You're problem probably stems from the fact that you are propping that engine way too big!

a 14x6 is WAY too big for a 70. It's more like a 90 size prop.

Here are the recommendations for the 70 surpass: 11x8-9, 12x7-8, 12.5x6

Personally, I like to use a Graupner 12 x 7 on an OS 70