Compression and frustration

Ok bought an 82 kz750 ltd. just did a compression test and my one and four spark plugs give me 110 my two and three holes aren't reading. The bike ran great. I even drove it home let it sit and it started right up again 4 days ago. But now there is gas in the air box. The old spark plugs are black and so I replaced them. I'm stumped and frustrated

If there is gas in the air box it means the carbs are flooding gas. You need to make sure to turn the petcock off when you turn the engine off. If it's a vacuum petcock, it's not working right. The carbs probably need cleaning and fuel levels adjusted.

Also check you valve lash.

Assuming your bike has the stock carbs, each one should have an overflow tube running out of the bottom of each carb bowl. These must not be plugged up or you will have fuel run into the airbox, and worse, into the cyliners and then crankcase oil. So I suggest you start by ensuring those tubes are open to the air so excess fuel can drain out.

If the overflows are not plugged up, and if no fuel drips from them, it seems unlikely that the fuel level in the carb bowls are set high enough to flood fuel into the airbox, so there may be a different problem.

If when unplugged fuel does flow through the overflow tubes, and assuming you have the stock vacuum operated petcock and it is not set in the "Prime" position, you have 2 separate problems. The first problem is that the petcock is not shutting off fuel flow when the engine is turned off. This most likely means you need to rebuild or replace the petcock. The second problem is that the floats in the carb bowls are not shutting off fuel flow. If that is true, and if the floats haven't been damaged or installed upside down (odd but possible with newbe carb rebuilder) there are 3 things to look for as the cause of the second problem.

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.

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

Ok I determined it is definitely the petcock. So i am rebuilding that this evening. As far as the carbs go, I drained a significant amount of gas out of each bowl using the brass bolt at the base. Is that what you are referring to as an overflow? I am a newby and haven't tackled the carbs yet. So this information is incredibly helpful.

No, those are the carb bowl drain screws. The overflows do not (at least should not) have any screw or other obstruction in them. Each should just be a nipple on the bottom of the carb bowl, and each should have a small hose attached to it which then is routed down behind the engine to the ground for disposing of fuel that may overflow. In the diagram below you can just barely see where the overflow nipples are (red arrows); green arrow show the overflow hoses. Ed

Okay that would explain it. I have no drain hoses!

Do the carb bowls not have the overflow nipples on the bottom, or is it just the hoses that are missing? I think some carbs didn't have the overflow nipples, but I cannot imagine why. Ed

They do have the overflow nipples and Gas has flowed out of them when I tried starting it originally, but now the carb bowls are empty.