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Old 19-05-06, 01:36
jim sewell jim sewell is offline
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Join Date: Jul 2004
Location: guildford perth western australia
Posts: 305
Default Fuel problems in older vehicles

Just to add a few points to what has been mentioned before about vapour locks , fuel boiling and starvation .

Mechanical fuel pumps should produce about 3 to 5 PSI pressure idling and have about 10inches of vacuum suction . the pressure is determined by the amout the diaphram spring is compressed so worn links and arms will cause the pressure to be lower and the volume pumped to be less .

Placing the plastic inline filter between pump and carby has several advantages , it acts as a serge chamber of the pump pulsations and may prevent needle and seat problems and also acts a expansion area ( usually these filters are part filled with air ) when the vehicle stops after a hot run and the heat from the stopped engine raises the temp in the fuel line thus expanding the fuel and compressing the trapped air and not forcing the needle open at the carby ( flooding ) .

In some cases vapour lock can occur on the suction side of the pump when the suction fuel line and fuel pump reach higher temperatures , the reason being that pressure increases the boiling point ( vapour ) whereas vacuum or suction will decrease it so we can have vapour lock occurring in the suction line if the vehicle has some sort of restriction in the pick up line ie partially blocked filter or pick up screen .

Fitting an inline electrical pump on a remote switch as close to the tank as possible is a good safe guard and still use the mechanical for every day use . the electrical can be turned on when the vehicle has not been used for a while or when the vehicle runs out of fuel to save those long cranking spells and if there is a suspected fuel supply problem as stated above .

An electric pump makes it quick and easy to check if there is a fuel supply problem by using the remote switch and disconnecting a fuel line near the carby and safely observing the fuel flow with engine stopped .

Regards
Jim S.
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