Petcock

[GS450inSF]

I picked up a 1980 gs450e a few weeks ago. I want to go though it and make sure the 25 year old beast is up to par before riding it all over. I have had dirt bikes and atvs my whole life but I have never seen a petcock with two fuel lines and only and on & reserve setting w/ a diapram. I have sceen a post that said sputtering at low rpm can be dirty carbs or the petcock diapram. The guy I bought it from said he just cleaned the carbs. So I am lead to believe it's the diapram. Is thier anyway to test it and if it's bad where can I get one? I have searched high and low... Also if someone could tell me how this style petcock works that would be great.

Shawn

Ps I am new to the site and am glad I found it.

[KuriosIesus]

I'm new here too bro. And I've only owned a GS (and my first motorcycle) for 2 month.

The second line on the petcock (which should be smaller) is not a fuel line. I thought so too. It is actually a vacuum line which connects to the carb. The carb draws air through that line to open a valve which allows the fuel to flow...unless it is set on Pri.

Well, I'm pretty certain of it since there is no way that I can see for the fuel to reach that line. I took my petcock apart.

You can find a brand new petcock for around $50.

[LW_Icarus]

thats right, the smaller hose pulls the diaphragm open, and lets the fuel flow in the "on" position, there should also be a "prime" position which allows the fuel to flow full-time.

about the only way I can think of to check it is to see how much fuel flow you get at the prime position, or to pull it out of the tank and visually inspect the screen on it.

don't trust that the PO actually fixed anything when he had the carbs apart either

[GS450inSF]

Petcock part 2

The petcock on has 2 settings "on" & "res". What is prime for and why don't I have it???

[LW_Icarus]

well, your model could be different than mine, but it has a prime position to run gas into the carbs if they are empty, so the bike will start .

reserve setting may do the same thing on yours, youll jsut haveta take the fuel line loose and turn it onto reserve and see if fuel runs out. I have a difficult time believing they wouldnt have a setting on it that runs free without vacuum.

[Red_Phil]

It sounds like the your bike has the same or similar petcock to the 1980 GSX400E.

It has a 'res' and an 'on' setting.
these allow fuel through (from either the main or reserve outlet on the tank)
when a vacuum is applied by the engine to the thinner of the 2 tubes on the petcock.
below and to the right of the petcock 'dial' is a small screw.
You screw this out 1/2 way to bypass the vacuum activated valve and get
the petcock to prime your carbs.

If your carb float bowls are empty then the engine won't run
so you get no vacuum
so the petcock won't let fuel into the carbs,
so the float bowls stay empty,
so the engine won't run,
so...............

This is why you have a prime setting. So you can refill your empty float bowls.

[Red_Phil]

oh, and my GSX400E petcock has a knackered diphragm too.

This allows fuel to drip into the vacuum line and get sucked straight into the outlet of the right carb into the engine.
This makes the right cylinder run rich.
It also lets fuel run under gravity into the engine and on into the sump
where it knackers your oil.
This route bypasses your float needle valves, so no matter how good a seal they get, you'll still get fuel into the sump.

To test for this you can either detach your vacum hose at the carb end and see if you can suck any fuel through it. (not advised, it tastes BAD)
or replace the vacuum line with a clear one, then you can see the fuel running through it.
The vacuum line should have no fuel in it, ever.

I'm thinking of removing the vacum activated diphragm.
Then capping the vacuum line nipples on the carb and petcock.
finally I'll fit an inline fuel tap on the fuel line so I can turn fuel off
when the bike is parked to avoid fuel into the sump.