I have read about this in many different forms and have also helped other RC users wth this problem ..This will work on all nitro engines.
You need to install a OFNA oneway check valve in exhaust line to fuel tank..This will keep a constant air pressure in the fuel tank..
You will have to lean out the engine more both high side and low side needles a little more..You can get this at
www.towerhobbies.com item # LXEYG3 Install bulb side too exhaust side..I know it says it is a primer type unit..But there is a small ball inside to make it a check valve.so will not back up any air to exhaust and will keep the pressure in your fuel tank...I use this oneway check valve my self and have no problems with it..
Any questions feel free too ask..