I have had the same question for a while. My wife rides a KX-125 and she is constantly fouling plugs. I am running MC-1 oil at 50-1. I have leaned out until it lost power and then bumped the jets back up a little, so it would run good. It still fouls plugs. I have told her to push it harder, but that doesn't seem to work. The bike runs like a bat out of hell. I just can't keep the plugs clean. I looked for info on Blue Marble on the net. All I found seems to be published BY Blue Marble. I finally gave up and ordered a gallon. I will try it myself. If it works I will try it in my KX-250. If not, I may be rebuilding a 125.
Elf, I am running an 8 already. The 9 will foul in about 10 min. The 8 will last about an hour. We usually ride about 2-3 hours. That means we carry 2 extra plugs. On the plus side, my wife can now change plugs as fast as anybody.
This is a Blue Marble update. I have been using Blue Marble in my wifes KX-125 for two rides now. This is about 1 1/2 tanks of fuel. She has not fouled the plug yet. I think it is too soon to make a judgement, but it seems better. I don't see any difference in smoke, oil on pipe, or performance. I will wait until I have used most of the gallon of oil to make my conclusion.
I am cautiously optimistic. I rode the 125 myself just a few minutes ago and it started right up. I warmed it and rode it pretty hard for about 20 minutes. It doesn't stumble or have any indication of plug problems. The other factor is the temp. It is pretty warm today. 71 deg right now.
another idea...how bout if YOU warm up the kx125, get it cleaned out, then turn it over to the wife. Turn the choke off right away, then 1 minute of idling is PLENTY, then gently ride the bike, as the engine comes up to temp, find the powerband and hold it wide open for a bit in 3rd gear or so. This is how these bikes were designed to run! If the pilot circuit/air screw are proper, as well as the jet needle clip position, once the bike is cleaned out as I describe above it should be fine for the rest of the day on a plug.
50:1 will tend to wear out parts prematurely, I suggest 32:1!
You are describing what I usually do, but I can't control the way the bike is ridden during the day. My wife is comming along pretty well, but she is having a hard time understanding the difference between her new bike and the old bikes she was riding. One was a four stroke, the other a low tech 70's two stroke and to her it sounds like she is riding hard when she is running 7 grand.
She did not foul the plug today.(she did push it a little harder) I will keep using the blue marble for a while and see how it goes.