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.
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EGR stands for exhaust gas recirculation. The EGR valve will generally make the car run poor at lower speeds or idle, but can effect it anywhere. The valve should not open at idle. Operate at midrange speeds and quit again before wide open throttle. The egr valve is usually located on the intake manifold or head. There is a diaphram in the egr valve. The valve uses manifold vacuum on the diaphram to lift or close a valve allowing exhaust gas to enter the intake manifold. An easy test for the diaphram is to remove the vacuum line that runs to the egr valve. You can normally reach the diaphram area with your finger. Move this opposite from the direction it normally sits at. Then cover the vacuum port located on the egr valve with a plugged hose or your finger. The diaphram should either stay in this other position or slowly move to the normal position. If it won’t hold vacuum and instantly returns to it’s origional position, either a vacuum port is still not plugged or the diaphram is leaking. If it is leaking replace the egr valve. If the egr valve holds vacuum then remove the valve from the manifold.
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The cam is a very critical part of an engines valvetrain. The cam is what lifts the valves into the combustion chamber allowing air to enter the cylinder. Cam profile selection is very critical on how an engine will run, and how log the cam will live. The cam along with the cylinder heads control how much air can enter the engine. The more air in the engine, the more power that can be made (within reason). This does not mean the most aggressive cam is the best one for you. You can get away with a fairly agressive cam on a carbureted vehicle. Be careful on a fuel injected engine as too much overlap will drop vacuum and will not allow the fuel injection to work correctly.
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A basic car’s engine can be broken down into terms that anyone can understand. The goal of an engine is simple, move the car, which is accomplished by turning the wheels. The wheels are connected through the driveline (transmission, rear end gears), which connects to the engine. The rotation comes from a part in the engine called the crankshaft. The crankshaft is connected to a straight “bar” of metal called the connecting rods. On the top of the rods are pistons, cylindrical aluminum pieces that have rings of metal around them. The rings seal them close to the cylinder bore. The cylinder bore is simply a long hole that the pistons can move up and down in. This up and down motion pushes the rods down and rotates the crankshaft. Make sense so far? Next, the piston gets it’s motion from an “explosion” that happens when it is in the top part of the cylinder bore. This makes the piston move downward very quickly. For an “explosion” to occur the engine needs two things, air and fuel. At the top of the cylinder bore, it is sealed, to force the piston down, with a cylinder head. The cylinder head also allows fresh air to come into the cylinder bore and after the “explosion” allows the used air (exhaust) to leave the engine so more fresh air can come in next time.
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The variable valve timing used the best of two camshaft profiles. Some of the most common abbreviations for this are the Honda vtec and Toyota vvt-i valvetrain. Although I believe the Acura nsx was one of the first to use it. Other systems are now being used by all manufacturers. Some systems actually use two separate camshafts to benefit from low end torque, good emissions, and top end power with the other cam. The vtec uses a second lobe on the same cam with another profile. Others use a wide cam lobe with a different profile on each side of it. They then move the cam side to side. The problem with using one cam is compromise. Most single or dual overhead cam vehicles are tuned for mid range power and low emissions. This means reasonable lift numbers, but not optimal. Lift is simply how high the cam lifts the valve into the head chamber, to let air pass through it. The overlap is the amount of time that the intake and exhaust valves are opened into the chamber at the same time.
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The valvetrain includes the camshaft, lifters, followers, rocker arms, pushrods, retainers, springs and valves. It determines, along with the heads, how much air can pass into an engine. Most valvetrain system are very mild from the factory. This is why alot of horsepower can normally be extracted from a mild factory setup. The cam is usually the best place to extract power and can be purchased for a reasonable amount.
The oxygen sensor or o2 sensor measures the oxygen content in the exhaust. The more fuel placed into the combustion chamber (rich condition) the more oxygen that will be used in the combustion process. When the oxygen content in the exhaust is low, there is a chemical reaction in the sensor. This produces a higher voltage up to 1 volt. Lower voltage is for more oxygen content and can go down to .2 volts. The oxygen sensor will not work when it’s cooler than 600 degrees F. At less than this temp it displays .45 volts same being if there is a problem with the sensor. The computer will show an error (engine light)if at this value longer than what it should take to normally warm up. The o2 sensor continually shifts above and below .45 volts. If you believe the o2 sensor is bad you can test it. First run the car for 5 minutes. Disconnect only the wire running to the oxygen sensor. Then connect a ohm/volt meter with black to ground and red to the o2 wire. Set the volts to MV (millivolt) setting, the o2 should fluctuate between 100 and 1000 mv. If it dosen’t, it is not good.
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