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.

[TonyGee]

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

[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

[TonyGee]

34 minutes ago, gorbys said:

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

if i remember right theirs a hole in the bit you turn and lets the fuel flow through channels, but ive got the exact same tap to strip and clean as its full of crap so will know for sure  

[TonyGee]

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

 

[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...

[TonyGee]

4 minutes ago, gorbys said:

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...

TBH i couldn't get me head round it either 

[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. 

[TonyGee]

the 50mm taps don't come with a lever as far as i know !!!!  ive looked many times and not found one. 

[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 by cscott84
Spell correction