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.

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:

 

http://www.birdman308.com/service/floats/float_height.htm

 

- 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.

 

Carb tuning #1: Weber DCNF carbs and their adjustments explained

With the car built and now on the road one of the most important items is ensuring that the carburetors are properly adjusted. This subject is often treated as some sort of black art that only the most skilled and special are able to do; TOTAL NONSENSE.

The reality is that carb adjustment can be done by the home mechanic to a very high level with minimal tools. What is necessary is a clear understanding of the various adjustments and how they affect the running of the engine. It is this understanding that a shocking number of 'experts' lack and our series of carb tuning posts are aimed at dispelling many of the myths out there giving Dino owners the confidence to tackle this project themselves.

To start we will look at the Weber 40 DCNF carb fitted to the Dino and identify and explain the parts that are touched when tuning the engine.

*Note that we will only address the high speed running circuit late in our posts. For now all the focus is on the items that affect the running of the engine below about 3000 rpm.

Below is an image of the carb fitted to a Dino. It is a dual throat construction where each throat feeds a single cylinder. As such the Dino has 6 cylinders so 3 carbs are fitted.

Now for an explanation of the marked parts:

Idle jet
- There are 2 idle jets per carb (one per throat & the 2nd one is not visible in the picture). Each jet acts totally independent of the other.

- Located under the screw in the photo is one of the idle jets. It is nothing more than a brass tube with a calibrated hole in it. These jets are available with larger or smaller holes depending on the amount of fuel you wish to introduce to the engine.

- Contrary to its name the idle jet supplies fuel to the engine up to about 3000 rpm so it is a vital part of the tuning equation. This will keep coming up so be clear on this point



Idle mixture adjustment screw

- Again there are two of these which act independently on each throat of the carburetor

- These screws can be adjusted down all the way to fully closed where no fuel gets to the engine during idle

- The adjustment of these screws are to be used ONLY to adjust the mixture at idle! Mechanics often turn these to correct low speed running problems and this is totally wrong. The idle screws are for idle ONLY.



Air bypass screws

- One per throat

- Allows for a compensation between throats of a single carburetor to equalize the flow between the two barrels.

- When properly adjusted, one of the air bypass screws is closed while the other is open a measured amount. We will discuss this setting later but if both are open the carb is not set correctly. In some instances both may be closed but for both to be open is not correct.



Idle speed screw

-  Because the each carb has only one throttle shaft that operates both throttle butterflies there is only one speed screw per carb.

- Think of the throttle butterflies as doors that allow the air/fuel mixture to enter the engine and the idle speed screw is a stop that controls how much the door closes. Because the engine needs a certain amount of air/fuel mixture in order to idle, the speed screw allows for a fine adjustment of how close to closed the butterflies are and is essential in setting the idle speed of the engine.

- The idle speed screw is one of the most tinkered with screws on the carb and is often opened far too much. Generally speaking if the screw is in more than 1.5 turns from the moment it contacts the throttle then it is turned too far.



Below are some more photos of some of the parts we mentioned allowing for a different perspective.

Summary

- Idle jets control the engine up to about 3000 RPM
- Idle mixture adjustment screws are to control idle only
- When properly set only one (or neither) of the air bypass screws are open; not both.
- Idle speed screw is used to set idle speed only and should be kept between about 1-1.5 turns from the first point of contact.

With the basic adjustment points explained the next in our series will look at the tools needed for the job of carb tuning as well as outlining the basic approach when setting the carbs. Future posts will take us step by step (with lots of photos) through the whole tuning procedure hopefully giving blog followers the confidence to do this job themselves.