The manual says 4 ounces per gallon, 32:1 ratio. What do you use? I have a 05 KX65.
How is the bike ridden? Raced and strung out all the time, or is it a trail bike?
For racing, a richer oil mix will be needed. Keep in mind, when you richen an oil mix, you have to rejet. A rich oil mix makes the A/F ratio lean out. Meaning richer jetting.
A leaner oil ratio will richen the A/F ratio. So that would mean leaner jetting.
This bike will be used by my 8 year old for trail rides. Maybe a hare scramble but not ridden hard.
I'd definitely stick with at least 32:1 in the little bike. Most of the time the little bikes get ran a lot closer to the limit than a bigger bike like a KX250 would, so, more oil needed.
Manual calls for, and I use, a 32:1 mix in my KDX even.
+1 on the trail bike vs race bike mixture....we have a 60 and an 80 and we run 40:1 all the time...both bikes run very well and are very reliable. good luck!
The KX65 is notorious for losing the lower rod bearing when pushed hard on lean oil ratios. If the kid is really just lugging it around, 32:1 will be OK. If the kid starts getting fast and consistantly screams the engine, you should be running at least 28:1.
I run 32:1 in the son's kx65 for riding the trails. Same gas in my 250 - one less thing for my feeble mind to remember. The guy before me ran Amsoil at 50:1. The engine was worn out and took a new bottom and top end - the rod had noticeable up and down play and the piston skirts were well worn with some blow by half way down the skirts. I'm amazed it ran as good as it did. I probably need to replate when I do the next piston replacement. I'm a firm believer that a little extra oil will not hurt.
Pro circuit recomeded 32:1 when I got the engine back from them. Once I jetted it, I have Never, repeat NEVER fouled a plug. Piston has always looked good. IMHO you have nothing to gain by running super lean fuel to oil ratios. Dirt Bike magazine did a test back in the early 90's with an rm 250 running 50:1, 32:1, and 16:1 ratios. It made the most power with the 32:1 ratio, and all the pistons looked good. Just my opinion take it for a grain of salt.
oils have come a long way since the 90s!
But with the stock crank, just a matter of time from what I have been told no matter the ratio. I bought our 65 with a non running motor and promptly put in a new crank, bearings, seals, and top end. No worries about the stock rod issues.
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