Any ideas what my issue is?

Will idle all day if I let it idle from cold and dont ride it. Once I put load on the motor, (eg low speed riding/trail riding) it wont idle afterwards.

I'm also running an aftermarket TM28 carb instead of the stocker VM26SS, but I'd like to hear opinions as if it were stock? eg. air leak of some sort? bad gasket?

 

I'll put a small wager on it being your magneto. I had that problem for almost a yr,,,, with 2 different carbs,,, and many many many hours of tinkering n cussing!

Electrical parts have a way of not working right when they heat up.

Had a VW rabbit 15 yrs ago that did the same thing. Ran fine until it warmed up. A smart mechanic told me to check the dwell. Sure enough, the distributor bearings were worn out. Once they get warm, the bearings expand and the rotor in the distributor would flop all over and not send the spark to the plugs. A new distributor and it ran fine (by VW standards that is - POS from h3ll)

 

the stock 230 carbs are junk. the cdi boxes are iffy. my stock carb would idle but hunt rpm(up and down) to a extent changed out all the cdi system with new parts cause it would just die out. wouldnt kick start but pull started and then was fine. Ended up being valves were needing adjusted. now idles bout 1200 rpm according to my mini tach / hour meter