Yep. No leaking. I’ll install the 38 pilots. Then do the rear one size down - 140 to 138 keihin.I always check for boot leaks with spray carb cleaner, it might be worth a check after it has warmed up for those 10 minutes to eliminate that.
A mechanic I learned a lot of stuff from used to use a acetylene torch.
If that check doesn't show anything unusual I'd up size the pilots to 38's and reduce the main jet on the rear cylinder by one step and see what happens.
You have boot burner or backdraft pipes on that don't you?
you are correct about the backdraft