Carb troubleshooting
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From: South Carolina
Carb troubleshooting
1970 Ford 460cid, Motorcraft 4300 4 barrel carb. Motor will turn over but will not crank up unless I manually pour gas into carb. Disconnected fuel line going into carb and fuel is coming out of it when motor is turning over, so fuel pump is not the culprit... also, removed carb from manifold and it's full of fuel, so gas is definitely going into carb...
So what to check next? Not a carb guy at all - never even took one apart, so thanks for anything.
So what to check next? Not a carb guy at all - never even took one apart, so thanks for anything.
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Joined: Jan 2002
Posts: 4,907
Likes: 39
From: suburban atlanta
Is the "fuel" in the carb old?
Has the engine been sitting for a while?
Generally speaking, you have to press the gas pedal to the floor once before cranking. This uses the accelerator pump to squirt a shot of gas into the intake, and sets the cold idle cam on a step based upon temperature. It also releases the choke plate so it can close to enrich the mixture.
If the fuel is old, it may not want to ignite properly. If the idle jets are plugged from sitting, there will be no fuel to run on at idle speed, so you would have to hold the throttle open enough to possibly pull fuel through the main jets and venturis.
If the fuel in the carb is fresh, and has not been sitting around for months/years, I would remove the idle jet screws and squirt some carb cleaner into the openings. If you do, count the number of turns from the original position to being screwed in all the way GENTLY, so you can approximate their original settings when you replace them. In other words, you screw them to their seat while counting, remove, clean, then screw them into the stop again, and then loosen the same number of 'turns' originally counted.
To be more sure of clear idle passages, I would remove the carb, remove the idle mixture screws and ensure that the passages to the intake are clear using air pressure or carb cleaner. You can pretty much figure out where the passages go by following straight lines - they use drills to make the passages, and when they do 'corners', they drill along the next angle and use a metal plug to close the hole. The idle mixture screws likely will go into the sidewall of the carb just below the throttle plate. If you screw them in to their stop, you may see the tip of the idle screw protruding.
tom
Has the engine been sitting for a while?
Generally speaking, you have to press the gas pedal to the floor once before cranking. This uses the accelerator pump to squirt a shot of gas into the intake, and sets the cold idle cam on a step based upon temperature. It also releases the choke plate so it can close to enrich the mixture.
If the fuel is old, it may not want to ignite properly. If the idle jets are plugged from sitting, there will be no fuel to run on at idle speed, so you would have to hold the throttle open enough to possibly pull fuel through the main jets and venturis.
If the fuel in the carb is fresh, and has not been sitting around for months/years, I would remove the idle jet screws and squirt some carb cleaner into the openings. If you do, count the number of turns from the original position to being screwed in all the way GENTLY, so you can approximate their original settings when you replace them. In other words, you screw them to their seat while counting, remove, clean, then screw them into the stop again, and then loosen the same number of 'turns' originally counted.
To be more sure of clear idle passages, I would remove the carb, remove the idle mixture screws and ensure that the passages to the intake are clear using air pressure or carb cleaner. You can pretty much figure out where the passages go by following straight lines - they use drills to make the passages, and when they do 'corners', they drill along the next angle and use a metal plug to close the hole. The idle mixture screws likely will go into the sidewall of the carb just below the throttle plate. If you screw them in to their stop, you may see the tip of the idle screw protruding.
tom
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