Piston yamaha dt125
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The mixture has been leaned out. That's detonation.
https://www.reddit.com/r/Dirtbikes/comments/wnl4gt/does_anyone_know_what_could_be_the_cause_of_this/
You'll need to check the fueling as something has gone wrong here.
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@tfsv8 Exactly the right thing to do holding off from fitting the big bore kit until you've sorted this, I'd start off taking a look at your jet sizes and compare these to what the Yamaha repair manual says they should be (Haynes managed to get their VM26SS '88 carb mixed up with their '89 onwards TM28SS so it's well worth getting a genuine Yamaha manual online).
I once bought a sub-5000 mile French import DTR in excellent condition which ran awful, on stripping the carb I found:
270 main jet fitted (stock TM28SS is either 210 or 240)
Float height about 5mm out ensuring the float bowl was constantly pretty much empty
One of the float bowl breathers blocked with silicone sealant
It also had a pattern coil which cause all kinds of running problems on DTRs, and the air filter element hadn't been cleaned in what looked like years (this went straight in the bin; a well-maintained £10 Yambits one is easily as good as a genuine Yamaha item)It ran OK at low rpm but was asthmatic as soon as you tried to rev it and was pretty much unrideable as it was set up so a basically empty float bowl was trying to supply that fire hose main jet. As soon as I put the jetting/float height back to stock and fitted a genuine coil/new air filter it became well-mannered and went really well like a well-sorted DTR should. These are the kinds of modifications "Biwt me own bowek" type characters make to try and make 125s faster before heading off to Bike Night to brag about how the manufacturer didn't know what they were doing when they designed this bike so they had a few beers, went into their shed and finished the job.
