15% is fine. Been running it for years.

Like others have said, check the fuel tubbing inside the tank.....OK, just saw you fixed it. OK, also check the metal tubing

Anyway, the first thing I do with an engine problem is check for:

Ignition check
1. Different glow starter
2. Different glow plug

Fuel check
1. Carb adjustments
2. Fuel line plumbing

Never had that list fail to find the problem