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Title: Engine Misses in Mid-range Operation
Problem: A street rod owner has 800 miles on a 1978, 350 small block engine with a Holly 4 barrel carburetor. The owner says that on a recent car run, the engine began to hesitate in the mid-range at about 35 to 40 miles per hour. At this speed, when the throttle was slightly open, such as going up a slight increase in grade, the hesitation occurred. If the throttle is further opened, the engine picks up and shows no signs of hesitation. The carburetor uses a mechanical choke that heats up electrically. What could be the problem?
Solution: The easiest thing to check is the setting of the choke. See arrow on photo. If the choke plate is just slightly closing during mid range operation, it may cause the hesitation. It could be that the bi-metal strip in the choke is starting to change its physical characteristics. This could cause the choke to be incorrectly adjusted even though the choke was correctly adjusted after the engine was rebuilt. Reset the choke so that in cooler weather, the choke plate is just completely closed. Do not go any further. Also, check the choke plate when the engine is at operating temperature. It should not be able to move toward the closed position freely. It should be solidly in place at this temperature. The reason that the hesitation went away when the throttle was further depressed was that the other two barrels on the carburetor were kicking in. If the choke is set correctly, and the hesitation is still evident, the next step would be to check the power valve in the float area.
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Problem: A street rod owner has 800 miles on a 1978, 350 small block engine with a Holly 4 barrel carburetor. The owner says that on a recent car run, the engine began to hesitate in the mid-range at about 35 to 40 miles per hour. At this speed, when the throttle was slightly open, such as going up a slight increase in grade, the hesitation occurred. If the throttle is further opened, the engine picks up and shows no signs of hesitation. The carburetor uses a mechanical choke that heats up electrically. What could be the problem?
Solution: The easiest thing to check is the setting of the choke. See arrow on photo. If the choke plate is just slightly closing during mid range operation, it may cause the hesitation. It could be that the bi-metal strip in the choke is starting to change its physical characteristics. This could cause the choke to be incorrectly adjusted even though the choke was correctly adjusted after the engine was rebuilt. Reset the choke so that in cooler weather, the choke plate is just completely closed. Do not go any further. Also, check the choke plate when the engine is at operating temperature. It should not be able to move toward the closed position freely. It should be solidly in place at this temperature. The reason that the hesitation went away when the throttle was further depressed was that the other two barrels on the carburetor were kicking in. If the choke is set correctly, and the hesitation is still evident, the next step would be to check the power valve in the float area.
Back to Troubleshooting Clinics
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