Yz65

Jetting 101 might help you

Ours seems rich down low. I've dropped the needle all the way down and run the smallest pilot jet that comes with the bike. My young fella is still getting used to it so we tend to get a bit of spooge out the exhaust but no fouled plugs. We are running silkolene at 50:1. I was talking to another 65 owner and he was fouling plugs too.
Depending on your elevation and weather codification you might have to change some jet's. I'm looking at getting the next size down pilot

He's trying to tell you it would be wise to learn "jetting 101".

Not sure what you mean by "heard there were problems with the jetting"... Jetting is going to be different based on elevation, temp, humidity etc., so maybe you will need to reject according to your geographic location first, then report back any issues and we can help you fine tune or at least get close.

I have gone to a hotter plug on most of my two strokes and it solves the fouling issue.

Sounds like the YZ65 is jetted for a WFO kind of rider. Cold plug, slow running will foul them quick.

We own a 2018. It comes very rich new. I dropped the main jet down 1 to 2 sizes (can't remember) and dropped the needle i believe 1 clip. Also we do not use Yamaha 2stoke oil, maxima k2 seems to burn cleaner in this bike. We run 36/1 at the track, and 32/1 in the desert. Have about 30 hours on it, and compression is still perfect. Also went up 1 sprocket size in the rear so my son would be on the pipe a bit more, and not lug it as much.
Now its a screamer, and we have ridden it everywhere from tracks to desert rides in SoCal. Plugs are perfect. My son has down well on the local races recently. Its a great bike, good luck.

Check with jdjetting.com and see if they have a kit. I always used their stuff with great success when I had carb bikes.

You need to figure out what throttle position is used most in his riding prior to jetting changes. If he's not running wide open throttle then the main jet shouldn't be the problem. Main jet is 3/4 throttle on up. I agree with the 101 sugesstion and starting with the basic tests, high idle adjustment, wfo throttle plug chops, etc.

I gave up trying to jet our 18. Messed with Jets, Moved Needles, changed fuels and ratios and kept fouling plugs and cleaning spooge after every ride. While having suspension done at JM Racing down here in Houston, I mentioned the problem, and they told me they had a different headspace gasket that they had found to fix the problem. Had them install it, and have had no issues since. Maybe give them a call and see if they can give you the specs or mail you one?

My son's KX65 back in the day would foul plugs almost every other day until i managed to get it dialed in.

They are horribly rich plus the fact that most new 65cc riders hardly get it on the pipe long enough to get cleaned out.

I've seen a couple of the newer Yamaha 65's at our track with black splooge leaking down the silencer, the plug has no choice but to fowl.

Lean it out to 40.1 use Motul 800 and break it in for an hour or 2 and get it hot and will start to run good may have to go thru a couple plugs like you said but it will start running good after that. A good premix is important.

Try running 60:1 with the Silkolene and check your float height. You already leaned the carb out pretty far. Like others have said, go with the hotter plug also. Good luck.