Regarding the Poulon 42cc, an 18" bar is just fine, I've been running one for 4+ yrs now. Helpful hints:
.....Wear PPE; ear,eye,face, good leather gloves,chaps.
.....Keep the cutters sharp,
.....Next new chain, try upgrading to a Carlton or Stihl chain, keep it sharp.
.....Get a shorter bar, 14" even, it'll run faster and put more of the power into cutting instead of pulling the chain around the bar. A 14" bar will cut through a 14" log faster than the 18", and the chains will be less expensive.
.....Can change the chain type, not the hardware store safety chain, get one made for you, semi chisel, non-safety type.
.....Try a 3/8-.050, NON-low profile, this will put a little more drag on the saw.
.....Radical change, mod the muffler, it has been done here on this saw before, using the search feature you will find info and pics.
24" bars are more suited for 60cc and up, really 70cc+ saws can run them to full effectiveness. There will be less money in your pocket and no smiles on your face if you try to run too long a bar on this saw, not to mention the strain on the saw, shortens the lifespan.
Learn to file, keep the chain sharp, give it some TLC and enjoy the saw for years to come.
Work safe.