1980 TROJAN 1900 speed in 2nd gear

Hello, I have a 1980 Trojan 1900 model wheel loader, in first gear (2 gears) it is strong and can make wheels spin if the bucket is against a rock load, but in 2nd gear, with nothing in the bucket, just going uphill a little it slows down to walking pace. I'm not sure but I think i have seen on the net that there is an ajustment for the transmission. Anyone can help me with this issue, please...

1980 TROJAN 1900 speed in 2nd gear