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I trail ride my YZ125, well, my son does now, and we keep a wrench and plug with us just in case. However, as I have the same year bike as you, I did go from a 32:1 ratio to just in between a 32:1 and 40:1 ratio and run an "8" plug as opposed to "9", which has helped in bogging and fouling. I also set the carb up a bit different, but I can't remember what I did now since it has been over a year since I did it.
That being said, you have to ride the bike hard in order to trail ride a 2t of this nature, and you need to have your jetting and needle right as well. Those 125s like to have their necks rung off, so make sure you are riding on the pipe more often than not nomatter if you are on a trail or on a track.
That is why I did not go to a 40:1 EEE299. A 32:1 ratio is 8oz of pre-mix to 2 gal. of gas. I use 7.5 oz. of pre-mix to 2 gal. of gas, which is just under 32:1, but not a 40:1. Funny enough, I ride the bike no different and the bike fouls far, far less. *shrugs* I changed the carb settings, using a 32:1 and trail riding, and used about two plugs a month. I keep the carb settings the same and just changed the mix ratio from 8oz. to 7.5 oz. and I foul plugs maybe once every three months now, and I ride the same. This is with an "8" plug as well. The YZ125 calls for a "9", but I went to an "8" for trail riding.
If you ask me, I think it is the carb itself and not the bike or settings. I was a huge Mikuni fan dealing with Harley-Davidson and was not a big Keihin fan. Now, I like the Keihin more so over the Mikuni. Just my $0.02 here as my KTM has the Keihin and it has fouled one plug, and I am almost ready to believe it was the plug that came with the bike in 2006, and it fouled this year.
I would always run a 32:1, especially on a race bike like a yz. I run it on my bike and it's perfect. Any time your bike dies the first thing you should check is the spark plug. Also, make sure you have enough fuel in the bike. If you find you're constantly fouling plugs, check and clean your carb. If it still doesn't start, I'd check my oil, and then my top end. The top ends on these things blow from anywhere between 10-25 hours.
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