Leaking Carbs caused by petcock?

I have a kz550 cafe racer with an almost completely stock engine and carb set up. with an odd symptom:, the left most carb pours gas intermittently....I'm talking a crazy amount of gas.

The only difference with the carbs from stock is that I installed a manual petcock in place of the stock vacuum actuated one. Since owning the bike I have had carb leaking problems. I've replaced all the fittings/screws/needles/rubber/etc in the carbs in an attempt to fix this problem. I've tested (and replaced) the drain screw and tested the overflow tubes for cracks. The float level is correct and verified by a mechanic, the float needles are all new, I've got 2 fuel filers (one in the tank and another inline.) I've cleaned the carbs multiple times... just can't figure out what's causing this leak!

My current theory is that there is too much fuel pressure coming from the gas tank and it's pushing right past the float needle, or maybe that float is bouncing around enough to allow gas to get by to cause this leak. Either way, will a vacuum actuated petcock solve this?

If the fuel bowl level is correct and the needle valves are new and free of debris that just shouldn't happen.

Where is the gas pouring from? Does your bike have overflow tubes on the carbs? If the fuel is pouring out of the overflow tube the fuel level is too high. That is the purpose of the overflow tube. If the overflows weren't there and the fuel level gets too high the fuel goes into the intake manifold on to the top of the piston and filters down into the oil pan.

Did the overflowing carb get a new seat (and new fiber seat washer if applicable) along with the new float needle? Or is the seat permanent and non-removable so that the new float needle uses the same seat?

Given non-cracked overflow tube and properly functioning float, seems likely that the overflow is due to the interface between the float needle and the float needle seat failing to completely seal when supposedly closed. Sometimes polishing the seat orifice (can use q-tip chucked into drill) allows a better fit for the pointed tip of the float needle (to achieve full closure when supposedly closed.)

A "correct float level" should produce a fuel level within specs, which fuel level is usually measured by the clear tube test. But a leaking float valve makes it difficult if not impossible to do a valid clear tube test because the fuel level reading is steadily rising while testing until reaching the top of the overflow tube which is way higher than spec.

There's no fuel pressure inside the tank. Fuel is gravity fed from the tank.

All the petcock does is to stop fuel flow from the tank to the carb. A properly functioning petcock does not "fix" a leaking float valve. Or a leaking drain screw. Or a leaking float bowl gasket.

A severely leaking float valve absent a functioning overflow circuit can result in raw gasoline intrusion into the crankcase while riding (when the non-combusting cylinder is flooded and just sucking in raw fuel that has risen up into the carb bore).

The possibility of a leaking drain screw is a different matter entirely. For example, an otherwise perfect carb with a missing or damaged drain screw o'ring may leak fuel from the drain screw outlet even when the drain screw is properly tightened.

Another possible leak source could be a faulty float bowl gasket, or loose float bowl screw(s), or bent float bowl ears preventing proper fitment of the float bowl when attached -- another different matter entirely.

Good Fortune!

thejameswilliam wrote: ..............................
My current theory is that there is too much fuel pressure coming from the gas tank and it's pushing right past the float needle, or maybe that float is bouncing around enough to allow gas to get by to cause this leak. Either way, will a vacuum actuated petcock solve this?

1. That theory is totally wrong. Overflowing carbs can be caused by any of the problems listed below.
2. A vacuum actuated petcock won't do anything to correct the carb problem. Every petcock, no matter whether manual or vacuum operated, serves the exact same function - a simple on & off valve.

Regarding the carb overflow -

Fuel coming out of the overflow hose (the little hose on the bottom of the carb) is the common symptom of the 3 possible problems described below. Remember - NO SMOKING or other activities that could ignite the fuel while working on carbs!

The float valve is not sealing. This may be (and probably is) just a matter of some minor dirt in the float valve and is very simple to remedy and can be done with the carbs still on the bike. Turn off fuel; drain carb by loosening big brass screw near bottom; and remove the 4 little screws on the bottom of the carb bowl. This will enable you to remove the carb bowl. You will then see the float. If you carefully remove the float you will see a stubby little needle that mates with a brass orifice; together, these two pieces are the float valve. Assuming there is no obvious damage to the needle or seat, use a Q-tip and some carb cleaner to clean the seat of the orifice and the needle. Be careful not to bend the tang that is attached to the float. This tang is the piece that the bottom of the float valve needle rests upon, and it determines the fuel level in the bowl, so bending it will change the fuel level. With the float valve cleaned it should no longer leak. To prevent a re-occurrence a quality inline fuel filter should be installed between the fuel tank and carbs. If there was damage the needle and seat should be replaced as a set; they are available.

The brass overflow tube inside the carb bowl is damaged. When you remove the carb bowl you will see a brass tube attached inside and rising toward the top of the bowl. This is the overflow tube and it leads directly to the overflow nipple on the bottom of the carb. Normally, the fuel level within the bowl rises somewhat close to the top of this tube. If the tube becomes cracked or separates from the bottom of the carb bowl fuel will flow through it and out through the overflow hose. Checking the condition of the tube is very easy after you have removed the carb bowl from the carb. Simply hold the bowl level and fill it up near the top of the tube with water or alcohol and see if it leaks. A slight crack in the brass overflow tube can be difficult to find. You can connect a rubber tube to the overflow nipple, fill the bowl with water, put finger over the open end of the brass overflow tube and blow in the rubber end and look for bubbles. If it does the best remedy is to replace it although some folks have found creative ways to repair them.

The fuel level is set too high. This condition occurs when someone has bent the float tang as mentioned above. This condition can be checked using what is referred to as the "clear tube test." The test involves attaching one end of a clear piece of flexible tubing to the carb drain hole and holding the other end of the tube above the bowl/carb joint. When the fuel is turned on it should rise in the tube to a level 2.5 - 4.5 mm below the bowl/carb joint. If the fuel rises higher than that level it may run into the cylinders or over the upper end of the brass overflow inside the carb bowl (mentioned above) in which case it will run out the overflow hose. This condition is corrected by removing the float and gently bending the tang to raise the float valve needle position. Doing this may take several tries before achieving the desired fuel level.

Correcting each of these three conditions involves removing the carb bowl. This task can be made easier if a short screwdriver bit is used, and depending on which carb you are fixing a mirror may help you locate the screws on the bottom of the carb bowl. Also, it is not a bad idea to have a new carb bowl gasket available in case the old gasket is damaged while removing the carb bowl. Ed

With credit and thanks to KZr member, loudhvx

click here > s3.amazonaws.com/gpzweb/TK22mainPage/TK22mainPage.html

Good Fortune!

1981 Kz550C2 with Tk22 carbs.

Might want to merge this current thread with the one below since they are basically the same topic.
www.kzrider.com/kunena/3-carburetor/604215-carbs-leaking-fuel

As mentioned, there is an overflow tube which can sometimes develop a crack which will leak gas regardless of the float valve. I realize you said you already inspected it, but have you tested it by filling the bowl (not attached to the carb) and let it sit for a some time? Maybe heat or possibly it leaks from some stress when the bowl is tightened down. If it's cracked you migt be able to solder it or put a piece of heatshrink over it, or seal it with some other ethanol-proof substance.

Does it start leaking after being parked on the side stand? Or does it leak while the bike is standing up and running?

The carb float should be inspected for a dimple on the tongue. That can cause errant metering of fuel. You can't really remove the dimple entirely, as that would probably remove too much material, but I find if you sand it smooth with very fine paper so the needle valve nipple can move across it easily, it'll work much better.

A properly working vacuum petcock is nice insurance. The 550 ones tend to be very reliable as long as you clean it up well.

Now if the tank vent is plugged, on a hot day, or after a ride, heat can build up in the tank and pressurize the gas. With a manual petcock left open, that extra pressure will put more fuel pressure on the carbs. But I doubt this is it. If the tank vent is plugged, you would have riding problems as well. Air would not be able to get into the tank to let the fuel flow out. And I think the pressure would have to be extremely high because it wouldn't just have to overcome a normal float pressure on the needle. Once the fuel fills the bowl, the float would be submerged and putting a lot of pressure on the needle valve to close.

Did the mechanic set the fuel level using a clear tube, or did he only set the mechanical height of the float?

Shut the petcock OFF
You can chase your tail round and round trying to make it right, in theory it should work that way, too often in the real world it doesn't
As ED pointed out, it's a simple ON/Off valve
Turn the petcock to the off position, problem solved :woohoo:

Thank you Ed. Nice explanation and method of checking the overflow tube. I also have, or may have, a similar situation on a '78 1000. Was wondering how I could prove to myself that the overflow was good (or bad).