Another thing to check.....That series DT250 was notorious for the silencer chambers clogging with carbon and seriously restricting power. I had some success with heating mine with a torch and hammering it to get the carbon out, but the best solution I ever came up with was to cut the stock silencer off of the tail pipe and weld a YZ250 can onto it.
As suggested do a leakdown test, its going to tell you more than simply measuring the compression. The compression was only about 6.5:1 on those anyway as a 2 stroke only makes compression from when the exhaust port closes, not over the full length of the stroke so if the port has been lifted by 1.5-2mm - as was a common mod - its hard to tell exactly what the compression should be.
Is it running weak? That would indicate an air leak.... as they were also notorious for the left hand crank seal letting go.
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