Let me see if I got this right. You have a BH Chipmunk with a .70 four stroke engine. .70 being equivelent to roughly a .45 to.50 two stroke. I could be wrong but the only BH Chipmunk that I can find is their 85 in 40 to 50 cc model. If this is the case, your about two lbs and two horse power off on the nose as the balance is designed to have a much larger engine up front. If this is the same airplane. Im surprised the .70 was able to even taxi it around!!