slmjim n Z1BEBE
CB750 Enthusiast
Petcocks are vacuum operated. Petcock diaphragm is opened to allow flow by vacuum pulses from #2 cyl. No 'prime' position that bypasses the need for vacuum as on many other vacuum petcocks.
When the engine is running, vacuum pulses are frequent enough to essentially hold the diaphragm open continuously.
If carbs are drained, much cranking can be necessary to allow enough fuel to pass with the petcock in the ON or RES position because flow is only briefly intermittent during each vacuum pulse. This can be overcome at dry startup by applying vacuum to a hose attached to the vacuum port on the petcock continuously, either by mouth (think sucking very thick milkshake through a straw) or, using a vacuum pump. Doesn't take much; ~ 3 in./Hg or less will do it. Only takes 60 ~ 90 sec. of continuous vacuum to fill the float bowls.
Some OEM petcocks also have a port on the petcock diaphragm casting that is vented to atmosphere. The same effect can be realized by applying gentle pressure to that port, which opens the diaphragm. Again, doesn't take much pressure or time. Think blowing something stuck in a straw out of it.
Let's assume a '91 - '03 petcock is otherwise operational. Has anyone tried modifying the needle on the diaphragm to allow fuel to flow when the petcock is in the ON or RES position, yet stop flow when OFF? In other words, make the petcock manual instead of vacuum?
Good Ridin'
slmjim & Z1BEBE
When the engine is running, vacuum pulses are frequent enough to essentially hold the diaphragm open continuously.
If carbs are drained, much cranking can be necessary to allow enough fuel to pass with the petcock in the ON or RES position because flow is only briefly intermittent during each vacuum pulse. This can be overcome at dry startup by applying vacuum to a hose attached to the vacuum port on the petcock continuously, either by mouth (think sucking very thick milkshake through a straw) or, using a vacuum pump. Doesn't take much; ~ 3 in./Hg or less will do it. Only takes 60 ~ 90 sec. of continuous vacuum to fill the float bowls.
Some OEM petcocks also have a port on the petcock diaphragm casting that is vented to atmosphere. The same effect can be realized by applying gentle pressure to that port, which opens the diaphragm. Again, doesn't take much pressure or time. Think blowing something stuck in a straw out of it.
Let's assume a '91 - '03 petcock is otherwise operational. Has anyone tried modifying the needle on the diaphragm to allow fuel to flow when the petcock is in the ON or RES position, yet stop flow when OFF? In other words, make the petcock manual instead of vacuum?
Good Ridin'
slmjim & Z1BEBE
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