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Joined: Thu Jun 02, 2022 6:30 pm


Post by Petetherod » Wed Jun 08, 2022 8:01 pm

I would like some advice on my fuel pump fitted to a 1933 10/4. Should I feel some pressure through the handle when priming the pump? it seems that there is no pressure, car starts after a few attempts and on the button when warm and runs fine.

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Location: New Zealand


Post by Rod » Wed Jun 08, 2022 11:01 pm

Hopefully not giving you any egg sucking lessons but:
Whether or not you feel anything on the priming lever depends on the rotational relationship between the fuel pump rocker arm and the eccentric on the camshaft which drives it. If the rocker arm is on the "low" spot of the eccentric you will feel pressure, as you are now operating the pump through a full working stroke; if the rocker arm is on the "high" spot, the pump is already on full stroke and you will feel only the resistance of the priming lever return spring and the internal anti rattle spring. So whether you feel any resistance or not depends on which phase of the firing cycle the engine has stopped at.
The default for most cars with this type of pump is that the camshaft grind (the pump eccentric is also part of the camshaft grinding operation) is arranged so that when the crankshaft is on TDC, no 1 cylinder compression stroke, the fuel pump rocker arm is then on the "low" spot of the eccentric. Hence the advice sometimes given to TDC the engine on cyl 1 compression before removing/replacing the pump, as this positioning minimised the difficulty of dismounting/remounting the assembly by reducing the loading on the rocker arm caused by the compression of the fuel pump diaphragm spring.
If you strike the situation where you can't get anything on the priming lever, the usual advice is to bump the engine over until you can. Some handbooks actually mention this.

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