definitely carb
The specifications for the carburettor of a German '86 XT are as follows:
main jets L/R: 130/125
needle / clip position left: 5C37/4th notch from top
needle / clip right: 4A71/4th notch from top
idle jet: /46
fuel level / float height: 6-8/25-27
idle setting: Basis 2,5 indiv. setting
synchronisation: 5 mm
I'd recommend to check the needle positions, as these have the most influence on the mixture at part load. This version of the XT seems a bit out of the ordinary as the needles are not standard in the middle. The previous owner might have changed the positions also, which would have been better for altitude adjustment than changing the jets. Also, there are some tiny washers and rings that should be under the clip, which obviously also works on the needle position relative to the slide psoition.
Then check the synchonisation, although the 5 mm indicated above was later changed to 6 by service bulleting from Yamaha. Does the vacuum operated slide in the RH carb work as it should?
Finally, although I do not think that that is the issue, adjust the idle mix setting to your bike, for any older XT the 2,5 turns out is way too lean.
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