I have considered many aspects and potential sources and you should know this.
Let's consider Mr Johnson's statements for what they offer.... Often... Report supporters claim that no officer had driven his bike all the way up the knoll to the pavilion like some have mentioned.
I agree. Certainly no pictures show this. So how could a motorcycle that was not up by the pavilion be emitting smoke next to the pavilion?
That was never claimed. As I posted: "I believe he's taking about the motorcycle escort for the President, and Hargis leaving his cycle." Nor did I content this was an explanation for what others saw, quoting Johnson: "A lot of them said they saw smoke come out of the grassy knoll, but I didn't. Maybe it's because I wasn't looking that way, and I was looking down in the car."
But who knows? Smoke can travel. Accelerating vehicles may emit more exhaust. Did WWII-era rifle emit noticeable smoke? Why wasn't the "smoke" under the trees, or a portion of it, captured in the Moorman photo, and the Muchmore and Nix films during the assassination? Why did witnesses much nearer the fence corner, like Sitzman and Hudson, report no "smoke" under the trees, gun report from there or smell of gunpowder?
Was a motorcycle literally driven onto the knoll for a brief period, presumably by Hargis?
- "Mr. JOHNSON stated that white smoke was observed near the pavilion, but he felt
that this smoke came from a motorcycle abandoned near the spot by a Dallas policeman."
-- Clemon Johnson Affidavit - "SIMMONS said that he recalled that a motorcycle policeman drove up the grassy
slope toward the Texas School Book Depository Building, jumped off his motorcycle
and then ran up the hill toward the Memorial Arches. SIMMONS said he thought he
saw exhaust fumes of smoke near the embankment in front of the Texas School Book
Depository Building."
-- James Simmons Affidavit - "Mr. WINBORN stated that very shortly thereafter the motorcycle escort dispersed
and one motorcycle was abandoned by the police officer riding it on the grassy slope
on the north side of Elm Street and this officer rushed up the steps leading to the
pavillion and was lost from sight."
-- Walter Winborn Affidavit
It seems unlikely this could be Officer Haygood, who arrived some while after the last shot, parked his cycle and ran to the juncture of the railbridge and wooden fence.
Mr Johnson didn't seem to know that there was a shooting at all so he could not state where the shots [that he didn't hear] came from or how many there were.
His affidavit states: "He stated he first realized something was wrong when the motorcycle began moving from their regular groups and at or just before this he heard sounds that could have been shots."
Mr Johnson didn't seem to know anything and was not called to testify.
Some [as the guys I mentioned above] did see something of interest but they were not called to testify.
Oodles of people [too many to list] who didn't know anything were called to testify however.
So [as I figure] this non-investigation was nothing but a sham 
If we're going by the initial report of how Johnson described things ...
- Walter Winborn: Didn't know where the three shots he heard came from and didn't report seeing "smoke"
- Thomas Murphy: Heard what sounded like two shots from west of the Depository and didn't report seeing "smoke"
- James Simmons: Heard three shots from the direction of the Depository and saw "exhaust fumes of smoke".
