How do you prime this type of petcock?

[gorbys]

 

Colleague of mine bought an 80' gsx750 I'm helping him get it back but I've never seen this type of petcock and I wonder how to get it to prime? I tried to turn it with a screwdriver but nothing happened.
Fuel flows if applying vacuum to the vacuum port so it's working in that sense.

[bunk]

I use a vacuum tester.

[Thierry]

When my 1979 GS 1000 has not been ridden for a few days, it shows its unhappiness by refusing to start. In addition to turning the petcock a quarter of a turn with a screwdriver to prime it, I use two tricks:

One based on a tip from a member of this forum: squeeze the front brake handle and rock the bike back and forth

Another based on a tip from my mechanic: tap on the carburettors with the handle of a screwdriver (or anything else) so the floaters get unstuck.

Only then will  the engine start.

[gorbys]

14 hours ago, TonyGee said:

you turn it a quarter of a turn with a screwdriver to prime and it should flow, if not it'll need a rebuild. 

Just out of curiosity how does it do that mechanicaly speaking? I opened it up since it didn't flow to begin with(turned out the oring and piston was stuck in the bore) but i couldn't figure out how turning it would provide fuel since its on the vacuum side and also it isnt connected to the diaphragm or piston in anyway.

Just curios

[gorbys]

On 8/9/2024 at 6:19 PM, TonyGee said:

totally wrong on this tap  I'll let the factory manual do the talking. 

 

Nice find! But if I understand this correctly then even if you turn it to prime you still need some vacuum applied beneath the diapraghm before it'll start to flow,
either by cranking the engine a little bit or by giving the vacuum hose some sucky sucky action...

[Poldark]

Can you simply replace it with one having the normal lever?

[davidfinch1960]

2 hours ago, Poldark said:

Can you simply replace it with one having the normal lever?

Quite a few new ones available on a certain auction site.

Just be aware that most of those available now are circa 44mm between the centre of the mounting holes.

Some (like on my GS1000S) need the 50mm ones.

HTH

[gorbys]

21 hours ago, Poldark said:

Can you simply replace it with one having the normal lever?

Pretty sure you can as long as the cc between holes is the same. 

[cscott84]

I just got a 1980 gs850g this spring.  I fould that I need to give the starter a quick spin to start the primer fuel flowing,  then it will flow until I turn the tap back to the on position. 

I was also able to make a 44mm tap work by carefully groveling out the mounting holes to fit the 50mm holes and you have to enlarge the hole in the tank to except the filter post.  I then made a gasket to fit, and coated it with gasket maker sealant.  Works fine so far.

Edited September 7, 2024 by cscott84
Spell correction

[Anders74]

On 8/8/2024 at 11:21 PM, gorbys said:

 

Colleague of mine bought an 80' gsx750 I'm helping him get it back but I've never seen this type of petcock and I wonder how to get it to prime? I tried to turn it with a screwdriver but nothing happened.
Fuel flows if applying vacuum to the vacuum port so it's working in that sense.

Did you get any answer how you set it? Ive looked but cant find it! Is the PRI 5 before 12 a clock? And on 12 a clock?

[TonyGee]

these are a very odd tap, in the PRIME position you have to turn the engine over a few turns to open the tap, they need a vacuum to work, once the carbs are full you turn to the ON position that only works with vacuum.  as I said very odd 

[Anders74]

i have problems with this becouse when i start the motorcycle and the vacuum starts and then stops the engine, it still comes petrol! So i go nuts! = )

Where is the ON position and is the PRI  5 minuts before 12 a clock!?

[gorbys]

On 9/25/2025 at 9:58 PM, Anders74 said:

i have problems with this becouse when i start the motorcycle and the vacuum starts and then stops the engine, it still comes petrol! So i go nuts! = )

Where is the ON position and is the PRI  5 minuts before 12 a clock!?

see the pictures in my post from august 13.