Petcock ignorance

[SiP]

I am going to show my lack of understanding here but....

Can someone explain the petcocks to me? Why can't I use an open / close valve to just turn fuel flow on and off when I stop?
Do they perform some mysterious function that I am oblivious to?

Thanks,

[skids]

In the special's factory configuration, if they are left "on" then they are supposed to open and close fuel flow automatically. This works by the vacuum from the engine acting on the diaphragm, which in turn pulls open the valve on the other side of the diaphragm. The petcock will bypass the automatic function by switching it to "prime." Of course, you have to be able to stop fuel flow when you pull the hoses off of the petcock, like when you remove the tank from the bike, so there is an "off" selection on the special's petcocks. The diaphragm of the standard is built into the petcocks instead of the octopus, hence there is no need for one on those models.

[TopCatGr58]

Hey SiP,

The Standard's Petcocks are vacuum controlled, and so they are not supposed to flow when the engine is not running, that's the special control, the petcocks are connected to the engine vacuum via the synch ports on the intake manifolds.

The Special's don't have that feature, they are just on/off valves, that's why they incorporated the OCTOPUS vacuum controlled valve assembly, but many of us with specials got rid of the Octy, and just remember to turn off the petcocks when we park it, JUST IN CASE the carbs float valves start to leak which could cause fuel in the oil!

So...you can use a standard on/off valve alone, but like you said, would be best to remember to turn off when parked!
T.C.

[SiP]

Thanks folks.

[81xsproject]

Sip, notice in the pics I sent you that the prime ports on my petcocks are blocked with JB welded bolts. I just turn the gas on and off. Plus, if you do the routing like we talked about you can run on just one.

[Hired_Goon]

Originally posted by 81xsproject
Sip, notice in the pics I sent you that the prime ports on my petcocks are blocked with JB welded bolts. I just turn the gas on and off. Plus, if you do the routing like we talked about you can run on just one.

I did mine differently. I blocked off Reserve for the off position, mainly to leave prime available to fill the carbs up after having them removed. I did this the day I found 20 litres of gas supplementing my 3 litres of oil.

After all, Prime and reserve both work from the same part of the tap. When my bike starts running out of gas I always switch to prime instead of reserve anyway. Seems to fill quicker.

But this method may only work on a standard because IIRC I had to swap the internal peices of the petcocks from left to right to make it work correctly.

[TADracer]

Well after about 23 years of leaky petcocks I switched to aftermarket cocks a few years ago. Considering what a rebuild kit cost, it is not much more to just buy a cheap pair of cocks instead of rebuilding the factory ones.

I installed the ones that use a 3/8" NPT thread so I needed to mount some adapters to the tank but that is a one time thing. I put on cheap ALFA petcocks and they are now alomst four years old and have not leaked a drop. And, they flow about 20% more fuel over the stock petcocks (yep, I did a test before I removed the factory cocks).

Yeah, you have to turn them on and off manually but I had to do that anyway. If I ever decide to change them out and want to step up to Pingles, they will just screw right in. But then, a new pair of ALFAs is only about $50.00 compared to $OUCH.00 each for Pingles.

[ross]

tadracer....that sounds like that plan worked for you...but if u r a cheap b*stard like me, i just went down to the lawnmower shop, bought a couple feet of fuel line and two plastic shutoff valves...yes, just like on a lawnmower...route the fuel line around the outside air box boots, put the shutoffs on the outside of the boot, and route into the carb...tie it off to the boot with a black ziptie and u r done.....about $7.....has worked perfectly for 2 years.....(sad to say i learned the hard way and drained a lovely fuel/oil mixture when i did the oil change)...hope this helps all involved....if ya need pics of it, let me know and i should be able to demonstrate....ross

[corey]

I just finished something similar to what Ross did, except mine was a bit easier cuz of pod filters. Also, I fabricated some metal plates to mount to the tank and soldered some brass nipples in the middle of 'em to let the fuel flow.

Look at the tech article for more info on this.