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78 GS750 petcock

  • Thread starter Thread starter shadowfist
  • Start date Start date
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shadowfist

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what exactly does the 'pri' position on the petcock do, i understand that it is sposed to make gas flow somewhere etc... but what is the purpose of it? and how does it work? IE: do i have to put it to 'pri' and crank it a few times? or do i put it to 'pri' for a few seconds with the electrical systems on without cranking and then put it back?

the reason I ask is my bike right now is stranded at a friends house i forgot how cold it got durring the night and i couldnt get it started again that night so i had to leave it there (drained the battery trying to start it) but when i went to grab it the next morning i couldnt get it to kick start either. the guy i bought it from used to start it when he had problems getting it started by spraying starter fluid or something of that nature into the air filter and then he said 'it starts right up that way'. isnt that what the 'pri' position is for?
 
The petcock you have has a diaphragm that is 'sucked' inwards when the motor is running. Said diaphragm has a little piston attached to it that then moves out of the fuel route through the tap allowing the fuel to flow. This operates when the tap is on the on or reserve position. When you turn the tap to prime the diaphragm is moved mechanically by a little prong inside the tap, allowing fuel to flow without the need for the engine to be running (or cranked).

The idea of the prime is to refill the carb float bowls after they have been stripped and rebuilt or to allow the carbs to fill after the bike has sat a while (= fuel evaporated from the bowls). Once the motor is running the tap should be switched to on - don't leave the tap on prime when the motor is switched off as the float needles aren't designed to hold back fuel flow under this circumstance and you could end up with fuel getting into the motor.

Having said that, if your battery is ok your bike should have started without the need to swap to prime as you would have had the vacuum operating on your tap for plenty long enough to fill the float bowls - especially if you had ridden the bike that day. Bad starting is usually either due to a poor ignition set up (dirty / out of spec points etc) or your valves need adjusting.
 
Weak battery can do it. Just does not spin it fast enough. Usual procedure on GS is to put it on prime for few seconds- turn back to On, full choke, NO throdle. start bike. The petcock has a seperate passage that allows fuel to flow in the prime position. On position is turned on by the engine vacuum.
I would jump it with a car battery If it is real cold, car off!
 
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I've only used prime after a carb cleanup, or after sitting for the winter. To start my bike when it's been sitting for a while or if it's a bit frosty out there, I do the following:

With the ignition off, kick the engine over 5 times with the kick start. Lever the choke to about halfway, then fire her up with the electric starter. It takes only a few seconds before she catches and growls to life.
 
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