Phillip Hogg, 43, who was convicted of drink driving nine years ago, caused the death of James Burns, 53, in the head-on collision on the A70 near Carnwath, South Lanarkshire.
Hogg's trial at the High Court in Glasgow heard how he was witnessed overtaking a car just before the accident in heavy rain.
Hogg claimed a "black shadow" had become apparent through the spray that appeared to be moving into his Alfa Romeo's path, but said he could not remember the impact that left his car on its roof.
He claimed he only became aware it was not a deer when his then girlfriend, Katy Penman, a passenger, was interviewed by police.
Hogg admitted having one glass of white wine before setting off from Roberton, South Lanarkshire, and was overtaking a car when he crashed into the bike.
Hogg, of South Queensferry, Fife, was originally charged with causing death by dangerous driving.
Mechanic William Gourlay, 29, told the trial he overtook a car and pulled in before he saw a motorbike coming towards him.
He looked in his rear view mirror and added: "There was a collision. The Alfa Romeo was on the wrong side of the carriageway and level with the Golf. I couldn't believe what I'd seen."
Steel erector Alan Bertram, 29, said he was overtaken by Mr Gourlay and said the bike was on the same side of the road as the Alfa Romeo. He added: "I just saw a dark shape and a dark figure and then there was a collision."
Judge lord Brodie said: "The victim in this case was entirely guiltless and faultless."
He deferred sentence until later this month at the High Court in Edinburgh.