I've not yet had the petcock apart on my bike (see signature).

However, a while back, I developed what appeared to be a diaphragm leak that was letting gas into the petcock vacuum line, thus making the bike run rich intermittently.  Luckily, Serowbot diagnosed that one for me.

Well, I then disconnected the vacuum line from the petcock, plugged the vacuum port at the petcock and then plugged the top part of the vacuum line.  The bike's been running great ever since.

When I shut the engine off, I position the petcock to ON.  Before I start the engine, I position the petcock to PRI.  I realize that I'm essentially running on RES at all times, but I want to emphasize two things:  This is not a problem for me, the way I ride the bike, and I do not consider having to reposition the petcock all the time an inconvenience.  As an old-school biker, this is "normal ops" for me.  As a matter of fact, I find myself wanting the shut my Gold Wing's non-existent petcock off when I shut the engine down on that bike.....and it has fuel injection!    

Are you still with me?  Here's my question:

I realize I should either do the manual petcock mod, or buy a new petcock.  However, is there any way the petcock could leak gas into the petcock fuel line when the petcock is set to ON and there is no vacuum being applied to the petcock vacuum port?

If gas can't leak out under those conditions, I see no need to do the manual petcock mod.  I can happily continue on doing what I'm doing (remember, having a RES capability is not important to me).  However, like I said earlier, I have no way of knowing this as I've never had one of these petcocks apart.

I do, however, plan to remove the petcock vacuum line altogether and plug the intake manifold vacuum port itself.

TIA!

Yes, If the diaphram has a hole in it then it will leak, it doesn't need vacuum to do that.  100% sure.

The diaphram is constructed of a thin rubber sheet, backed up with a plastic washer and preloaded with a metal leaf spring and a compression spring.  This assy has a plunger on the gas side with an o-ring to shut off the gas.

Guess what? this area loads up with sediment and corrosion.  Enough sediment and white gas boogers and it's going to hold the valve open.

If you converted the petcock to manual, what you could do is replace the diaphram assy with a rubber sheet.  Then you wouldn't need to worry about a leak.

You guys with balky engines at speed... try tightening up the four screws on the back of the petcock.  It's possible the vacuum leak is there.

ls650v wrote on 08/12/08 at 09:22:11:


Apparently your diaphram is whole and you developed abit of vacuum under you capped port keeping the gas on.

The problem of 'venting' the cap as you've done, when the diaphram does fail, you'll leave a trail of gas.

Digger wrote on 08/12/08 at 21:52:27:

Well you lazy b'tard, at least take the tank off the bike!  
And take the time to clean out the sand that might be in there.

I've thinking of adding a tour tank on the bars.  And I'm thinking of converting the petcock to manual.  Corking up the vac tap on the carb.  take out the diaphram, replace it with a open gasket.  And use the vac tap on the back of the petcock to feed in the tour tank.

verslagen1 wrote on 08/12/08 at 11:37:54:


I will take my chances.  How many people on the forum have reported gas pouring out of the petcock vacuum port?  None that I know of.  Better to dump gas on the ground than onto a hot engine.

Serowbot wrote on 08/13/08 at 01:21:50:

You did plug one of the ports with a set screw didn't you?
Off for the tour tank will be a sepparate petcock for it.

verslagen1 wrote on 08/12/08 at 22:45:07:


The tank will already be off, as I will be checking valve clearances and torquing head bolts.

I also intend to check the cam chain adjuster on this next maintenance cycle and perhaps install a Verslavy, if required.

Now, does anyone know where I can purchase some Nitrile rubber gasket stock?