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.

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