1975 yamaha DT 175 it floods when the gas tank

I had the same problem - actually more than one problem:
1) Floats in the carburetor bowl (at the bottom of the carb) are getting stuck. Remove the carb; carefully disassemble the bowl & float assembly (pay attention to gasket orientation and don't lose the needle valve that the float-tab presses on!); use a carb cleaner from AutoZone or similar, and a toothbrush to clean all the parts. If carb is not too dirty, leave the jets and mixture-adjust screws alone (otherwise will have to re-adjust later).

2) Overflow tube is plugged (hence fuel in the cylinder instead of on the ground). Clean both ends of the brass-looking tube from inside and outside the carb bowl. Make sure you can blow through it or it is still plugged. Toothbrush and carb cleaner should do the trick. Replace the short flexible tubing that connects to the overflow on the outside of the bowl - transparent polyurethane tubing is less than $1 per foot at your local ATV/motorcycle shop.

3) Air Vent tube is plugged. Same thing as the Overflow tube but mounted higher up in the carb (not in the bowl). Clean it the same way and replace the flex tubing with another short piece of polyurethane.

4) Make sure the float-tab and needle-valve move smoothly. Mine was catching and getting stuck so I carefully "polished" the face of the tab with 400-grit emery paper, and then made sure to clean the area from grit. Smooth as a hot knife through butter now.

5) Set the float level by CAREFULLY bending the tab that pushes on the needle valve. On my 1981 175MX, float height spec is 21mm when just making contact with the needle. Yours may be different though.

6) Reassemble the carb after making sure all dirt and debris are gone. Use a new bowl gasket or at least put a good gasket-sealer compound on the old gasket. Attach the short lengths of transparent polyurethane tubing to the Overflow and Air Vent. When installing the carb, the Air Vent tube should just be 2" long and point out to the side; the Overflow tube should route through the frame near the swing-arm and point down to the ground.

7) MAKE SURE the Oil Feed tube coming from the AutoLube oil pump is connected to the carb (brass-looking fitting toward the cylinder-end). I replaced mine with transparent polyurethane tubing (different size than the Air Vent and Overflow), so now I can see that oil is being presented to the carb!

8) If the carb was nasty-dirty, you may need a new air filter. Mine was disintegrating. I recommend also putting an in-line fuel filter between the petcock and the carb, again using short lengths of TRANSPARENT polyurethane tubing (so you can see that fuel is flowing). You may also need to adjust & prime the AutoLube oil pump.

I highly recommend getting a Clymer manual (check eBay) which shows all these procedures and gives you the right specs for float height, etc. based on your model / year.

I would say is your carb,probably the needle is stuck . recommend cleaning or new tune up kit (needle) set,etc.) or new carb. i am not an expert but have dealt with this alot. good luck - Jason.

Replace your needle and seat and reset your float height.
If the needle and seat is OK just set your float height

For an immediate fix tap the side of the carb bowl with something. Most times it will free up the needle. Does it to me the first time after I've stripped and cleaned a carby. Only need to open the carby again if it keeps doing it.