Engine Maintenance and Modification Bradley Artigue

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FIAT 124 Spider Engine Maintenance + Modification meter being set too low, so that the pulse of air during idle keeps turning the pump on and off. Adjust the rod as described in the “Air Flow Meter Failure” section above. The fuel pump may begin behaving erratically or running “all the time” (draining the battery). The pump is activated by a chrome rod inside the air flow meter. When the AFM flap moves the pump turns on. Therefore when your engine is cranking, idling, or cruising, the pump is on. Over time the rod may move out of position; it can be fixed by lifting the black cover off of the AFM and bending the rod so that the pump is on only when the flap has moved. 58
FIAT 124 Spider Engine Maintenance + Modification 6. The Cylinder Head If you open your hood and look at the center of the engine you’ll see four spark plugs. The aluminum thing that your spark plugs are screwed into is the cylinder head. The cylinder head on a FIAT 124 Spider is of the semi-hemispherical, dual overhead cam (DOHC) type. It is called semi-hemispherical because the operation occurs along a 180 degree arc. The two camshafts sit on either side of the spark plugs and rotate in time with the engine main crankshaft to draw in, burn, and expel fuel. A cylinder head has three major components: Camshafts are metal poles with lobes on them. The lobes press against valves that open and close, allowing gases into and out of the engine. Lobes are engineered into a specific pattern, resulting in the camshaft being the mechanical brain governing the Otto cycle. Valves cover ports between the intake and exhaust manifolds and the combustion chamber. Valves are held closed by a high tension double-spring assembly. The spring is held down by retainers installed in a donut-shaped cap. The entire assembly is capped off by a bucket. The camshaft strikes a shim on the top of the bucket, opening the valve. The shim is adjustable to maintain proper valve clearance. Spark plugs descend into the combustion chamber and create a spark that ignites the air/fuel mixture. We start this section with the intake of fuel from the fuel system and finish with the expulsion of spent fuel into the exhaust system. Assume your engine is just starting the intake cycle – the camshaft has depressed the intake valve, drawing air and fuel into the combustion chamber (figure 41.1). As the camshaft rotates the valve closes. The piston is pushed upwards, compressing the mixture (figure 41.2). Ignition occurs (figure 41.3), exploding the mixture and forcing the piston down. Finally, the camshaft opens the exhaust valve and the piston is drawn up, expelling the burned mixture (figure 41.4). 59
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