A lot of running problems on DTRs are caused by sticky powervalve cables due to water and dirt ingress. As well as affecting your enjoyment of the bike, this puts a greater load on the servo which as we all know are becoming difficult to get hold of (particularly 3-wire versions). A genuine set costs £40; pattern ones are half the price but all the ones I've tried are the wrong legth meaning only a few adjuster threads engage in the pulley housing when they're adjusted correctly. However, it's possible to upgrade the cables so they're stronger than stock and you can clean and oil them externally in seconds. You'll need:
MTB inner and 4mm outer gear cables (stainless steel inners are tougher than galvanised and the nipple is the same size as the one that fits into the pulley on the barrel)
MTB 4mm inline cable oilers
6mm solderless nipples for the servo end
Some old powervalve cables as you need to save the threaded adjusters for re-use
Cable cutters (the type used by cycle shops)
Take the old cables and do whatever is necessary to remove the inner and outer cable from the adjuster; this usually means chopping the inner cables and carefully hacksawing the very top of the adjusters (the crimped portion above the 8mm hex) to free the outer cable from the adjuster. Then put the adjusters in a vice (using soft jaws to protect the threads) and carefully file or Dremel the top to tidy it up so the new outer cables fit into them easily. Also give them a good clean up so they screw into the pulley housing smoothly and make sure they're not bent as this is a right PITA when setting up the PV.
Screw the adjusters into the pulley housing and leave about 11mm of the threads exposed above the locknuts. This is so you can disconnect the outer cables from the servo later (to give you enough room to take the inner cables out of the pulley) if you need to.
Make up the outer cables the right length. Remember to factor in the length of the inline oilers and the little ferrules that fit onto each end (these are supplied with the outer cable and are a pretty snug fit into the servo's cable stops and the modified adjusters). Also the outer cables are a very tight fit into the inline oilers (they have to be so the lube goes where it should) so be ready for this.
Set the PV to the fully open position by putting a 4mm drill bit through the U-shaped part at the top of the pulley and locating it in the hole in the barrel behind it. Then turn on the ignition and allow the servo to do its cleaning cycle; this causes the servo to rest in the fully open position as well. You're now ready to install the inner cables, but switch it off and take the keys out of the ignition BEFORE FITTING THE INNER CABLES, and don't switch it on again until the inner cables are fitted AND YOU'VE REMOVED THE DRILL BIT FROM THE PULLEY as doing this can destroy the servo instantly.
Fit the inner cables starting at the pulley end and threading each one through the 2 lengths of outer cable and the inline oiler. The 6mm solderless nipples will need to be sat in the servo pulley ready so you can thread them through those as well. They're quite long so most of it will end up being cut off but don't worry, most cycle shops supply a pack of clamp-on cable ends to prevent fraying (these are fitted very last thing).
Pull the inner cables tight with pliers when tightening the grub screw in the solderless nipple; remember if the outer cables need to be disconnected from the servo you've already provided 11mm of adjustment for this purpose (10mm is about how far the outer cable fits in the cable stops on the servo).
Cut off the excess cable and fit the cable ends, and then adjust the PV cables according to the Yamaha/Haynes manual. Then fill those cables with lube! Watching clean spray grease come out of each end is strangely pleasurable.
This is a fiddly job which a lot of people CBA with, but the problem with YPVS is on most bikes the cables are too short and vertical to retain lube for any length of time. Plus a lot of people never lube them because this means readjusting them. This doesn't stop them acquiring grit and water however, and it is this which is responsible for most DTR powervalves ending up poorly adjusted and not working very well. Fit these properly and you'll have a set of PV cables stronger than stock which take about 30 seconds to clean/lube every time you wash the bike, all for about the same cost as buying a new genuine set.
I love inline cable oilers. I started using them on my mountain bike a few years ago, and they made the drivetrain so reliable I eventually forgot how to set up gears! Yes, I actually forgot and had to sit through the GMBN Tech how-to guide on YouTube. Most embarrasing
https://www.sjscycles.co.uk/cables/jtek-cable-oilers-4mm-black-3-pack/