This writeup describes the process for setting the carburetor fuel levels, commonly referred to as wet float-heights.
The following procedure will expose you to gasoline and gasoline fumes. Gasoline is highly combustable and the fumes can be explosive. Any time you work with gasoline, keep a fire extinguisher within range so that you can extinguish a fire should one occur.
Gasoline is also a carcinogen. It will cause cancer. Avoid exposing your skin to fuel and avoid breathing the vapors. If working in your garage, keep the door open and ensure proper ventilation. Wear nitrile gloves to prevent skin exposure.
Gasoline is an environmental hazard. Do not pour gasoline on the ground or down the drain. Catch all gasoline into catch cans and reuse them in your lawn mower or other small engine. Rags that become soaked with fuel should be laid out to dry in a safe place, far from where you are working.
Be responsible and be safe.
The factory spec for fuel level is 4mm to 6mm below the line cast into the fuel bowl, for US and Australia spec carburetors. For Diversion and XJ600N carburetors, this should be 5mm above the line.
The fuel level within the carburetor float bowl is a critical setting to ensuring the bike runs correctly. Fuel levels that are too high will result in a rich running condition through the throttle range while fuel levels that are too low will result in a lean condition. It is also important that the fuel levels in each carb are set to the same level ensuring consistent A/F mixtures across each cylinder.
Fuel levels should be set anytime the carburetor is opened and modified. This includes installing new float valves or after float height adjustment. Fuel level setting is the second (and more critical) adjustment that should be made to your carburetors following such activities and should follow a bench float height setting.
Tools and Process Overview:
The following tools are required:
- Pair of jackstands to support the bike in an upright position
- Piece of wood for use as a wheel chock
- Ratchet straps
- Short carpenter's level (less-than 12" long)
- 10mm socket for removing the front tank mounting bolt
- 12mm socket for removing the rear tank mounting bolt
- #2 JIS screwdriver
- Needle nose pliers
- Remote fuel tank
- 18" length of 3/16" ID clear vinyl tubing
- Calipers for measuring float settings
- Sharpie marker for marking fuel levels
- Secure the bike in an upright position
- Remove the fuel tank
- Disconnect the hoses from the carburetor
- Level the bike side-to-side
- Install a remote fuel tank
- Measure fuel level
- Adjust fuel level
- Confirm fuel level
Secure the bike in an upright position:
In order to accurately measure and compare the fuel levels, the carburetors should be level. To do this, we need to hold the bike in an upright position. If you have a center-stand, simply put the bike on the center-stand. If you have a swingarm stand, use that.
Use the following procedure only as a last resort! The intent here is to hold the bike upright but not lift it off its wheels or otherwise try to raise it. To do this, you want to find a way to support the bike and hold it securely in an upright position. This is how I did it. Use caution when doing this. If the jack stands slip, the bike WILL fall over, possibly crushing you in the process. I knocked mine over on the righthand side before adding the straps but fortunately it wasn't damaged.
Begin by chocking the front wheel with a piece of wood and place the bike in first gear.
Wrap the mufflers with a protective surface to keep them from being scratched up. I used pieces of foam for wrapping water pipe and secured them with masking tape. Starting on the left-side of the bike with the side-stand down, lift the bike to an upright position and place a jack stand securely under the left-side muffler. This will hold the bike nearly upright.
Move to the right side of the bike and place a jack stand beneath the right side muffler. Push the jack stands toward the front of the bike until the bike is held securely upright.
Once the bike is upright, run a ratchet strap between the two jackstands and around the front of the bike. This will keep you from accidentally kicking out one of the stands, preventing the bike from falling over. Repeat the process with a ratchet strap up over one of the rafters in your garage. This will support the bike long enough so that, if you do knock out one of the jackstands, you will have enough time to catch and right the bike.
Remove the fuel tank:
Because my bike has a non-stock method for mounting the tank, the following photos will not match what you have. To remove the fuel tank from a stock bike, follow this how-to to remove the fuel tank: How to remove the Seca II fuel tank.
With the bike upright, remove the fuel tank. Using a 12mm socket and ratchet, remove the rear mount and place the hardware aside:
Using a 10mm socket, remove the front mount and place the hardware aside:
Using the #2 JIS screwdriver, remove the fuel handle:
Disconnect the petcock vacuum line from the #1 intake:
Using needle-nose pliers, remove the hose clamp for the fuel hose:
Place a piece of paper towel below the fuel hose to catch any spilled fuel. Disconnect the fuel hose:
With everything disconnected, lift the fuel tank from the bike and sit it on a piece of cardboard, away from the work area.
Disconnect the hoses from the carburetor:
Using needle-nose pliers, remove the clamp from the carburetor inlet fuel line and disconnect the line. Be especially careful with the brown fuel tee. It becomes brittle over time and can break. Yamaha no longer supplies this part so if you break it, you're hosed.
Level the bike side-to-side:
This is important. To ensure that the fuel levels are properly measured, the bike must be level. Place a carpenter's level on the carb bracket, just above the float bowls, and ensure the carburetor bank is level. Adjust each jack stand to move the bike slightly left or right until you have the bubble in the middle of the level. .
Install a remote fuel tank:
In order for fuel to flow properly and for the fuel level to be measured, a remote fuel tank should be used and that fuel tank should be mounted above the bike. This ensures there is sufficient fuel pressure to fill the carburetor float bowls and seat the float valve. If you don't have a remote fuel tank, you can make one. Here's the write-up on that.
Ensure the fuel-valve on your remote fuel tank is closed. Fill the tank with a small amount of fuel and mount the remote fuel tank above the bike. Connect the fuel hose to the brown fuel tee on the carburetor.
Measure the Fuel Level:
Finally, after all that prep work, we are ready to measure the fuel level.
Connect a short length of 3/16" clear vinyl tubing to the float-bowl drain and loop the tubing back up toward the top of the carb, aligning it perpendicular to the float bowl line.
Using a #2 JIS screwdriver, open the float-bowl drain screw and allow fuel to flow into the tube. If fuel doesn't readily flow, an air bubble is usually trapped. Lift the vinyl tubing a bit to push fuel back into the float bowl. This will dislodge the air bubble and allow fuel to flow back into the tube. Hold the tube up against the side of the float bowl and note the fuel level. Use a Sharpie marker to mark the fuel level.
In this photo, I've added tape to show the maximum fuel level. The tape is at 6mm below the fuel line.
Once you've marked the fuel level, close the fuel drain screw. Pinch the carburetor-end of the tubing to prevent fuel spillage and disconnect the vinyl hose. Drain the hose back into your remote fuel tank.
Repeat the process for the remaining four carbs. When done, you should have all marked. In this photo, you can see that all of my carbs require some adjustment and that the #3 and #4 cylinders are much higher than the #1 and #2.
Adjust the fuel level:
If you only have one or two carburetors which require adjustment, you can do it in-situ by removing the specific float bowl and making the necessary tweaks to the float bowl tang. If, however, like me, you find that all need adjustment, it is much easier to remove the carburetors. I chose to remove my carburetors.
To adjust the fuel level, the float tab must be tweaked to change where the fuel shuts off. If you want to raise the level closer to the line, bend the tab so that it is closer to the float body (less float height). If you want to lower the level, bend the tab so that it is further from the float body (more float height).
Begin my measuring the current height of the floats and write this number down. It will help you determine where you started.
To adjust the float height, remove the float pin and remove the float:
The location of this tab sets the height of the float:
Grasp the tab with a pair of pliars and very carefully bend it. Bend it in small increments; a little bit goes a long way:
Repeat this proces for all four carburetors, bending the tab slightly to raise or lower the fuel level as indicated by the initial measurements.
The measurements below are where I found my carburetors. These carbs were bench set with an 8mm float height. You can see that that static setting didn't produce consistent results for fuel level. You do see a trend, too; the lower tab heights result in lower fuel levels.
How do I know how much to bend it? Well, I had the same question. During this process, I took measurements and compared the fuel level with the tab height. Using some interpolation, I came up with a formula to determine how tab height impacts fuel level. Using that formula, I came up with the following guide. This guide provides the tab measurement based on desired fuel level. Measure each tab distance and use the guide as a reference to figure out how much to bend things to get the desired fuel level. For a target fuel level of 5mm below the line, the tab height should be somewhere near 0.224". Note that this is only a guide to help keep you in the ballpark. You will need to adjust the tab hight based on your actual measurements.
My desired fuel level was 5mm below line. I set each tab height to 0.224 inches. This actually worked and all four carburetors were at the appropriate level when rechecked. This may or may not be a reliable method.
Confirm Fuel Level:
Refit the carburetors, if you removed them. Or reinstall the float bowls. Reconnect your remote fuel tank (again, if you removed the carburetors) and fit the clear vinyl tubing to the float bowl drains. Make a mark at your desired height, on each carburetor bowl below the line. In this case, my desired height is 5mm, so the Sharpie line is marked 5mm below the line.
Check float heights using the procedure detailed above. If your fuel levels are spot-on, you're done. If not, remove the float bowl and readjust the tab height until the fuel level is where it should be. This is a long and iterative process but one that you will only have to do once. Fortunately, my float heights were all consistently at 5mm so I only had to do this once.
In the words of the Hayne's manual, reassembly is reverse of disassembly.
- Remove the remote fuel tank: Drain your remote fuel tank back into a fuel can and set it out to dry.
- Reconnect the hoses to the carburetor. Ensure all float bowl drains are tight. Install the fuel supply hose back onto the brown carburetor inlet tee.
- Install the fuel tank: Install the fuel tank and reconnect the hoses.
- Final inspection: Ensure all connections are made and that the fuel tank is properly mounted.
- Lower the bike: Make sure your sidestand is down. Remove the ratchet strap from over-the-rafter and the ratchet strap installed between the jack stands. Carefully push the bike to the left and remove the righthand jack stand. Holding the bike upright, remove the lefthand jack stand. Let the bike tilt leftward onto the side stand.
- You're Done: That's it, you're done. Warm the bike up and take it for a spin. See if the fuel level adjustments helped.