First, let me start with why ride a 2-stroke, and why a 300 in particular.
1) Reliability and durability and cost of running: To my knowledge, there is no other bike that is as performance oriented as the KTM 2-strokes (light weight, premium components) that has anywhere near the reliability they show. I ride a lot, and I ride pretty hard, and they take it like they were designed for it (because they were). No monkeying around with unrefined systems, the bottom ends last forever (a friend, Fast with a capital F, has a 2000 380 e/xc that he rides as much as I ride mine. He's done a top end a year and has yet to see the inside of the bottom end).
Doing a top end on the 2-strokes costs about $150-250, depending on OEM or aftermarket piston, and takes a couple of hours (or 20 minutes if you are an ISDE racer). The modern 4-strokes, especially those from Japan, have a shorter top-end life than the KTM's do, and the job costs at least $500. Not to mention, if something goes wrong on the 4-strokes, you're into $1500 by the time you start replacing everything that needs it... 2-strokes have about 4 moving parts, so it's pretty hard to break them.
So, given that brake pads, chains/sprockets/tires are the same as anything else, the overall cost of owning and racing a 2T KTM is basically cheaper than anything else out there. The 2T Jap MX bikes are just not as durable- trannies, bottom ends, and top ends are all underbuilt compared to the KTM stuff (though certainly workable).
2) Weight and rotating mass: The 2 strokes are light and they feel it. The 4 strokes are heavier and try to disguise it... but what they can't disguise is all that rotating mass. Step back and forth from a 2 stroke to a 4, and you'll instantly feel the difference of having a several pound gyro (crank and valvetrain) spinning at reasonable RPM's.
3) It's damn fun. It is zippy and playful and makes you feel like a hero, even when you are a gear high or a gear low, even when you miss the apex and have to pick it up with your foot, even when you stall it. It starts easily, always, and the motor can be ridden aggressively at the start of a race or chilled out at the end of a long day of trailriding, and it's happy either way.
Now then, on to setup:
The weak point of the motor, for sure, is the exhaust mounting. It's mickey mouse and it's basically impossible to avoid all leakage:
There are a couple of tricks to minimizing it, though:
- Clean everything really well with carb cleaner, which is the only thing that cuts spooge very well.
- Check pipe alignment- if one side is cocked out, it will never mate well. Use a broom handle inserted into the pipe to "convince" it to go straight.
- Line the pipe with hi-temp RTV sealant before assembly. Watch it come out the exhaust for the next 2 rides.
- Double spring it if you can't get it hung quite right.
As I said earlier, I prefer the stock pipe on the 00-03 300 motor to any alternative I've found. And the eline pipe guard is the ticket for protecting it.
The stock silencer is good, and when fresh, will test at around 89 dba (which is quiet!). I also have an FMF Q, which is a touch louder... but offers less leverage in the case of an "issue" for tweaking the exhaust system. And I think it looks better. I run them both, and can tell no power difference between them.
Carefully clean the junction between the expansion chamber and the exhaust pipe. Assemble with no sealants or zip ties on the rubber gasket- they don't seem to help. If it is a good fit on both ends, it shouldn't leak. If you have to torque it into place, be prepared to spend some time cleaning. DAMHIK.
The KTM's are unique in coming with a jetting chart. It's conservative, but not overly so. If I have a debate for which box I fit into (it's a temp/elevation matrix), I err to the lean. This will be a good starting point and you won't seize the bike. Some folks like to go to a #7 slide cutaway and run the N85D/E needles, some folks run some of the newer needle tapers... I find that the KTM book gets me close and leaves me confident that I won't blow it up, and that's worth something. Summer jetting for HIGH elevation (8k +) is NOZI/3rd, 40/162, about 1 turn on the A/S. Winter is NOZH/3rd, 45/175, 1 turn. Whenever I rejet (which is a lot) I write what's in there with a sharpie on the side of the float bowl. It'll wash off with gas and it means you never remember wrong and leave summer high elevation jetting in for a winter ride at sea level.
The OEM Mahle Forged Pistons last a long damn time. Wiseco Cast pistons are a bit lighter and rev easier. Note that KTM provides 2 different sizes of cylinders- they are very close, but your original piston will have a "1" or "2" stamped in the head. If you go OEM, get a matching piston. If you go aftermarket, they aren't precise enough to care.
I have been running no-toil products in the air cleaner for a long time with good success. I'm sure the petro stuff would have worked fine too, but no-toil is much nicer to work with.
I have an enduro engineering clutch slave protector. It's got a lot of marks from chain slap. No idea if any of those slaps would have hurt the stock unit... but I'm glad I didn't find out. Another $20 well spent.
The bike comes with a crappy guard between the chain and tire, which falls off shortly after you start riding the bike. Preemptively take it off, and use some silicon to seal the holes so you don't fill your swingarm with water and sand.