This bike is in pretty good condition but it had been sitting for a few years when we got it. It started right up, but there was a gas leak. (We only kept it running for a minute or two.) We then drained the tank, put in new fuel, and cleaned the carb. It didn't start. Many of the brass parts in the carb were corroded and the bowl drain nut was seized so we bought a new carb on Amazon for $30. It didn't start.
While the carb was out, we realized that the petcock was leaking while in the off position, so we disassembled it and replaced the worn parts, which fixed the leak, but the bike still won't start.
Is it possible that the crankcase is flooded with fuel because the petcock was leaking? Why did it start at first?

Any help would be greatly appreciated

 from the UK.

My money's on the cheap carb as being the culprit.

If the brass parts in the original carb were beyond saving, then most are still available through Yamaha or a Mikuni dealer.

Yes, it's possible the crankcase flooded, not particularly because of the petcock but more because one of the carbs may have been leaking via the float needle. If it was, you can either take the exhaust off and kick it over a lot, or take the exhaust off and run it down a hill in 3rd gear and that'll clear it out really fast.

$30 carb could be complete junk, they are hit or miss. Personally I'd try and get that bowl drain nut loose and clean all the brass parts of the old carb and try it again. Typically a beat up Mikuni is still 10x better than the cheap Chinese carbs unless you get a good one at random (buy a lottery ticket that day too).

I am also a fan of cleaning the points (still points on the 75 250?) and a new spark plug to give it the best chance of firing well.

Thanks a ton. I'll try this tomorrow and report back.

Had to take a break from the bike for a bit, but I'm back. We ran it down a hill in 3rd gear a few times with the plug out and the exhaust disconnected. We then tried bump starting it but that failed. Right after we tried bump starting it, I pulled out the spark plug to examine it and it was wet with fuel, so this means it should eventually turn over, right? (It's a brand new spark plug, too.) I think we're going to try towing it with a truck and bump starting it again. I will also try bump starting it with the original carb, which I cleaned a bunch.

How did you connect your 2T oil injection line to the Chinese carburetor? You are aware that line must be connected unless you are running pre-mix in the fuel tank?

Has the flywheel been off. Wonder if the crank key has let go & then timing goes wherever it likes. Would have thought it might backfire though with what you've been doing even if crank key is gone. If its got a belly full of fuel & 'os' sparking at the right time then just hold the throttle open while pushing down hill or towing. Just make sure you can stop it, or that the throttle is bottoming out when shut & back off the idle slide stop screw as with no exhaust back pressure it'll likely not slow right down. Could have blown the right crank seal but they're pretty robust, but if that happens it'll flood the crank with trans oil==but doubt its that.

The bike works! We towed it with a truck for about 500 feet and it fired up. It runs great, too. The Chinese carb is working well. (It has a port for the oil injection line.) Kicked it again next morning and it fired up again. Thank you all for the advice. 

Congratulations!!!