Hi,
My Brother responded to your youtube vid the other day...He copied/pasted my reply to him after emaiing me, onto your vid.
This really really looks like a Cylinder drop issue. One of the posters here, like we put onto your Youtube comments, says to use a timing light to monitor spark. I still feel like you are initially running on all 4, then dropping one...Or just running on 3. Erratic spark or no spark is not always evident. There are proven ways to monitor spark correctly, and rule it out. Also CDI Corp has very good Ignition troubleshooting steps on thier website.
You squeezed the bulb and no difference, try spraying a mix into the throats of each carb when its failing...Probably no difference, if its not a fuel delivery issue.
The way its running, seems like a non-firing cylinder. Could be all wrong here, but those components like to to idle, or when cool...they breakdown when hot and under load.
Pease reply so we know were this goes.