What are the petcock positions?

[Guest]

My '79 GS400 has a petcock with three marked positions:

PRI - ON - RES

Can anyone shed some info on these positions and what they offer?

PRI = Primary tank I assume?
ON = errr... ON?... what was wrong with Primary? What's this for?
RES = Reserve fuel, I guess this just switches to another pickup lower in the tank? How many litres is the reserve usually?

I assume there is no OFF on this?

[Sci85]

I have an '82 so not 100 percent sure it's the same but PRI is "prime" which means flow gas all the time (prime the carbs). On is normal setting and requires carb vacuum to open the petcock and allow fuel to flow. Reserve is just that.

[azr]

Ok.....PRI is for when you need to prime the carburetors with fuel (the bikes been sitting for a long time and the bowls on the bottom of the carbs that hold the fuel have dried up, or you've drained the bowls from having the carbs off, ect...) ON is where it will sit almost all the time, fuel will only flow out of the gas tank in this position when the bike is running. This is done by vacuum. If you look at your petcock you will see 2 lines attached to it. The large one is the fuel line and the small one is the vacuum line, which provide vacuum from the engine only when you have the bike running, hence fuel will only flow to the carbs when running (unless the petcock is toast then it will flow on it's own which is bad...very bad). RES is as you guessed your reserve, and yes it just draws from a lower hole. How much you have depends on what bike you have. OFF is basically the ON position as it will only allow fuel to flow with vacuum, when the bike is running. To test if your petcock is still working properly, take off the fuel line and see if gas drips out. If so you are screwed well you need a new petcock anyways and need to change your oil as that is where the gas will eventually go.

[brettule]

Ahhh ok. I recall fuel ****ing out when I had the petcock set to PRI and I removed the fuel line. I've just been riding it for the past couple of days on PRI not knowing the difference. This arvo I switched it to RES and rode for 15 minutes just to see if there was any difference, I couldn't notice any.

[azr]

Opps...check your oil for the smell of gas. If you rode it on prime for a couple of days there may be a chance gas overflowed into your oil. If so, change it immediately. The test is simple, pull off your oil cap and smell.

[brettule]

Slight smell of petrol, certainly not like a tank of fuel. Won't the fuel burn off over time? The odometer tells me I rode for 30 kilometres on PRI. Am I safe?

[azr]

Change your oil!! oil is cheap, replacing your pistons and rings is not. The gas thins out your oil and that is bad news for your pistons. You are safe right now but do yourself the favour and change your oil. Then check your petcock and make sure it's not leaking. This is all routine maintenance with these bikes.

[brettule]

Bugger, I just changed the damn oil.

Set the tap to ON and pulled off the fuel line, a little bit of fuel came back out the tap but then stopped. I assume that means it's still good to go?

PS: Who makes a bike where (if you don't have the manual) you can inadvertently thin out your oil with fuel? That's just evil! Tell me there is no sugar compartment that releases into the fuel tank if I don't tap dance to the 10th integer of Pi on the gear shift lever?

[tkent02]

Who leaves a bike out in the rain for 35 years and expects it to work perfectly with zero maintenance and not even knowing how it works?

[koolaid_kid]

Bugger, I just changed the damn oil.

Set the tap to ON and pulled off the fuel line, a little bit of fuel came back out the tap but then stopped. I assume that means it's still good to go?

PS: Who makes a bike where (if you don't have the manual) you can inadvertently thin out your oil with fuel? That's just evil! Tell me there is no sugar compartment that releases into the fuel tank if I don't tap dance to the 10th integer of Pi on the gear shift lever?

Yes, a slight amount of fuel is appropriate when the petcock is set to ON.
The GS owner's manual instructs the owner to not leave the petcock in the PRI position for very long or it may leak at the needle/seat.
I know you have been working on this bike for quite a while to restore it back to reliable condition, but you may wish to rebuild the carbs at some point. Use the rebuild procedure on BassCliff's web site, where you can also find the Owner's Manual for a 450A. Not quite your bike, but pages 10-11 discusses the petcock, which it calls the "Fuelcock".
I would also use Nessism's guide as a superb overall document for carb rebuilds, including tools required, sources for parts, etc.

[bwringer]

On is only on when the bike is on, otherwise it's supposed to be off. Pri is always on, even when the bike is off. Res is just like on, except you have access to more gas than on.


Got all that?

[bwringer]

Seriously, now:

A few things:

1) The floats and needle valves in a GS are not designed or capable of holding back a tank of gas with absolutely no leakage. Under ideal conditions, they will still seep a little bit each day. If you stick a GS in the garage with the petcock on PRI, some gas will end up in the crankcase. That's why Suzuki installed a vacuum-operated petcock -- you don't have to remember to turn it off.

2) You are not dealing with ideal conditions here... you'll notice that you never have to touch the petcock under normal riding conditions. When you move a petcock lever that hasn't moved for a few years, you will inevitably release crumbs of corrosion have built up inside the petcock. These often then block the float needles open, so the bike pees gas on the ground when you fire it up. So watch for that -- it's very common when someone acquires a GS. You also often get some leaks in the seals in this situation.

The best solution is to install a new petcock and worry about something else for the next 20-30 years. The rebuild kits usually don't work.

[Brendan W]

Back in the day, before vacuum operated fuel taps, I regularly found bikes parked up in Dublin city centre piddling fuel on the floor. Invariably the fuel tap was ON and I was kept fairly busy turning them to OFF.

[storm 64]

Your a good man Brenden.