I'm trying to change the fuel tank on my bike but don't have an owner's manual! Help?!

How about the manual below?

https://www.google.com/search?q=2006+Honda+CBR125R.+fuel+tank+hose+diagram

Fuel Tank - Honda CBR125R Owner's Manual [Page 143]

https://www.manualslib.com > ... > Owner's manual
Disconnect the fuel tank breather hose. FEED HOSE. BREATHER HOSE. Remove the bolt and fuel tank. Install the fuel tank in the reverse order of removal.

Hello,

If you have not done so, please check the fuel filter for good fuel flow. Also, pull the output hose off the fuel pump to see if it has good fuel flow. One more thing... pull the fuel line from the carburetor to make sure that there is a steady flow of fuel.

If all the above components are working correctly, then you might try this; fill the tank with premium fuel (NO Ethanol additives) and add 1/4 to 1/2 ounce of carburetor cleaner (B-12) and add Sta-bil (the recommended amount) to the fuel tank. The B-12 will clean the fuel path and the Sta-bil will stabilize the fuel. Start the bike as you normally would. It will take a bit to get the bike running so be patient. Once you have it running correctly, buy a can of Sea Foam (only if your fuel contains Ethanol additives) and add the recommended amount every other tank for about 4 tanks.

If you are still having issues after doing all the above, please reply with more information so that I may help you further.

Hope this helps. If you find this information useful, please vote on this post. Thank you.

Regards,

Oz

Hello,

You can buy a fuel stabilizer, Sta-bil, and add to the fuel tank. This will keep your fuel from gunking up in your fuel system. Start and run your bike, after adding the Sta-bil, and let it run for a few minutes to get the Sta-bil throughout the system.

Change the oil and filter, change your clutch and brake fluids and remove the battery from the bike.

Place the bike upon something to get the wheels off the ground and cover the bike with a tarp or bike cover.

Give your gear oil a look, but it should be OK for that length of time.

Some people drain the gas out of the system, but if your tank has any inside damage, it will begin to rust and you will have to clean the tank before running the bike.

I wrote a tip on what to do when a bike sits for an extended period of time. You can find the link here; Bike sitting for extended period

Hope this help. If you find this information useful, please vote on this post. Thank you.

Regards,

Oz

Try changing your spark plugs, this should get you started but also drain out old fuel a put fresh fuel in, use a fuel system cleaner like redex etc in a couple of tanks of fuel.