My '02 225 has been having trouble starting. It begrudgingly starts occasionally with a full full battery (fresh off the trickle charger). Runs fine after that.
Today I rode it for 30min after charging the battery over night and it wouldn't start up after that. I took it to a shop and they are stumped. Battery doesn't seem to work now even after we took the fast charger to it. It turns over and makes the occasional thump, but doesn't start up.

Here's what we know:
- jets appear fine; just cleaned the carb
- gas is getting to the carb (unscrewed float bowl valve)
- compression fine, >150PSI
- pulling the spark plug revealed intermittent sparking— coil issue? I have ordered a new one
- battery health is fine.
- didn't start even with a blast of carb cleaner in the air box

Hoping someone here can guide me in the right direction, as I've been stumped for a while on this.

I had this problem which was difficult to diagnose. It turned out to be a Bad Stator assembly. The XT225 has a magneto style ignition. You'll see that as the larger coil in the assembly. Wire is fine like human hair. Varnish can break down, short windings and reduce output. To make matters worse, performance can be temperature dependent and intermittent. I struggled with this for about a year before finding the problem. My bike wouldn't start when hot (sometimes). When Jeannie's stator went south it just quit suddenly. You can take resistance measurements per the service manual to check the windings. This hit both our bikes at approximately 10k miles. Hope this helps. Good luck.

First of all, does the starter turn the engine over at a normal speed? If the starter will just barely turn the engine over, then there is either a problem with the starter, or there isn't sufficient power reaching it. That could be either voltage or amperage. You might get 12V+ or more at the starter terminal, but not be getting enough amperage. First check for voltage at the starter while pressing the starter button. If you get at least 12V+, and does not drop significantly almost immediately, your battery is probably good. If that turns out ok, the next thing I would do is check the amp draw of the starter motor, but I cannot find that information anywhere. It does not seem to be in the service manual. Without knowing what it should be, checking it won't tell you if it is correct or not. But typically, if the amp draw is too high, there is either something mechanically wrong with the starter motor, the starter drive, or the engine that is preventing the starter from turning over properly. That will result in the starter motor itself, and the cable getting hot to the touch. It will usually blow the main fuse.

If the starter motor turns the engine over normally, then the starter motor, battery, and all electrical connections to the starter are likely ok. If that is the case, remove the spark plug, and with the plug wire connected to it, and the threaded part of the plug touching the engine, check for a spark while turning the engine over with the starter. If you don't get a spark, or get a very weak spark (it should be a nice fat blue spark) then the problem is likely in the ignition system. It could be the stator, the CDI, the coil, the plug wire, or the plug itself. I've never had an electrical problem with my XT, but the stator not only provides a spark for the ignition, but also power for the charging system, and there is probably a pickup coil on it that generates a signal to the CDI to trigger the spark. A properly equipped shop and a mechanic who knows what they are doing should have easily been able to check all that. In most cases there will be ways of rigging up ways of checking these things if you don't have all the proper test equipment, but I am not that familiar with the XT ignition system. I did find a bad stator on another bike once that was not sending a signal to the CDI by using a 1.5V LED connected to the pickup wire. My meter would not register such low voltage. The LED would have blinked if it was working, but it didn't.

I just reread your post and saw the part about intermittent sparking. That is definitely an issue. I would suspect the stator, the CDI, or the pickup coil that sends the signal to the CDI over the actual ignition coil itself. When an ignition coil fails, it typically fails completely. Not always but usually. Could also be a bad conductor in the spark plug wire, a bad connection at the coil, or a problem in the ignition switch itself, which sends power to both the ignition system, and the starting system, through two different circuits. If the starter works ok, that pretty much rules out the emergency cutoff switch, clutch safety switch, or sidestand safety switch, as those are part of the starter circuit, not the ignition system. A problem with any one of those would prevent the starter from working.

Need to start with a multimeter on the battery, should be 12.8v a couple of days off charge or having been run.

What you describe sounds like an electrical problem. Multi-meter on the battery. As Paul says, it should hold a charge for a least a week (more like 8 weeks) with very little discharge.

Keep the meter on the battery when you try to start it. Voltage shouldn't drop a much while starting. If it drops below about 10V, the battery is bad.

Once it's running, you should get 13-14V across the rev range. Sometimes 12.5V right at idle, but it should come up to 13-14 pretty quickly and stay there once above idle. If not, it could be the stator or the regulator/rectifier. There's a trouble shooting process in the shop manual for those.

The plug itself could be bad.