I know I'm a little late to the game, but...
I was having kickback and hard start problems with a Predator 224 build. I literally spent months trying to lash and re-lash at .003. I contacted everyone in the mini bike game I knew or had a channel. They all had the same advice. Check your lash, check your compression release. One night I had torn down my entire engine to check rings and compression release. When I was reinstalling the rods and rockers I noticed it was significantly easier to hand crank with 0 lash. I did some searching and found this and another thread about 0 lash and finger spinning the rods. I set set the lash at 0 using the EOIC method and being able to finger spin the rods.
It absolutely worked first time. It is in a whole other universe. It starts easy, first pull, every pull. No hard start, no kickback, no problem.
I have started and stopped the bike several times while tuning the carb with a Wideband 02 Sensor and Tachometer. With no problem what so ever.
The bike is tuned at 1800 rpm and a cool 14.6ish stoic on the carb. It has a mean .308 cam growl and zero bog, poping, or backfire. It seriously doesn't sound like a mini bike at all. The person who came up with this is tops in my book. I'll never do anything else on other builds.
My build:
Predator 224 block.
Governor removed.
Low oil removed.
ARC connecting rod.
Dyno Cams .308 cam.
Flat top piston.
Big valve, shaved head - polished and ported.
32# duel springs
Chrome 5.11 push rods.
Thin head gasket
3:1 stamped rockers, lash caps on both sides.
Push rod guide.
AR 3910x plug.
Makuni 24mm flatslide - 25 pilot, 135 main at Washington State sea level.
Tuning with Bosh Wideband 02 sensor and Tachometer. Again, 1800rpm and 14.6ish stoic at idle. I might need to enrich a little after riding.
Adjustable coil bracket with unregulated coil - I put 2 pennies behind the coil to get the business card gap correct.
3 magnet flywheel at 6 degrees advance.
First generation Baja Warrior frame with the headlight working off the flywheel.