My friend had bought a 997 and installed it on his Ranger. He had interferance and put the factory "choke kit" on and it still had some interferance, then we found that if we ground the T/M it got better. It still was not clear but usable. Hummingbird couldn't find anyother reason it had the interferance. Then TODAY he tried to think out side the box. It was fixed today----------here's how He has a 109 (36vt) M/G and it has the "metal' prop on it. He put the Plastic one back on and it stopped all the clutter. He even unhooked the ground trick and it was still cystal clear at all speeds of the T/M.
So if you have a M/G T/M w/ Metal prop and have clutter/ interferance ----try the plastic prop and it should clean it up.