Try the easy things first.
-new plug, let us know what the old one looks like, wet, dry, fouled?
-clean the carb and check float height
-check compression, should be around 150psi
-how old is your fuel??
If none of the above seem to help then you may have a weak spark, check all your electrical connections, your bike will run with weak spark but will be hard to start.
After you get the bike running with starter fluid does it stay running?
Great suggestions guys :) , I'll add a couple things. Is there any way this could be a lean pilot circuit issue? Is your air screw cranked way out, is the bike jetted for high altitudes or hot temperatures and you've relocated it to a lower altitude or cooler temps?
Try tilting the bike over until you see gas at the carb vent then give it a kick.
C Dave you say a lean pilot. I kept going down on my pilot to help low end, as I did so my starting got harder. I thought the choke would take care of fuel at cold startup?
Also do low tension reeds help starting(Boyesen Dual stage) since they are more sensetive at low RPM?
No it's not a KDX but the theories behind this still apply.
- And lets hope we get a little rain before we burn down the province - just hope a bike never causes it or we'll be banned from everything.
Will Boyesen reeds help? I expect they would by they likely aren't the solution to your RMXs hard starting problems unless the reeds you are presently uses are worn past their service limits then changing the reeds to either the stock or Boyesen reeds would be a big help.
At start up the engine is cold so the incoming charge or fuel easily condenses on the cylinder wall, crank, case etc etc and not enough fuel is vaporized to allow the engine to fire. The fuel is there just not in a form the engine can fire. The choke dumps additional fuel into the engine so that there is enough vaporized fuel to fire due to the shear volume of fuel. Fuel is drawn into the cylinder via the pilot circuit so as the size of your pilot is reduced so is the amount of fuel at start up.
Try to get MX Tuners attention on the Performance Forum. He ran RMXs for a number of years and can likely give you the guidance you're looking for. I'd also suggest calling Jim Couster at CC Cycle in Westlock. He has a pile of experience with the RMXs including trips to the ISDE.