Tuning Carburetors

Importance of a Carburetor:

In this installment of “This Week in the Shop” were going to look at the process it requires to tune the carburetor system, one of the most important parts of your vehicle is the engine, as is the performance of your engine.  The whole idea of a carburetor is to blend your air and your fuel and to regulate your engine speed. So of course, bigger and more is always better and just like anything else when it gets old it needs adjustments, now comes the lost art of tuning 3 carburetor systems.

We have a couple different carburetor projects going on right now in our shop. Mike is DSC_0259was busy mastering the triple carburetor system in the Jaguar E-type we talked about and Gary put a brand new two carburetor system in the Morris Minor we bought a few weeks ago. Mike finished the tune up on the Jaguar this week in a matter of of a little less then four hours.

An engine only will perform as well as it’s been tuned. You can build a beautiful engine using all the best parts and machine and assemble everything with precision, but if it’s a carburetor engine and the carburetor isn’t properly set up or tuned right your engine will not live up to it’s full potential. Basically put, a good carburetor tuning can make an engine, and a carburetor that’s not tuned properly can literally destroy your engine. A good tune means the engine should start and idle without stalling, and respond instantly when the pedal hits the floor. The carburetor will also deliver the optimum air/fuel ratio throughout the engine’s RPM range to get peak horsepower and torque.

An improper tuning, on the other hand, can create many problems. A slightly rich fuel mixture is necessary to make peak power, but if the A/F mixture is too rich, fuel can flow down the cylinder walls and dilute the oil. It can also cause the spark plugs to foul. Having the mixture too lean is even worse. It kills power and increases the risk of engine damage. If an engine is hard to start, barely idles, stumbles every time the throttle opens, gets horrible fuel economy, spews black smoke from the exhaust pipe or never seems to run quite up to its standards, chances are the carburetor needs to be tuned.

The Difference Between a Carburetor and Fuel Injection System:

The main difference between a carburetor and electronic fuel injection is that a carburetor is solely dependent on the intake vacuum for fuel metering and delivery. The intake manifold vacuum pulls the fuel through the metering circuits and the atmospheric pressure provides the push. Fuel injection is sprayed into the manifold so it is not dependent on an intake vacuum. The (PCM) powertrain control module controls the fuel mixture using inputs from the O2 sensors, throttle position sensor, airflow sensor and the manifold pressure sensor.

Electronic fuel injection is constantly readjusting the A/F mixture and therefore is also self-tuning and the ratio is based on feedback from the oxygen sensors. Barrel carburetors lack that ability, although in the early 80’s some could vary the A/F mixture. these were electronic feedback carbs. Consequently, mechanical carburetors require constant fiddling and readjusting for prevailing weather conditions and track conditions.

Steps to Adjusting the Carburetor:

A few adjustments can be made before installing the carburetor on the engine. One adjustment is to adjust the idle mixture screws to about one full turn out. Do not over-tighten the screws as doing so can damage the tips! If there is a choke on the carb then preset it to the “0” or midpoint setting. Also, make sure you leave the locking screws for the choke housing cap slightly loose so you can easily readjust the choke if needed. Adjust the idle speed screw on the throttle linkage so the throttle plates are closed, then tighten the screw one full turn for the initial idle setting.

Mike has it down to a science when he does his carburetor adjustments, and the first thing he likes to do is to check the throttle linkage and to check the throttle butterflies and how they’re opening. Once he has done this he then sets the carburetor mixture and puts the high and low idle back to base factory settings and then hes separates the throttle intakes so they’re independently working.  Next step is to warm up the temperature, once the temperature is good you set your base idol and balance the air flow with a flow gauge, make sure you adjust back and forth between the carburetors so theDSC_0262 air flow system is balanced properly. In the case of the triple-carb Jaguar the front  carbs run the first two cylinders, the middle carb runs the middle two cylinders and the back carb runs the last two cylinders. Once your flow is balanced and your intake is locked down you adjust your mixture.  A good way to do this is to lift the piston in the carb about 1/32 of an inch and give it a good listen. if it dies your mixture is too lean, if throttles up too much then you have it to rich, you want it to throttle up just slightly and then to balance itself back out.  If it dies or accelerates way to much then you need to continue to adjust it.  You also have to make sure you work all the carbs together back and forth. get them in sync with each other and talking back and forth. After you’ve finished adjusting it’s time to take your vehicle for a test drive. If you find your vehicle pops back when you accelerate then you are to lean, still, and need to adjust again. If you find you vehicle making black smoke and sputtering then your mixture is to rich and you need to adjust again.  Mike likes to do his adjustments on the road while hes test driving. This is one of thousands for him and I’m sure there will be plenty more for Mike to adjust. Everyone does it the same but it takes a certain person to be able to grasp the concept and be able to differentiate between separate vehicles and systems, and to become a master like Mike or Gary.

Speaking of Gary, he himself also had an adventure this week in the land of carburetors.DSC_0256 DSC_0260 The Morris Minor he purchased weeks back underwent a carburetor conversion this week, and you couldn’t ask for a better person to do it then Gary himself!  The Minor came with a single carb system and as I stated above bigger and more is always better, especially when it comes to vehicles.  The single carb system was removed and we replaced it with the two barrel carburetor system. naturally Gary had to do the exact same steps we talked about above and the Minor is now in great running shape and happy with it’s new carburetor system.

Things to Keep in Mind:

The engine needs to run at fast idle 1,000 to 1,500 RPM for the initial break-in period. Once the rings have seated, you can make the speed adjustments and initial idle mixture. The goal here is to achieve the fastest idle speed by turning the idle mixture screws in and out to achieve the fastest idle speed, then you adjust the idle speed screw to set the idle speed.

Now you can use a handheld A/F meter with a wide-band O2 sensor to measure the A/F ratio in the exhaust. Remember that the meter readings are an average for all of the cylinders that are dumping exhaust into that pipe. If you have a dual exhaust check both exhaust pipes. Keep in mind as well that the differences of fuel distribution in the intake manifold may cause the end cylinders to run leaner than the cylinders in the center. Another thing to check before you continue is the fuel level in the fuel bowl.

If you’re tuning the carburetor the old fashioned way, test driving the vehicle will quickly tell you if your engine feels strong or not, as well as how well it responds to throttle changes. If the engine feels sluggish when the driver steps on it, the carb may be either too lean or too rich. Trial-and-error changes to the accelerator pump, main jets and/or power valve should sort out the problem eventually. Reading the spark plugs can also give you a rough idea of what’s happening to the A/F mixture. Black carbon deposits on the center insulator and electrodes would indicate an overly rich A/F mixture. A blistered or yellowish appearance on the spark plug electrode tells you that the A/F mixture is much too lean. Heavy black discoloring in the exhaust or tailpipe would also indicate a much too rich A/F mixture.

Keep in mind that the jet sizes, power valve and accelerator pump adjustments that provide the best performance today more than likely will not produce the same results tomorrow. As soon as the weather changes or other changes are made, the carburetor will be out of tune and in need of further adjustments. With carburetor engines, it’s an ongoing process of tweaking and adjustment to find the settings that deliver the best performance. So unless you’re ready to swap out the carb for a bolt-on fuel injection system get used to making the adjustments.

~Matt Beal- Writer/Editor

~Gary Gammans- Publisher/Owner




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