbs while maintaining an idle speed close to 900 rpm does not take very long provided you follow a methodical approach. After achieving three equal readings, be sure to accelerate the engine a few times by hand allowing the throttle mechanism to cycle and check your work. This step is complete once the engine holds a steady idle near 900 rpm while having one throat of each carb equal to or 'synched' to the other carbs.

Below is an example of what to shoot for when setting the idle speed. Remember the ACUTAL NUMBER DOES NOT MATTER and does not need to match what is in the photo. What you do want is for one throat of each carb to be equal to at least one throat of each of the other carbs. In the image below you will note that each carb has at least one throat that reads 3.5 when the engine is at 900 rpm.

Synching the throats with the air correctors

If you remember our lessons so far then we know that (when properly set) at least one air corrector on each carb is left closed. Like their name implies, the air corrector 'corrects' for differences in airflow between the two throats of a single carb.

At this stage we have one throat of each carb equal to at least one throat of the other two carbs. Lets call this value the SYNCH NUMBER. Also (if we followed the procedure), the other throat will have a number EQUAL TO or LOWER than the synch number. Unscrewing the air corrector on the 'low' side allows us to RAISE the value up to the synch number ensuring that both throats on the carb flow the same.

The photo below illustrates a hypothetical synch number of 3.5 and the green arrows show the throats that require their air correctors opened in order to achieve a value of 3.5 on all 6 throats.

To do this work only on the air corrector that is on the low reading throat. Here are the steps:

1. With the engine idling determine which throat on a particular carb reads your synch number.

2. Now take a reading on the opposite throat of the same carb. If the number is the same you are finished and can move on to the next carb.The number will otherwise be lower on this throat so you need to loosen the lock nut on the air corrector of that throat and unscrew the air corrector until you achieve a reading equal to the synch number.

3. When you have achieved a matching number be sure to fix the air corrector in place by tightening down the lock nut.

4. Repeat the procedure to the other two carbs.

Once this is done, your synchrometer should read an equal number on all 6 throats.

At this stage you now have your idle speed set and all three carbs are properly synchronized. It may seem like a lot when you put it in writing but in reality this entire procedure can be carried out in much less than 30 minutes.

One final step is to turn the idle speed screws on Carb #1 and Carb #2 to the point where they 'just' touch the throttle mechanism. Again a piece of paper can be used between the tip of the screw and the mechanism to be sure of the point of contact. Be careful to ensure that the speed screw does not alter your synchrometer numbers in any way. If your numbers change it is because the speed screw is turned in too much and needs to be backed off a hair.

The next step (and subject of our next blog posting) will be setting the idle mixture screws which will conclude the basic setting up of the carburetors.

Sunday, August 17, 2014

Carb Tuning #2: Tools needed, pre-tuning prep, & general pre-requisites

With a basic understanding established of what the various adjustments do on the Weber carbs fitted to a Dino lets now get ourselves ready to tune. In order to do this we will cover three items in this post:

- Tools needed
- Pre-tuning preparations
- General pre-requisites before getting started

Tools needed

In addition to a pair of ears to listen to the engine, there are only a few tools required to tune the carburetors.

#1 - A flat headed screwdriver to adjust the various speed and mixture screws. As a tip I make a small mark on one of the flats of the screwdriver to help me keep track of the number of turns and half-turns that I am making when adjusting the carbs. Remember these adjustments are made while the engine is running so it can be a little distracting and the mark helps quite a lot.

#2 - A carburetor synchrometer. It may sound fancy and expensive but a synchrometer is nothing more than a tool that measures airflow in the carburetor and costs less than $50. It is an essential tool for setting the carbs and makes you look really knowledgeable to casual observers when you use it.

#3 - Not totally essential but I like to use a digital timing gun to have a quick reference of the engine RPM's. It saves constantly looking into the car to reference the tachometer, acts as reference to check the accuracy of the tach, and is an essential tool anyways in the toolbox of someone who works on old cars. You may not need it to set up the carbs but it is required to make sure the ignition is working properly.

#4 - Lastly are some 8mm and 10mm wrenches to adjust the throttle linkage and the air bypass screw lock nuts. Simple stuff that should already be in the tool box.

Pre-Tuning Prep

Before getting started it is necessary to get your Dino (or other classic) ready to tune. The following is a checklist of things to do.

- Warm the car up to full operating temperature
- Remove the rear engine cover
- Remove the airbox cover
- Remove the nut (27mm wrench) that holds the crankcase breather assembly to the airbox. It is much easier to leave this entire assembly intact and remove it from the airbox rather than disconnecting all of the hoses that lead to it where visibility is greatly reduced.
Note the tape placed over the carburetor inlets to eliminate the chance of dropping anything down the carb openings. 

- Remove the nuts that hold the airbox to the tops of the carburetors. There should be 12 of them and can be removed using an 8mm socket. Also loosen the large hose clamp that connects to the ribbed inlet hose to the airbox.  

- Pull the airbox out being very careful not to loose any of the tube 'top hat' spacers that go under each of the nuts you previously removed. Some of these spacers may stay behind on the studs on the top of the carbs while some may come on the rubber sealing gaskets that are on the airbox. This is ok as long as you have all 12 of them accounted for once the airbox is removed.
- Re-connect the entire crankcase breather assembly. I used one plastic zip tie to hold the assembly in its finished position.
- I will add that some people disconnect the throttle linkage where it meets the #3 (left most when viewing the engine from behind) carburetor while others leave it connected. I have done it both ways with equal results always assuming that the assembly is free of un-necessary play or binding.
- If you are using one, connect your electronic timing light to act as your tachometer and take the pulse from the #3 cylinder. This is the rightmost cylinder on the rearward bank (the one closest to the distributor).
The previous list may seem like a lot but it can all be done in about 15-20 minutes. You are now ready to get started with your tuning.
General pre-requisites
All of the steps outlined in this post make some assumptions before tuning begins in earnest. Please be certain of the following before proceeding as carburetion is often cited as the cause of other problems:
Note that this list also works as a good troubleshooting guide if you struggle to get the carbs to setup properly. Setting the carbs is easy but you need to remember that everything around them need to also work well in order for the carbs to properly do their job.
1. Make sure your ignition system is working properly. This includes:
- Spark plugs are clean, of the correct type, and gapped appropriately
- Spark plug wires are in good shape
- Ignition cap and rotor are in good shape and not cracked
- Ignition advance mechanism is not seized and is operating properly. This can be tested on a running engine using your timing light. Testing that the distributor is properly advancing is very easy but is beyond the scope of this tutorial (we will cover this in a dedicated post in the future). We will proceed assuming that your distributor is advancing properly but note that this is one of the leading causes of poor running engines that are improperly diagnosed as carburetion issues.
2. Make sure the fuel system is working well. This includes
- Fresh fuel in the tank
- Fuel filter that is clean
- Fuel lines that are in proper shape and not cracking or deteriorated
- Throttle linkage that is properly lubricated and free of any play or binding.
- Make sure your float level is properly set on the carburetors. Here is a link to an excellent tutorial outlining this procedure:
- Make sure the carburetor has matching idle jets, main jets, and air correctors. Jetting may be different to stock but as a baseline here are the factory installed components on the Weber DCNF 40 carbs fitted to the Dino
Note: Do not always trust the sizes stamped on jets! These cars are old and lots of people have likely messed with them over the years. Quite often jets have been drilled out so their size is larger than what they show. I always check jets (even new ones) using a simple jet gauge. This is a tool with calibrated pins that either fit or don't fit into the jet openings and can be bought for about $15 on eBay or from whoever you buy your carburetor parts from.
3. Make sure the engine makes proper compression.
With everything apart this is a good time to check the compression of the engine. Again this is a separate procedure not covered in this tutorial but it is easy to find a YouTube video showing exactly how to test for proper and even compression. An engine with low compression can be tuned but having one or more cylinders with very low compression relative to the others will make proper tuning almost impossible. It is also good to know the health of your engine so a compression test is always a good idea if you do not know what condition your motor internals are in.
It may seem like a lot but really it is not. Being careful and methodical in setting up the carbs may not be glamorous but it is where the magic really is. Proper preparation and understanding of simple individual tasks is what will make you look like a factory trained carb tuning rock star.
Our next two carb tuning posts will cover idle and air correction adjustments followed by idle mixture adjustment. This will all be carried out on a running engine and again follows a structured approach that leaves little room for error. Stay tuned.