Initially my whistle smoke was weak, also.
I did blow down the stack several times and that helped a great deal.
It does take a few whistle blows before it heats up enough to see the steam.
Every time I add fluid, I blow down the stacks several times.
I think now my whistle smoke is excellent.
I had the same issue with my new PRR M1 and I also now blow down the stack every time I add smoke fluid.
It now has excellent whistle steam.
Also, I always add the max smoke fluid (20 drops). It seems the newer Lionel locos can take the full amount and begin smoking right away after a filling. Maybe they have a larger fluid capacity? I don't know.
Older locos may not start smoking for a few minutes if they are overfilled.
Final comment, I use Lionel smoke fluid in my Lionel locos, as recommended to me by Dean Brasseur.
