Fuel cock

My fuel petcock on my kz650 is kinda worn down and i cant tell what position is what i took the tank off and when its facing all the way up no fuel comes out of the valve and to the left fuel flows out and all the way down fuel flows out. I dont know if mine is broken or what but it looks like all the way up says RES but no fuel comes out when i have it in this position. Im kinda new to this all so sorry for these simple questions just wanna make sure i know how to get to res incase i run out of fuel

Please put the year and model of your bike on your signature line. That will help folks help you.

IF your bike is a 1978 KZ650, and if it has the stock petcock the petcock will have 3 positions. One position is on, one is prime, and one is reserve. The image below should be similar to the stock petcock.

Prime should be used if the bike has been sitting a while as it will allow fuel to flow to the carbs even if the engine is off.
On should be used whenever the engine is running. It allows fuel to flow when the engine is running because vacuum from a line to the #2 carb opens a diaphragm in it. When you turn off the engine the vacuum stops and the petcock stops the fuel flow.
Reserve should only be used when you run out of gas with the petcock on "on." Reserve lets the fuel remaining near the bottom of the tank flow to the carbs. Ed

Awsome so when i took tank off the fuel didnt flow when on the on position which makes sence because of vacuum but in prime and on res the fuel flowed freely is this a problem ?

No, that's how it is designed to work. Ed

awsome thanks i got 200km on 10l of fuel does this sound right ?

insutama wrote: Awsome so when i took tank off the fuel didnt flow when on the on position which makes sence because of vacuum but in prime and on res the fuel flowed freely is this a problem ?

RES -->> fuel flows freely?

81 KZ650-CSR:
ON & RES -->> fuel only flows when engine is running.
PRI -->> fuel flows -->> engine off or engine running (PRI bypasses the diaphragm assy).

Sounds GREAT!

200 kilometers = 124 miles
10 liters = 2.64 US gallons

124 / 2.64 = 47 mpg

That is excellent! My fuel economy is never that good. Maybe the ethanol in our crappy fuel has something to do with it, but I'm very lucky if I see 42 mpg. Ed

martin_csr wrote:

insutama wrote: Awsome so when i took tank off the fuel didnt flow when on the on position which makes sence because of vacuum but in prime and on res the fuel flowed freely is this a problem ?

RES -->> fuel flows freely?

81 KZ650-CSR:
ON & RES -->> fuel only flows when engine is running.
PRI -->> fuel flows -->> engine off or engine running (PRI bypasses the diaphragm assy).

Thanks for the correction. My bike has a manual petcock, and I was not aware that on petcocks with a diaphragm RES requires vacuum. I wonder how the diaphragm can be shutting off fuel flow properly for the RUN position but failing to do so for the RES position? I would expect it to fail on both if the diaphragm was damaged. Ed

Maybe he's mistaken & only PRI flows freely?

The owner's manual may have the info???

I just went for my first real ride on this bike and it runs great ill check again with the fuel flow but ya when i took tank off the fuel seemed to only stop flowing when i turned it to on

I was just looking in my manuel and it looks like i actually have been riding my bike in the RES position i thought the long tail part of the cock was were it pointed but looks like it oposite does this make sence how could i ride 100km on RES or would this work ?

Inside the tank the petcock has 2 ports to the fuel. One port has a brass tube that sticks up in the tank about 1". This is the one that is normally used when riding. When you are riding with the petcock in the ON position this port will allow all but about 1" of fuel to be used before the carbs run dry. The second port is the reserve port and it is pretty much even with the bottom of the tank; this is the one that is used when the petcock is switched to Reserve.

So when the fuel level gets down to about an inch from the bottom of the tank while riding with the petcock in the ON position the port that has the brass tube sticking up will run dry and you then switch to Reserve so you can use the last 1" or so of fuel in the bottom of the tank.

If you ride with the petcock in the Reserve position the bike will run fine even with a full tank of gas, but you will not have a reserve supply of fuel if you run dry. Ed