28 degrees ignitiontime at high rpm, wat kind of engine is this.....4stroke ?
If it isn't a 4stroke, please set the ignitiontime back to 15 degrees or maybe even 10 degrees at high rpm.
If the ignition will be to early, you get to soon ignition and the engine get a flash back.
The piston don't have enough energie to go TDC and push back.....there is your limiter !!