Hi recently bought a 1985 DT 125 in fairly good condition for my son.

No leaks, starts first time on kick. however the problem comes in when riding its not idling properly and when reaching high revs it seem to cutout.

2 questions really;

Is it compression related?

Is it carb related, if Carb, how does one get instructions on setting the carb appropriately.

Help

You need a compression tester to be accurate on that one, pull the plug out and hav a look at the colour!!!

While the bike is idle and you give it a little flick of the throttle does the revs linger then drop?

Try looking for a downloadable Haynes manual for the original settings but that won't help you much if the bike has been derestricted.

Do the older dt's have powervalves + servos?

Ken

Nope no pv on a 85 bike. Didnt happen to the dt till 92 i think.

Have you done the basics first. Checked the plug and things like the small filter in the fuel tap.

Carbs dont change much apart from getting dirty.

Daft question time, it does have an air filter right? (an original type with the air box, not pod filter/s).

As above, check basics first, fresh clean fuel, filter not blocked, new spark plug (keep the old one and post a picture or describe exactly the colour & condition). Also, (don't ride too much as you may have badly lean mixture) when riding & it starts to die, does putting the "choke" (cold start lever) on help or make it worse?

Welcome purley" i think at that age, Crank seals will be sucking air, [ poor idle ] its impossible to set carb up if this is the case, .

pull the plug out , it will tell the story if lean or not,,,