Well, you aren’t going to get 25-30 hp out of that 18 hp without swapping power heads and I don’t believe any 25-30 powerhead that will swap. At best you could swap in a 70’s 25 hp powerhead, but I don’t think it will be easy. Plus it won’t have the mounting points for the cowl.
24:1 shouldn’t be as messy as you are experiencing. If it’s still too dirty and you aren’t willing to use 50:1 then I suggest finding a newer motor in the hp range you want. I don’t mean that to sound harsh. I just don’t see an alternative.
Wayne
Upper Canada Chapter
uccaomci.com
