For what it's worth, I had a similar issue 4 years ago right from the initial installation of my Model 6244 (20kW). Took a while to get it sorted out but finally was resolved by replacing the internal Regulator Assembly (which included the "reservoir")
. I posted about the issue on this forum here: https://www.zillerstore.com/post/hard-start-on-new-20kw-9945621?&trail=15
I also posted videos of the hard start on YouTube here:
and the resolved start here:
Not sure this will help but thought you might want to look at it anyway...
Did that already. I think it was your post that promoted that. Only a slight improvement. I really need a parts manual. But that's what has been in the back of my head.
It acts like there is a void in the fuel flow during a cold start. Even if there was a a small line feeding it, it should still start normally. The pipe size issue should become a problem under a load. The 15KW ran for years with no issues. According to what I can determine, the newer units use less gas