Hello, i am new here in this forum. I hope you can
help me with my issue with my Yamaha DT 175. 

I have been driving a good distance when 
it suddenly went max on RPM. 
It's not the cable, cable housing neither 
is it carberator i have looked on that with help. 

What can be causing this now?
It's a 2 stroke, 171cc with single carburetor. 
 

Air leak if there are no cable/carb issues.

Could be a gasket, carb rubbers, even the crank seal behind the mag. get squirting with wd40, that will show if its a gasket/seal leak.

One thing you can rest easy on, we know exactly what bike your talking about, many of us have had one for years. I got mine 30 years ago.

Cynic is always spot on, you can trust his advice phil.

Similar thing happened on my DT and cynic knew it was an airleak then.

One of my old posts might help you..