Hiya!
I've had this problem for half a year now; I posted about this last July, though I accidentally flodded the motor because I forgot to shut the pet valve & had to let it dry all Fall & Winter. Now that the engine is dry, maybe I can finally fix the d#$m thing! 
So, here's the lowdown: I've got a Yamaha Enduro 100 motorbike from around 1972-73; The engine bearly starts, when you do get it to fire up it won't idle. I was able to ride it on my driveway for a few seconds, but that all stopped when I tried going uphill. Then the engine just sputtered out, I only got it to run for another 4 seconds by starting it going downhill. Now, the engine won't turn over at all. It's been like this ever since I got it for my birthday last July, same problems as then. My pop got it for me from a guy & he had to ride it a full mile at least to get it to us, so we know it worked. I just replaced the spark plug about an hour ago, as well as putting in fresh gas & oil. We suspected it was a dirty caurberator, & that me accidnetally leaving the pet valve open would clean it, needless to say I don't think it worked.
I'm truly at the end of my rope here, fellas. 2 whole seasons of waiting, for no ? Any & all help is apreceated; Thanks in advance.

UPDATE:
I checked it again, the fuel line was loose, practically disconnected. Maybe that could be my issue?
Only problem is, I flodded the engine again. Apparently, fuel is still flowing if the valve is fully up or fully down; it only stops when it's in the middle. Now I gotta wait for it to drain again (Maybe not a full 2 seasons, considering theres still fuel in the tank & I didn't empty a full gallon into the engine block) before I can test it again. Keep you posted!

To get the fuel out of the engine, take the spark plug out, stick into 2nd gear and push the bike for a few hundred yards.

The petrol tap has three positions and fuel is meant to flow in two of them - "On" and "Reserve". The other is "Off", but I won't insult your intelligence by going any further! There should be a cut off valve in the carb float bowl.

If you can, remove the carb and check the pilot and main jets - you should be able to see through the hole in each of them. Post here if you need any further guidance on this.

In the float bowl there should be a tiny hole to assist with cold starting - if this is blocked, the choke won't function as it should. I have attached a random float bowl image just to use as an example - the tiny jet is at the bottom of the hole in the centre of the bottom edge. Spray some carb cleaner, WD40 etc into the top of the hole and see if it trickles out of the jet at the bottom.

UPDATE:
I was able to get it to run for another 15 or so seconds; I squeezed the choke as hard as I could & had it on full throttle, but the instant I let up on the choke, it stopped instantly. I've got a hunch this has something to do with

So What your saying is, it might be a problem with my carbeurator?
I have a Chilton manual for my bike, & it should tell me how to remove the carb. Do you have any tips for inspecting the pilots & main jets? Suppose I can't see through the hole in them, what should I do then?
How do you mean "the choke won't function as it should" if the float bowl is clogged? Where would I be able to find it on my bike?

Getting late here in the UK now and I'm just about to check out.

I'll look in again in the morning and, if no-one else has chimed in, I'll answer your questions. 

Blocked jets.

Video is good so we can all get an idea of what it's realy doing or not doing. There's certain things it could be like crank seals pushed out of the case after which it all goes down hill fast if it actualy runs at all but you need to stop the flooding or at least turn off the fuel when it's not running. To load a video--long enough video for us to get a good handle on it--,you need to link it through "Imgur" or similar external thingy. Maybe that's only for photos but I just link to youtube for videos. Crank seal out are common when they're old & the motor gets a belly full of fuel then get pushed sideways when kicking over. Not saying it is that for sure but food for thought.

My guess is that you have a LT2 or LT3. On that basis, see link below to an online parts list from a very reliable source. Have a good look at the enlarged version of the exploded parts diagram. Also take photos at each stage of the process, so that you know how to reassemble!

Yamaha LT2 1972 1973 USA CARBURETOR - buy original CARBURETOR spares online

Before you remove the carb from the bike, slacken off the banjo bolt (item number 7 on the diagram) on the bottom of the float bowl - the main jet sits on the end of that.

Remove the carb from the bike.

Remove the float bowl from the carb by undoing the 4 screws. Try not to rip the gasket, otherwise you'll need to find a new one.

Remove the float (item number 9) by tapping out the small pin (item number 10).

Item number 1 on the exploded diagram is the pilot jet. Once you have the carb removed from the bike and the float bowl removed from the carb, you will find the pilot jet sitting in a recess in the underside of the main carb body. It can be unscrewed with a small flathead screwdriver. However, don't force it if it won't move. If you can remove it, can you see daylight through the small hole? If yes, it isn't blocked. If no, it is blocked and so you need to unblock it with a thin, stiff wire - something like a wire removed from a wire brush will work, whilst others have used a guitar string. Some folks like to soak it first (in Berryman's?). With a magnifying glass, check to see what number is stamped on it - should be 25. If you couldn't remove the pilot jet, use the wire method with it in situ. If you spray a little carb cleaner into the pilot jet recess, it should trickle out of another part of the carb, if it isn't blocked.

Item number 6 is the main jet and this sits on the end of the banjo bolt you have previously slackened off. Remove it from the banjo bolt with a small spanner and check and clean as above. Also check that all of the holes in the banjo bolt are clear. Check the number stamped on the main jet - it should be 120.

Item number 5 is a rubber 'O' ring - replace this with a new Yamaha one (they are still available).

Item number 2 is the valve (cut off) seat assembly. With the float back in position, turn the carb body upside down and gently press on the float - the small pin should spring back up when you release this gentle pressure.

I think that I covered the choke question in my earlier post.

Reassemble the carb, put back on the bike and fingers crossed! 

If the jets are wrong, then genuine Mikuni ones are likely still available from Yamaha. The Mikuni logo is a small square in the corner of a larger square.

I am going to mention about getting a set of JIS screwdrivers and not using regular philips head on these motorcycles. I never heard of the JIS screw drivers and was stripping the heads on the screws. The JIS screwdrivers made a world of difference.