The button is actually a primer for the vaporizer if you have to hold it in that tells me you aren't getting fuel through the vaporizer
Possible causes would be faulty vaporizer, faulty mixer ( not pulling enough vacuum to close diaphragm in vaporizer) as engine turns over vacuum pulls on diaphragm to let fuel in
You can turn off fuel remove front of vaporizer and check diaphragm for a tear or hole