constantly fouling plugs?

Rebuild/go through your engine to make sure its ALL with'in spec to run properly. You will need the manual to go through ALL the variables. You are reducing the amount of premix oil, and increasing the amount of gas? You did not try reading any older posts about spooge/jetting or even how to mix fuel? Even like last week? There is still one getting beat to death from a month ago! Nice bike, it will run better, lots better! A few sheet metal screws still left in? Vintage Bob

You are inadvertently making it way richer! http://www.dirtrider.net/forums3/showthread.php?t=177404 . Vintage Bob

18:1 is the power mark, yes. The jetting baseline for your bike, in the manual, is a great place to start. Its usually rich. And they are set up for running 32:1. If you are dead set on 50:1, you will be out in nowhere land and need to lean the entire carb. That will really shorten the engine life! If your blowing spooge, there is something else wrong. Compression, ignition, and draw the fuel in and out, no spooge and no fouled plugs. The float valve needs to be replaced yearly, or can cause all kinds of jetting issues. Floats do not move by there-selves, but, not being careful taking the bowl on and off can knock them out of whack! If the cylinder has been ported wrong, you will never be able to jet it. Dents in the pipe, silencer packing and the reeds closing? A bore out of round will run/mimic rich also. And not 1 soul has taken me up on just pulling the right crank seal, no more mixing in the tank, fill the tranny up with premix oil? That one really needs a leak down test to rule out. Should have seen some seal wear marks on a rebuilt crank I seen the other day. Surely that left seal will leak soon, and run lean. Who wants to tell the guy he needs a 400 dollar crank, after he just got a new rod kit installed? Who told him he needs just a new rod, not me! Vintage Bob

Probably no damage, you will tell me? And the packing soaking up spooge, yes. Are any other pipe joints leaking black gold? The silencer pipe joint, cylinder to pipe? Check the plug on each circuit? Wet or dry? Vintage Bob

Do you know the age of the engine? It's likely you could jet it and it'd be fine but it is possible you have bigger issues. If you try to jet it and it doesn't respond quit jetting and start looking for mechanical problems. A leakdown test will tell you if the engine is sealed up good enough for tuning without costing much, less than some top end gaskets actually. It won't tell you if the piston is about to shatter from age though.......

Fine metal particles can melt on the spark plug. Everything from crank thrust washers to a rusty gas can. Not so much the rusty gas can these days.
This is assuming the jetting is correct and clean, and the plug heat range isn't way off.

No power may be a sign of lean somewhere. Rich hits slow, but hard. Lean fast and weak. For the engine, and the hit, I like it a little to the rich side. Vintage Bob