In our case, this kept me occupied for a full day: Float Height
What is float height?
The float bowl has those floats in it, which act like the float in a toilet to close a valve when the liquid level gets too high. The adjustment of your float is commonly measured by your "float height" That is, remove the float bowl and turn the carb upside down, and measure the distance between the tip-top of the float and the rim of the bowl (the rim of the top of the bowl, really). |
| Not the right carb, but an example of float height. From the rim to the top of the float, upside down. |
In the case of a float this small, precision counts:
- If your float height is too high, the floats stick into the bowl too low. A small amount of liquid will close the valve too early, cutting off the gas before the bowl is full.
- If your float height is too low, the gas won't be cut off until it has already started overflowing out the top of the carb.
Trust The Manual -- No Wait, Don't!
The manual for the XS400 states that the float height should be 32mm, plus or minus 1mm. Then again, the manual says that the blinker unit is under the seat and that the floats are made of brass, so you can't always trust this manual. To achieve 34mm, I would need to bend the floats to nearly a right angle, and it would be impossible to fit the bowl on. |
| The manual says that my floats look like this. |
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| My floats really look like this. |
I didn't note my original float height exactly, only that it was "about 20mm or 25 maybe". In an effort to richen the mix, I reduced it to 18 -- which resulted in gas dumping out my airbox when I tried to start. Pull off the carbs (again), empty the bowls (again), adjust the floats (again) to 25. This time the thing acts like it's out of gas, because it essentially is.
Keep in mind too, that the float height isn't REALLY what matters, but the angle of the tang that presses on the needle. If your float is at 22mm but the tang is at 20 degrees, that's not even the same as your float at 22mm and the tang at 15 degrees. The floats themselves aren't in a fixed position either; that metal bends if you look at it funny. We quickly found ourselves in a situation where the float height was basically irrelevant.
So, How To Figure It Out?
During the hours we spent on this, we figured out a few tricks to "shorten the development cycle" -- in other words, to do more testing with less time spent tightening the airbox and fender and throat clamps, then undoing them again.Pull out the float pin and the floats, and try to get them straight and flat. Flat tang, evenly balanced floats, all 4 as close as you can get them. Then put them back in.
A quick experiment. Get a soda straw, put it to the fuel intake on the carbeurator, and blow. (a friend can help) You should be able to blow through, and feel the air going through both of the needles. Push upward on the float, and feel the air being cut off, first one carb then the other. Okay, that was the practice.
Put one of the float bowls on, and with your finger close the un-bowled float valve. You should still be able to blow through -- the bowl should NOT close the float valve. If you can barely blow through it, or not at all, then your float height is too high and that bowl wouldn't fill with gasoline even if you were to put it onto the bike -- lower it. If you can blow through it, it may not be high enough -- raise it just a millimeter at a time until you can't blow through, then lower it again just 1mm.
Do the same for both bowls: raise the height until you can't blow, then lower it 1mm.
Go ahead and attach the bowls, and install the carb into the bike. Your "blow test" should get your float settings just about perfect. At any rate, with only 1mm between dry and flooded, that's as good as you're going to get.
