I'm working my way through a new used boat. Rebuilt the Wallas heater and got better results, but still not all that thrilled.
I was relocating the tank. When I got ready to pull the lines, I couldn't. Of the gallon and a half or so of fuel in the tank, more than a gallon was frozen solid - at our 28 degree lows, or so. I'm guessing the previous owner had added some alcohol??? There was no separation until it froze.

I poured off about 2 cups of liquid and thawed the tank out. Fresh fuel and it lights much more quickly and burns hotter. It was taking a couple of tries before to start. Now it's hitting on the first time.

Just a thought. If you haven't empited your tank in a long time, might be worth it.