Another Leaking Gas Valve Problem

Question:

My fuel petcock is leaking, really it's pouring fuel in all positions - ON, PRI & RES. Is it possible that this can flood the engine even after it's running?

I just bought the bike, which has been stored for a year or so. I dumped about 4 qts of fuel/oil (mostly fuel) from the crank case, and the fuel tank was empty. Clearly, fuel has been getting from the fuel tank to the crank case.

After putting in fresh oil, I got it running but after about 5 minutes it quit and wouldn't restart. When I pulled the carb, it was overflowing with gas. I would have thought that the petcock couldn't flood the engine after it's running. I considered adding an on/off fuel valve downstream of the petcock, but I don't think this is going to help.

Bottom line, is the petcock capable of flooding the engine after the engine is running? Doesn't this carb have a float valve & tank? Or do I have other problems?

Obviously, I'm not sure exactly how this vacuum controlled petcock is supposed to work. Does it modulate flow, or is it supposed to allow full fuel flow anytime the engine is running?   {Doug, NC  ~ 1996 Suzuki DR200}

Answer:

This is not an overly complicated or costly problem to fix.  There are two possible answers to your leaking gas valve question.


First lets explain the petcock. Petcocks are commonly and interchangeably referred to as fuel valve, gas valve, gas cock or petcock. A mechanical petcock is attached to the fuel tank and has one outgoing fuel line.  It is simply an on off valve. When in the off position there should be no fuel flow to the carb.

The second type of fuel petcock is vacuum operated.  It attaches to the fuel tank and has one or more outgoing fuel lines and one small vacuum line going to the cylinder head or carb to cylinder head air boot. Snowmobiles sometimes use a vacuum port off the engine block.

Vacuum type gas valves, when in the on and reserve positions, use engine vacuum to open a diaphragm inside the petcock. Generally a small vacuum line is attached to the number one cylinder vacuum port.  Every time the piston goes down, it creates a pulse of vacuum to allow gas to enter the carburetor bowl.

Therefore, if your engine is not running due to a crash or being stored, no fuel should get from the tank to the carb.  This prevents fires and shuts the engine off after an accident. It also prevents engine run-on after a bike goes down.

Newer bikes are equipped with electronic bank angle sensors that shut off the engine if the bike lays down.



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How to Diagnose a Mechanical Valve

Remove the supply hose from the carburetor.  Is gas being supplied to the hose, through the valve, when the lever is in all positions?  If so your valve has failed.  If not, the problem may be in your carb's float chamber (see below).

Many mechanical gas petcocks can be rebuilt with a repair kit that contains replacement o-rings, springs, rubber diaphragms, etc.  These small parts are available from OEM dealers or aftermarket sources.  The older your bike is, the more likely it is that these parts have been discontinued by the manufacturer. At that point your only option is to find a used petcock or an aftermarket rebuild kit

The top two aftermarket kits that we see are K & L Supply and Keyster.  I do not have any personal experience with Keyster, but I have not heard anything derogatory about them.  We have always used K&L Supply for our carburetor and fuel petcock rebuild kits.  K&L is the supplier for many manufacturers OEM parts. 

I checked the fiche card and K&L Supply catalog.  It looks like you have a mechanical, non-rebuildable valve.  As with all older bikes, this part may not be original to the bike so read the entire page before drawing your conclusions or attempting any repairs.  You may be able to find a used OEM or new replacement valve online.  Try Ebay or Craigslist.


How to Diagnose a Vacuum Operated Valve

If your bike is equipped with the vacuum type of gas valve you need to check 2 things.

First, remove your fuel supply line from the petcock. Then unplug the vacuum line at the back of the fuel valve. When the bike is not running, the fuel valve should only supply fuel in the prime (PRI) position. When in the on or reserve positions no fuel should be supplied.  If fuel is flowing freely, your petcock has been either altered or has internally failed.

Check the vacuum line port on the petcock. You should not have fuel coming out here, in any position, ever. Not even a drip. If fuel is leaking through the vacuum port this is very dangerous. Do not attempt to run this bike again until you have determined the cause of the fuel entering the engine oil. You can start a fire or gas wash your engine bearings, ruining the engine.

Why is fuel inside your engine bad?

A leaking gas valve can do more than just waste gasoline. If a leaking gas valve or carb float needle cause gasoline to fill the engine crankcase and you attempt to run the bike, the fuel is then pumped through the system washing oil from vital areas such as bearings. This situation is called gas washing the engine.  This removes the lubricating motor oil causing friction, which can easily damage soft aluminum parts. Even just trying to start the motorcycle engine can do damage.



If your gas valve tests good, or has now been repaired or replaced and tests good, the next area for fuel control is the
carburetor float needle.


Reminder

After all repairs to your leaking gas valve have been completed, remember to change the motor oil and filter. Be sure to check the airbox for fuel puddling and empty any drains on the bottom of the air box.

Also, it is a good idea to be sure any carburetor float bowls that are equipped with a drain nipple, have the hoses going from the bowl nipple to the ground. This prevents any fuel that is trying to escape to reach the ground. Avoiding any electrical components or hot pipes along the way.


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