Hi all,
I finally got my DT3 motor back together. I installed a PVL ignition and have spark. I think it is timed as good as I can get it. I get nothing when I kick it. Nothing when I prime it directly through the spark plug hole.

The original Yamaha piston has 2 holes in it facing toward the rear. 
I replaced it with a Wisco unit which does not have these holes.

The compression is good and seems fine mechanically.

Are the 2 holes essential for operation? I am beginning to think that they are.
I am learning about MC's as I go here.  

Thanks for all the help I have received from the members on this site.
I will sent pics when I get a chance.

Those two holes are used to open the intake port and pull air/fuel in, without them it's probably not pulling enough air + fuel for combustion. In theory you could add them to the Wisco piston, but porting is a pretty specific field and they'd need to closely match the OEM piston. Probably cheaper to buy an OEM piston.

If it has spark and a good fuel/air mixture, then the mixture should ignite when the spark happens.

It will run on it's own if the spark happens at the correct time and you have compression.

My opinion is that it will 'run' without the reed holes in the piston... just not at its best performance.

If you spray starter fluid in the cylinder and it never fires, then I would suspect the ignition system.

Even if the wrong piston it'd still run. Is it a go fast piston with a large cutaway in the skirt instead of two holes, or sounds like nothing from what you're saying. Either way she'd run. Wonder if you got hold of a DT1 Wiseco piston to have 'no' holes, in which case it is probably 6mm short of the top of the cylinder at tdc. Or am i thinking back to front & piston should pop out the top--so obviously not that. I'm sure there's a gudgeon height difference between reed & non reed motors for 250 but not 360.

can you take a pic of the piston box or link to where you got it. Even if 6mm short up top it'd still go puff in a soft way to try & do something--i think. Timing at 3.2 or in the ball park at least.