Rebuilding a petcock

[Anonymous]

I had a badly leaking petcock on my 1978 GS1000E and decided to do a rebuild. The valve was dripping fuel all down the side of the machine and must have been doing it for quite a while judging by the stains. The previous owner's neglect was sickening.

After ordering the standard rebuild kit from a source on eBay, I cleaned up my unit, threw in the new diaphragm, o-rings and such and put everything back together. Threw it back on the tank and started filling the thing with gasoline, only to discover that fuel was flowing out of the valve with the engine off. I took the thing apart and reassmbled it several times and was quite at a loss as to why it should not function as designed.

Of course, I referred to the tech info section here on the GSR and found this to be a common problem and a continual source of frustration for many otherwise happy GS owners. Most, if not all of the leaky petcock problems were solved with a new Pringel valve and I thought about that as an option for a while.

Being an Engineer, I resolved to find the answer to this problem, that was my quest. After some measuring and experimentation, I discovered that the o-ring at the tip of the plunger that actually blocks the fuel flow was too small!!! This was the o-ring that came in the rebuild kit.

I went to the local hardware store and got the next size up in o-rings which increased the O.D. a bit as well as the I.D. the thing sealed perfectly! It sits in the groove of the plunger a bit loosely as compared to the o-ring that came in the kit but, this was much better from a functionality stand point. Please bare in mind that the difference in size was almost imperceptible, perhaps .5mm or less. I also stretched the spring about a 1/2"-1" to get more tension on the vacuum side of the diaphragm in order to get a real good seal. I tested the whole assembled unit for proper flow at about 5-10 in/hg and was very pleased with the result.

I believe that the stock petcock on a GS1000 is the safest and most convenient form of fuel flow control and was very happy to discover that I would not have to replace it with an expensive after-market unit. It is a simply designed, very serviceable device and there is no reason that it should not function perfectly if the correct replacement parts are used.
I believe that the o-rings in the rebuild kits are the wrong size and this leads to great confusion and frustration.

BTW, the o-ring cost me a mere 19 cents. Hope this information helps out those that have had similar problems.

[Billy Ricks]

I still have my stock petcock. Over the years it has failed to shut off a couple of times. Just take it apart and make a slight adjustment and it's good to go again. All it takes is disassembling it and figuring out what makes it tick.

[KEITH KRAUSE]

Glad it worked out OK for you. You took a chance by stretching that diaphragm spring though. It will close tighter but it WILL take more vacuum to open fully. Hopefully, there is more than enough vacuum present and some margin for error. Otherwise, you'll have fuel starvation most noticable at higher speeds.

[Anonymous]

Glad it worked out OK for you. You took a chance by stretching that diaphragm spring though. It will close tighter but it WILL take more vacuum to open fully. Hopefully, there is more than enough vacuum present and some margin for error. Otherwise, you'll have fuel starvation most noticable at higher speeds.

It is amazing how much work even a few inches/hg of vacuum can do.

I did check operation with a calibrated vacuum source.

Thanks for your input.

[KEITH KRAUSE]

Glad it worked out OK for you. You took a chance by stretching that diaphragm spring though. It will close tighter but it WILL take more vacuum to open fully. Hopefully, there is more than enough vacuum present and some margin for error. Otherwise, you'll have fuel starvation most noticable at higher speeds.

It is amazing how much work even a few psi of vacuum can do.

I did check operation with a calibrated vacuum source.

Thanks for your input.

Hi. I'm just curious and always like to learn if I can.
To check for the diaphragm fully opening, you say you used a calibrated vacuum source. Does this mean you measured the vacuum the motor provides and then set your vacuum source the same? How much vacuum does your motor provide? Thanks.

[Anonymous]

Glad it worked out OK for you. You took a chance by stretching that diaphragm spring though. It will close tighter but it WILL take more vacuum to open fully. Hopefully, there is more than enough vacuum present and some margin for error. Otherwise, you'll have fuel starvation most noticable at higher speeds.

It is amazing how much work even a few psi of vacuum can do.

I did check operation with a calibrated vacuum source.

Thanks for your input.

Hi. I'm just curious and always like to learn if I can.
To check for the diaphragm fully opening, you say you used a calibrated vacuum source. Does this mean you measured the vacuum the motor provides and then set your vacuum source the same? How much vacuum does your motor provide? Thanks.

Keith,

I measured about 7"/hg at 1100rpm with a clamp style damper on the vacuum line to an automotive style dial vacuum guage.

As far as what it took to open the fuel valve, quite honestly, I don't remember now exactly but it was very low, about 2-3"/hg.

I was concerned about stretching the spring too much but at that point I felt that the valve not opening far enough was less damaging and easier to correct than not closing.

I also tested my inlet needle valve operation on all my carbs by removing the float bowls and blowing into the fuel line with the carburetors upside down (all 4 as a unit) and noting any loss in pressure in my less than calibrated mouth. :roll:

Now, I'm reasonable secure in knowing that my petcock is closing properly and as a safeguard, my needle valves are not allowing too much fuel to the float chambers. At the same time, I'm pretty sure I'm getting good fuel flow when the engine is running. I'll have to do a W.O.T. test to make sure.

[KEITH KRAUSE]

Okie-doke! Thanks.