Why do most if not all 2 cycle karts use syphon feed vs gravity feed?
Because the tank pickup is below the carb. Otherwise the pickup would need to be up around the steering wheel for a gravity feed to work.
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Where do you get that idea? What makes you think they use either one?
I am confused. Do you think they are not siphon feed. The tank is below the carb so that rules out gravity. That leaves pushing out of tank or pulling out of tank. Since tanks run vent line pressurizing is not happening. Fuel is siphoned most typically with pulse operated pump.
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Technically, Alvin is quite correct. Let’s not start the whole push pull debate again though 
. The upwards rising piston creates a vacuum, towards which atmospheric pressure acting on the fuel in the tank pushes the fuel, passing through the carb for atomisation along the way. Some engines aid this with a fuel pump.
A syphon draws a fluid through without assistance, using the weight of the fuel in the syphon tube, Alvin can probably explain how atmospheric pressure works to help a syphon, but probably will be a long read 
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I remember in my R.C. Nitro days the fuel tank was pressurized by a tube from the exhaust chamber. I believe the tank connection had a check valve that held pressure in the tank that aided in sending fuel to the carb. A very simplistic design.
I think the main reason for not using siphon or gravity is tank location. As you get into larger engine displacements and longer stints, the need for a larger fuel capacity means lower and more centered location for weight distribution/balance.
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Bingo. To much weight located high up and usually off-center.
Al, I might be using the wrong terminology and I could be totally wrong but it seems to me classes such as air cooled and tag are getting fuel to the carb with a suction created by the motor, hence siphon. This topic was split from my issue with air in the fuel line. I was wondering why gravity feed which would seem to eliminate the air issue wasn’t a solution to the issue of air in the line. What I didn’t consider was the height of the tank vs location of the carb as James pointed out. Just trying to learn.
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Actually, no kart engines use “siphon” to feed fuel to the carb, they all use pulse operated pumps. The pump can be built into the carb or mounted separately. The pulse energy to drive the pump can be from alternating crankcase pressure (most two strokes) or the pulses in the intake tract (some four strokes). These pulses cause a diaphragm to move back and forth. Flapper check valves upstream and down stream of this diaphragm only allow the fuel to move one way - to the carb. This style of pump usually works well for the type of engines we use. The output volume rises as the engine goes faster and the pressure is low enough that the carbs and fuel line we use can handle it. Maximum pressure a pulse style pump can get up too is equal to the pressure differential of the pulse source. The usual numbers are between 1.0 and 8.0 psi. . Float style carbs usually work with gravity feed ( if the fuel inlet seat is sized for it) but putting a tank over the top of the engine is inconvenient and and illegal in some organizations with out some roll over protection for the tank. Engines can “siphon” fuel from the tank if you cover the carb inlet completely while cranking the engine over. If this helps, you probably have a fuel pump that has dried out or stuck check flappers in it. The pulse diaphragm itself may be hard and inflexible too. Another thing to check is the fuel inlet needle and seat. These can become stuck closed from dried fuel/oil mixture and the even if the pump is working it can’t make enough pressure to overcome a stuck valve. The KT-100 racers know all about carb maintenance , it is essential to have everything working just right to get maximum performance from those engines.
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You missed one I think. The only one I know of is Comer kid kart engine. It has the fuel tank on top of the engine. I am pretty sure it is purely gravity feed.
Could be. I have not messed with the little Comers.
Comers, some Honda GX and GXH are gravity feed.
True. Most of the industrial based engines come with float style, gravity fed carbs. These can be fed from a pump if done right.
We started using gravity feed recently. The nipple on the tank is on the bottom, comes out the back. It has all but eliminated air pockets in the fuel line.
We also started doing the dynamic steering that was discussed on here back in the beginning of the year before it was a thing 