I bought a Husky 51 new back in 1991 and it came outfitted with a Zama EL-7 carburetor. I picked up another Husky 51 last summer and it has a Walbro WT-170 carburetor on it. The latest 51 is a bit newer model, as it has the 'air injection' parts in it. Why were different carburetor's used on the same model saw? Which carburetor has proven to be the better performer?