Linde forklift H25D Bj.75

Hello Have problems with a Linde H25D (Bauj. 75) low gradients to drive up. The 3 Zyl. Deutz is almost strangled, although the gas is pushed through to the stop – and the slope is taken with a running start. Tires is 7.00 x 12 and should in my opinion correspond to the original tyres. Did anyone ever have such a problem with such a type and white advice? In addition to that, the forklift is equipped with the additional weight provided by Linde. Thanks for Any help!