Car carburetor tuning

Carburetors or carbs were once commonly used on almost all car engines. Carbs come in all different types. The purpose of all carbs is simply to control the fuel going into the engine. The downdraft or draw through throttle plate carb was most widely used on passenger cars. These carbs usually consist of a low speed circuit used for low rpm or idle. Sometimes on the slightly more advanced carbs there are mid range circuits. The accelerator pump circuit assists in acceleration. The main jets are used the most and are very important to full throttle tuning. All production vehicle carbs (quadrajet, carter)usually have a choke circuit for warm up assist. Let’s start with the carburetor’s choke system. The choke operates usually by restricting the air into the engine. Along with changing the fuel to air ratio the manifold vacuum increases significantly, pulling even more fuel from the low speed circuit. The carb needs to run rich (alot of fuel) when the car first starts up because the engine isn’t warm enough to help vaporize the fuel. The carb uses idle, low speed or pilot jets (which are the same things) and air bleed screws to control the amount of fuel being sucked in at idle. The air bleed screws lean out the mixture as they are rotated out. Idle tends to be slightly richer than stoichiometric (perfect fuel to air ratio) of 14.7 to 1.

 

If the jets need to be changed, the higher number the jet, the more fuel will be delivered to the engine. In quadrajet carbs the metering rods control the fuel in the mid range rpm. The higher the needle is raised the richer the engine will run. The engine is usually tuned a little lean (less fuel than stoichometric) at the midrange. This is done because the engine is usually not working very hard at this time and dosen’t need the extra fuel. The tuning is very critical here to ensure good fuel economy as you drive up the road. The accelerator pump circuit sprays extra fuel into the carb as you step on the accelerator. If you didn’t have a way to add this extra fuel the engine would become way too lean as you started accelerating hard. The engine would make a lean whhhooo sound and stumble, misfire and would not accelerate well for a few seconds until the main jets caught up with enough fuel. A lean engine will also leave the spark plugs looking white colored on the electrode. This is usually controlled with the rod stroke like a holly carb, accelerator pump size or jets. Vacuum secondary carbs can also affect how fast the acceleration starts by controlling how quickly the secondary plates open and let air in. The main jet is the circuit that mostly affects the power of an engine. This circuit should generally be a little bit rich to make best power. A popular misconception is that more fuel equals more power. The truth is that too much fuel wastes gas and makes less power. Tuning too rich can also damage your pistons and cylinders over time as the gas washes the oil off of the cylinder walls leaving no lubrication. The engine will start to burble, stutter or misfire and turn the spark plugs electrodes black. The spark plugs should look light brown when the engine is properly tuned. Although the best way to tune an engine is with an aftermarket oxygen or o2 sensor to read the ratio. These usually have to be made to screw onto the exhaust manifold or headers where all the cylinders tubes meet. this insures that the reading is averaged. Race carbs like the holly, demon, predator and edelbrock carbs try to make tuning easier than production carbs like quadrajet, webber and carter.