Hi I have a 1969 Yamaha DT1B 250 that got a few weeks back and I am trying to get it running. I have tried many things and I am desperate. I changed the spark plugs, I fixed the magneto and I got a new ignition coil. The electric side works perfectly and I am getting sparks. I also replaced the carburetor. I checked the pressures and it I am getting between 100-110. The crankshaft has oil and it looks healthy inside. I managed to fire her up after I don’t know how many cranks and it only lasted two minutes with me giving some throttle. It was the only time It ran, just because I was giving it gas. That was the only time. Sometimes when I try to fire it up I can feel that it turns on and dies right away. Some other times I can be there for minutes trying and I see smoke coming out of the exhaust like is doing some combustion. I don’t know if I should be just trying harder or doing it longer. Have any of you faced the same problem? Any suggestions? 

You've said you have replaced your carburetor . What exactly did you replace it with ?

Could it have now broken the flywheel crank key. They eat them up if the flywheel isn't tightened Very Tight. Or has it now just dirtied up the plug with old oil that had settled in the crankcase getting sent up to the plug in the first run. Also--as Swoop56 says--what carburettor have you fitted, & is it nice & clean. You're lucky to find an early DT1.

Sometimes a bike that has not been started in years needs more.

If I had to make one suggestion it would be; “remove the spark plug, put the bike in second gear and run the bike down a long hill for at least 90 seconds of blow everything out of the spark plug hole rotation.”

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Yes at first i cleaned my carburetor with pressure air. But the previous owner messed up and the leveller was not in the right spot. I tried to fix it as much as I could but I still wanted to make sure that it wasn’t the carburetor. I saw on a different group chat that someone mentioned the VM28 works great with this motorcycle so I got it. I do want to point out I might be wrong but I think both carburetors were working correctly due to some Gas was coming out of the exhaust manifold connection.

The flywheel seems to be rotating with no issues and is generating power. I do need to check the plug to see if there is old oil in there, I will check that up. Thanks.

“remove the spark plug, put the bike in second gear and run the bike down a long hill for at least 90 seconds of blow everything out of the spark plug hole rotation.”
Oh for sure I will do that today. I have heard in other group chats that for a bike that has been seating for a long time it takes a lots of cranks to fire up. Is that correct?

Your replacement carb may not be correctly calibrated for your bike .
The stock carb is a pretty simple piece , and in most cases can be refurbished ,
after a GOOD clean . From your previous post I gather you're not too familiar
with the carb operation and settings .
We have members all over . If you post your location , there may be someone close by ,
who could assist in the setup .

Another , possibly related tip .
Don't leave the fuel tap in the on position , when not in use .
You can wind up with a belly full of fuel in the crankcase , which will give you grief with starting .
Asco previously made a suggestion of how to clear this excess of fuel .

I have a 69 DT1 too . I turn the fuel tap off a couple of hundred metres before I stop ,
so fuel level in the float bowl is low . When I need to start again , which could be months later ,
after turning the fuel on , it usually takes two kicks to fire up .

Remember, when you get to the bottom of the hill, then you might need to push it back up!!. Don't have the ignition on with plug out incase of sparks jumping. Me being old & lazy--stuffed knee-- i just spin mine up with a good drill & with both plug & blank off out of the head to let pressure escape easier. Wouldn't matter if your drill only goes clockwise, just don't accidentally touch the kickstart--but is on other side--. Exhaust off too of course, & will make a mess on your shed floor if loaded with fuel so 'have a plan'.

Thanks yeah I am located in Burbank, California. Yeah I was looking into having the stock carburetor being refurbished, but I wanted to make sure the carburetor was the issue and not something else. Thanks for the tip, yeah I always turn it off.