They can work in any direction but they won't idle for crap and you will have fuel dripping out the carb when you turn it off. On two stroke karts they mount them stack up and shut the butterfly when off the gas so they do not idle. This is so when coming off a corner when the throttle is lifted there is fuel buildup to keep from sticking the motor. Why the do it on 4 strokes is beyond me. A lot of tilly manifolds are made that way. If you look at the factory mounted WB engines they came stack down. Was yours stack down when in the original application? I've posted about this before and got a lot of negative comments by people saying they can be mounted in any position and that is true but the same people will say you will get fuel wash and poor idle. I have learned from working on these carbs for over 35 years and can get them ot idle fine and operate very well but on my mini's I've had to cut manifolds and use a rubber coupling to get the stack down. Here is a partial text from EC brit on stack positioning:
Now that your carb is all set, it is time to test. You plug your starter on and hit the go button or give a yank on the pull rope. The engine spins over and the fuel heads for the carburetor as it disappears under the fuel cap and the engine comes to life. You pull the starter out, and hit the throttle a couple of times, then one more time for good measure to make sure it will tack up. As your engine thumps away and fills the garage with smoke you notice raw fuel is dripping out of the carburetor mouth to the floor. You ground out the sparkplug and kill the engine as quickly as possible. Raw fuel is still dripping out of the carburetor and you can see it's coming from the high side discharge port. You think, “Where did I go wrong? I know I put all the parts back in. Something must be wrong.“
No nothing is wrong with your carburetor this is normal. With a Tillotson carburetor, we install the carburetor up side down from the way it was designed to be mounted. The pump plates are up on the top and not on the bottom. The low and high side needles are open and all of the fuel that is in the wet side of the carburetor is just draining out past the tips of the needles and out the discharge ports. It will keep dripping till all this fuel is gone from the wet side of the carburetor and only takes a bit of time to do.
Here is some more info regarding pop off from the same article.
Pop-off pressure has nothing to do with the fuel pump pressure of a remote pump or the pump that is part of the carburetor. What the pop off pressure controls is how much vacuum pressure it takes for the demand diaphragm to press hard enough on the foot part of the fulcrum arm to release the inlet needle off the seat. When this operation happens, it lets fuel into the fuel cavity above the inlet seat for the low and high side needles.
As the engine is running, the intake cycle air is being pulled in through the air filter and into the carburetor. As this air comes in the choke it crosses over the high side nozzle, or discharge port, then over the low side by-pass ports in the carburetor. This forward moving air created by the vacuum of the engine makes the fuel come out of the by-pass ports. As this is happening, a vacuum is being built on top of the fuel that is leaving. This pulls the demand diaphragm down.
Some carburetor engineers say that the pop-off pressure is a needless worry, and changing the pressure won't affect the carburetors performance. They feel that you are fine if the pop off pressure is high enough so the inlet needle will seal off the seat. NOTE... To some degree this is true as long as its high enough to seal off the seat. They are looking at maximum rpm and maximum engine vacuum. The demand diaphragm will come down no matter what you have the pop-off pressure set at, within reason, and the inlet needle will come off the seat opening the doorway for the fuel to come into the wet side of the carburetor through the inlet seat.
How do you check pop-off pressure?
With a pop-off pressure gauge which is basically a hand pump that mimics the effects of fuel pressure in the carb.
If you are doing bench work without the plates on the carburetor you simply fit the machined fitting into the fuel inlet passageway to the seat. If the carburetor is together, we put the line on the carb fuel inlet cap or some carbs require you to place the fuel inlet fitting into the carb body. If you're checking the pop-off pressure of your carburetor at the racetrack, between heats for instance, remove the high side needle so you're not blowing up a balloon. To get the right reading, we want the inside of the wet side of the carburetor to be at atmospheric pressure. The carb has to stay wet to give us the right reading. To do this, you need to replace the standard pop-off gauge line with a three foot long piece of tubing that is filled with WD-40 or any light oil. If you’re working on the bench, then just make sure that the carburetor is flooded with this mixture around the needle and seat.
What is the right pop-off pressure for your carburetor and why?
The pop off pressure is adjusted by changing the pop-off spring. The springs differ by length or by wire tension. We carry about 12 different pop-off springs at any given time. We have also designed shims that fit under the pop-off spring to split hairs which makes the job a lot easier than shaping springs by hand, heating them, or any of the tricks used before then. Never cut a spring off letting it set on a spring end that is not flat. Why? Because a spring with an end cut off will never do the same thing twice. If you have to cut a spring you have to take extra time to shape the end back so that it is at least flat enough to set on a workbench with out falling over. This is no small task. We also offer springs with double coils on the end so that one coil can be cut off and you still have a flat bottom for proper shimming.
Higher pop-off pressure usually makes the carburetor a bit more user friendly and lower pop-off pressure will give you more torque off a turn and can be faster. But you have to watch your needle settings a bit closer so that you don't go too lean on the top end and burn the engine. The following settings for a start position on the tune is only a guide line if the carburetor has been blueprinted and set up to fit your needs. In short if in doubt set the needles richer to start with... You never hurt or turn your engine into a onepiece unit by being on the rich side.
Again I am not saying you need to mount it stack down but if it is not stack down expect poor idle, dripping fuel. The fuel wash issue does not apply to two strokes.
