I picked up a 125cc quad. Guy said it had no spark. Had been sitting outside for a while, so I just pulled the old wiring and replaced with new. Still no spark. After trying other known good components I still got no spark. Figured it must be the flywheel, so I put a different one on. Got spark, but won't fire off. Checked timing and it was way off. Reset timing and adjusted valves. Put some fuel down the intake and cranked it over and it sounds like a gun shot coming back through the intake. I thought the timing must have still been off, but it was right where I set it. I'm out of ideas. Anyone have something else I can try?

 

Check TDC mark is actually TDC by piston, you may have got flywheel on wrong, easy to push key out as you push flywheel on. Bit puzzled as to what could be wrong with your old flywheel anyway, it is just a ring of magnets. If you have timed it with a false TDC the valve timing will now be out and a valve may have hit the piston.