Motorcycle Repair: DT 175 Yamaha 1979, dt175, petcock

Question
I'm not the most mechanical of minds and I'm not very good at explaning myself but here I go. My DT175 Yamaha 79, was working fine until it had a hard hit to the ground. Since then it won't start. I've pulled it to bits and cleaned all the parts out. It now starts if you tow it behind a car, but as you drive along it almost dies on itself but then picks up again. But when the revs get too low it just dies completly. There's spark there, just won't kick over. would you have any idea what is wrong?

Answer
There could be a few possibilities...Put in a new plug, yours could have fouled from the crash. Did the bike fall on the points side ? Could the points be closed beyond specs? They should have a gap of .014-.016" but if they are adjusted you will need to adjust the timing after adjusting the gap. Make sure the crash didn't somehow cause the exhaust to become clogged....the other thought I have is the intake/carburetion...make sure fuel flows from the tank-remove the fuel line at the carb and turn the petcock on...good flow there ? Now the intake ? somehow is the air restricted is the air filter clean and in good shape, not full of debris from the crash ? No? Then pull the carb and open 'er up and have a look....it is possible that it got banged up, or a jet got clogged with debris...Look for the obvious things that could have occurred from the crash.
If this stuff doesn't help then write back with specifics of the crash and the condition of the bike.
Thanks !