Senior
Posts: 7278
Joined: Oct 2009
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Check your spark for a nice blue spark about 1/2" long when the engine is idling and a plug wire is heald near a head nut. If it is orange or very short, your problem is in the ignition system. If the spark is normal, you may have a fuel problem. Insufficient fuel delivery: This condition shows up when driving at highway speeds as a miss or the engine dies. Vapor lock occurs when the fuel pump output pressure is very low and the engine is hot. It is much rarer than commonly thought. Most cases of suspected vapor lock can be attributed to another problem such as a weak coil or condenser, but can be caused by a weak fuel pump. To determine if you really have vapor lock, remove the air cleaner and look in the bowl of the carburetor, while operating the accelerator linkage. You should see two streams of gas in the carburetor if you don’t, you have a fuel pump problem. Fuel pump pressure is controlled by the return spring under the diaphragm, too little pressure means the spring is weak, too much pressure means the wrong spring was used. Pressure should be between 1.5 - 3.0 lbs at all engine RPMs. Too much pressure will not allow the float to control the amount of fuel in the carburetor bowl, low pressure will affect fuel delivery. I like to see 2.5- - 3.0 lbs of pressure. To check fuel delivery, you need a graduated 16 ounce or larger container to determine the amount of fuel delivered in a measured time. Disconnect the fuel pump line from the carburetor and direct it into a graduated container. Crank the engine long enough to get 2 ounces of gas in the container and then continue cranking for 15 more strokes of the pump. The pump should have delivered 6 more ounces of gas. Insufficient fuel delivery can be caused by a vacuum leak, plugged flex line, worn pivot in the pump arm, stretched diaphragm, plugged fuel line or tank pickup, plugged tank vent or wrong gas cap or a worn push rod. The push rod should be 8 7/8" long for the cast iron manifolds and 7 7/8" for the aluminum manifolds. It travel is about 3/8".
Tom
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