Just want to share my experiences with the XT225 Mikuni BST34 CV carb, in hopes of saving someone else the ridiculous amount of time I spent. Maybe this here thread could be a place to mention solutions, rather than problems.
My carb was leaking vacuum from the bottom of the diaphragm, where it’s clamped to the slide with a round white plastic part a bit bigger than a quarter. The clamp is held in place by two little round plastic tabs which are peened where they protrude down into the top of slide.
Now, after fixing, the bike will do 80+MPH (indicated, not actual) on level ground when tucked, and will accelerate up a hill in 6th gear from about 30MPH without bogging. This is with stock pipe and intake, stock gearing, 130 main jet (which is stock for a 1999), one .030” shim on the needle, Shinko 2.75/21 244 front tire, 170lb 6’0” rider, soft saddlebags, and good compression.
It seems like a questionable carb design by Mikuni; if your bike won’t do better than 75MPH indicated, it’s possible it has the same problem.
DIAGNOSIS:
Remove the airbox cover, air filter, and the screen if you have to (easy to get out, harder to get back in); watch the slide as you blip the throttle; if the slide doesn't go up (a fair bit) quickly and freely with even a bit of throttle, and maybe without oscillating up and down, you probably have a vacuum leak in the carb. If you have to rev the piss out of it to get the slide to go all the way up, you definitely have a problem.
Rotate the carb after removing the gas tank; remove the slide. If the diaphragm can be rotated under the clamp, it’s not clamped well.
Fold the diaphragm down around the slide, turn upside down, see if it will hold water--if it leaks water, it will leak vacuum. You need that vacuum to raise the slide, which pulls the jet needle out of the needle jet and allows more fuel to flow, up to the max allowed by the main jet.
SYMPTOMS:
Idles fine, OK power down low and particularly using the “choke,” but boneless using full throttle and/or high RPM.
Amazingly, it will still run kind of OK without the slide going up at all. But no fun, and weirdly, poor gas milage.
Mine also sounded strange. When running right, it kind of snarls, but when running badly, the snarl had a sharper, nastier sound.
REPAIR:
So, you have a vacuum leak in the carb. And it's from the diaphragm, and not:
* The black plastic coasting enriched cover, the vertical pipe of which seems to have nothing to do with the coasting enricher itself, instead being a second source of vacuum signal going from the intake tract behind the slide to the top of the carb (the other/primary source of vacuum being the hole in the bottom of the slide next to the hole for the needle), and depending on two small o-rings and one larger one.
* The carb cover, which is also black and plastic, and needs to be flat (perhaps fixable--not sold by Yamaha) to clamp the rather thin diaphragm lip tightly; and which also has to clamp down hard on the little o-ring to the coasting enricher mentioned above in order to fully clamp down on the diaphragm.
* The choke can also leak--my button-style choke knob needs to be pressed in with authority to not leak, but this seems to create a minor intake-style leak which affects idle, but not normal running.
So...order up a new slide assembly with diaphragm and clamp attached, $132 total from Partzilla. It’s a shame to replace all that, but it works, and is well worth it for more power.
Or, you may well be able to use heat to re-peen the little round tabs that clamp the diaphragm to the slide. And it might hold.
Or, you can do what I did (initially--I subsequently put in the new slide I had already ordered), and fix it by removing the clamp, snipping off the tabs, carefully drilling pilot holes, and using ½” #4 screws into the plastic, with o-ring to seal the screw heads, and the tips of the screws snipped off for clearance; there’s just barely enough room where the slide seats to allow this to maybe not affect how the slide seats when it's down. Another possibility here would be to use short machine screws with hex nuts.
Anyway, this DIY fix may seem bonkers, but it worked shockingly well. Maybe better than a new slide, initially--I was stunned at what my stock 225 could do. Became a little hooligan, breathing fire, and hinting at little accidental wheelies when hastily taking up 1st gear from a stop. (In fact, since I now have an extra slide, I might just try drilling out the vacuum hole on the bottom of the slide a tad.) It doesn’t seem so fast after a while, but I guess it's just perception, since it'll still do 80MPH.
SUMMARY:
If your XT225 won't do 75 or 80MPH indicated on level ground when tucked in, you may have a leak preventing the CV carb from doing what it's supposed to do.
