Carb leaking.

Alright... So I have a 78 kz400. While I write this up, keep in mind, this is my first motorcycle, and I am good with my hands, but not entirely mechanically inclined.

So... I noticed a while back my left carb was leaking fuel from the overflow tubing. I also noticed that it was still leaking after my bike was turned off. So I figured I had an issue with my vacuum petcock. I replaced it with a petcock that actually has an off. In the mean time, I cleaned my carbs, replaced the float bowl gasket. After all was said and done, I noticed my carb was still leaking.

My next step was to clean the carbs. Again. This time I noticed my floats were uneven. My left carb, the one which was leaking, was significantly higher (While the carb was upside down) So, I adjusted them to the same height, both roughly 22mm from the float bowl lip.

Here is the good news. My left carb stopped leaking through the overflow...

Bad news? My right carb is leaking through the overflow. I did not adjust the right carb float.


So... Today I tore them apart again. Went at the inside and outside with a wire brush, checked all the gaskets, all the jets, everything appears to be fine. I took both of my floats, and checked to see what happened when I put them in a bowl of water, both floated evenly. I put them back, and noticed my left was now a mm or so higher, lowered it. Put everything back together, crossed my fingers.

Still leaking from the right.


I'm not sure what I'm missing. There HAS to be something. The carbs look immaculate. Cleaner than they have been in probably 20 years. The float needles both push the floats back immediately. Not sure what else I can do.

Any advice?

I should probably also mention that I have added a fuel filter, and the bike has never run better. It idles smoothly at 1.5k rpm without any difficulty, usually starts on the first kick, always by the second. I cut off my muffles, added straight pipes, didn't rejet, but switched to velocity stacks, as it felt a bit rich, and it runs incredibly smoothly.

Just the stupid leak. From hell.

You need air filtration or you will eventually kill the engine by sucking in grit that grinds away the rings & cylinder walls. BUT, that isn't why the carbs are leaking.

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 of 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.

BTW, setting the FLOAT level with the carb bowls off is NOT an accurate way to set the FUEL level. The floats are not necessarily perfectly buoyant or equally buoyant between the different carbs. The clear tube method it the ONLY way to be sure the FUEL levels are correct.

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

kawasaki.com indicates three KZ400 models, each with CV carb, but B1 diagram shows a second drain screw.

KZ400-A2 (1978) Deluxe
KZ400-B1 (1978) Standard
KZ400-C1 (1978) KZ400 Special

Would do the following:
Check condition of the rubber o'rings on the drain screws (and replace if necessary);
Assure each drain screw is snugly tightened;
Assure that the floats aren't installed upside down;
Assure absence of a cracked or loose overflow tube (inside the float bowl)*;
Polish the float needle seat orifice area with a q-tip chucked in a drill;
Inspect condition of the float needle pointed tips to assure they aren't grooved or damaged;
Perform the clear tube test on each carb to determine whether fuel levels are within specs.

* May remove the float bowl and submerge it in water while blocking the top opening of the overflow tube with a finger and blowing into the overflow hose to see any air bubbles that might emerge from a crack in the tube or loose connection where the tube joins the bowl.

Good Fortune!

I'm gonna check my float while its still on my bike in the morning, that's a good idea, I have been cleaning with carb cleaner and compressed air, so maybe something is stuck to the side and a Q-tip will get it.

As far as the velocity stacks, there is a mesh screen covering the end of the velocity stack. Wouldn't that be enough filtration?

A screen will keep the big stuff out that would cause immediate damage (like a piece of gravel. etc.), but it won't prevent the fine grit, road dust, and other small particles from being sucked through the carbs and into the cylinders. That fine stuff can still grind away inside the cylinders and cause unwanted wear, so real filtration is a much better option for the engine. Racers run stacks, and stacks look really cool, but racers tend to rebuild their engines often. There are a couple folks on this site that use stacks on the street, but I can only assume the roads and air in their area are very clean. I know where I live if I wipe the frame, fenders, etc. once a month or so the rag comes out dirty, so I would expect if I rode for years without filtration the carbs would suck an awful lot of dirt into the engine. Ed