Are older 125’s THAT slow? How can I fix this?

Yeah, the YZ is about 36 HP.   Giving up 6HP on top is pretty substantial on any bike, especially a 125.  That's literally a 20% advantage.  You could dump a ton of money in that RM for porting and such and it will still not be as fast or easy to ride as the YZ.  

Nothing that you can bolt onto it will make it as fast as a newer YZ125.   The 2000 YZ125 had a better motor than the 2000 RM, and Yamaha has made slow but steady improvements since.

There are things you can do to help it though.   Checking the squish clearance would be a good start.   Make sure the powervalve is functioning properly, and advance the ignition timing slightly.   And of course jetting - if the main jet is too rich, it will kill top end power.

Beyond that, if you are looking for more mid and top end -  head work & porting.

A big bore kit will normally just add low and mid.

Zipty the throttle wide open.

I'm currently working on a custom 3D printed throttle cam and jockey wheel that takes around 5 or so degrees of turn out of the throttle, using the stock housing on my 2006 RM125 (may be the same throttle as you use)

5 degrees might not sound like a lot but it makes a small difference in getting WFO easier/quicker.

Like mxrose3 said above, its all about doing as many cheap/easy gains as you can to extract as much as you can. If budget allows then obviously port, head work, overbore, fancy fuel etc

Carb and Powervalve are important to look at. Ensure all the seals (jet block gasket etc) are fresh in the carb and the float is working properly. The entire powervalve system can wear which will lose power. 

Will do! Looking to get out on the bike and have a proper 'ride test' in the next 2 weeks.

I know it works fitted in the workshop, but I want to prove it in actual use before I potentially offer it up to people. I'm confident it will be a nice little gain though.

The only real/easy option is the G2 'Quick Turn' throttle, but I don't think that system reduces the amount of turn, it just speeds up the cable travel in the initial throttle opening. Which will help a little, but at a very steep price $$$

My friends and I all love the results we got from the STIC metering block, both in 150sx and YZ250 - I'm pretty confident it would help an RM125 too!

Throttle cam sounds interesting Russ, nice work.

There's one solution to make the 2000 RM faster than a YZ125...

Tom Morgan will make that engine as good as it can be….but never be as good as a newer 125…if you want an impressive older 125, pick up a 93-97 CR125, those HPP engines were dam good…I love riding my 93 Cr125, not much bottom but mid range & top end are strong…

The G2 quick turn for two strokes will reduce the amount of turn on the throttle slightly.   I have them on all of my 2 strokes and swear by them.

It is basically a modified domino throttle (like the older KTM's and 90's CR's), with a larger aluminum throttle tube/cam.   sort of like putting a zip tie around a stock throttle cam.  But if you can make something for less money, i'm all for it.

As mentioned, talk to someone like TMR. When rebuilding my 98 RM125, at first I was set on leaving it as a 125 but he informed me going to 134 or 144 setup would be at least 5hp more.

Secondly, I had a few early 2000s CR 125s which people love to bag on as having 3hp.  My point of bringing those up is if you find where the meat of the power is on your particular bike (even if it isn't that MEATY) and shift close to perfectly, they rip ok and can still be a blast on basically any track.

I raced RM's back then and remember the 01' being way "funner" stock to ride than my 99' because it was pipey and made to be revved to the moon.  Saying that, can't go wrong with TMR, but i'd talk with Lynks Racing.  He arguably builds the best small bore engines in the amateur scene. On top of that, was with Bills Pipe/Primal Impulse back in the day and learned those engines from Jeff Springman.  

Are you sure the powervalve is opening? I did a topend on my 2000 RM125 when I was a kid and I screwed up something on the exhaust valve. The bike was actually pretty fun to ride because it wasn’t pipey but it didn’t rev out at all. After a few laps the vibration must have popped it free.

Dude, the STIC Super X PWK 38/40mm carb is magic for small bores. The fuel uptake right off idle is very aggressive and it creates a cooled vaporous mix. I know 125 riders that have removed the less potent meter block version. The power delivery character is linear, but it is abrupt and will pull like a ported big-bore kit with more over-rev. The peak HP has a broader RPM range, so the shift point is much less critical. On trails it can be short shifted and not sign off, just keeps pulling without clutch input. No problem running a taller tire and drop a couple teeth out back. All this on pump has and stock engine. Imagine running an 80 pilot and 200 main jet, where stock would be 45p/178m! More power everywhere. Contact is vortex@sticfuel.us. goes for about $700, that's much cheaper than a full pro shop race build and it's simply a carb swap. I've been running 5+ yrs and could never return to any other carb. Good luck

This one has porting, head work, reeds, exhaust, aftermarket ignition and stic block fitted, and revs to the moon! Love this bike and great fun to ride, but no comparison to my 2019 TC125 with the same mods, and the gearbox is a real weak point on it.  If I was to do it again I’d have kept the RM stock and built up a steel frame YZ125 with modern SSS suspension, they’re the best mix of old-school feel with good performance and parts availability

Good to see another STIC user. What size tube and needle combo are you using?  I favor the 115 tube and JD white for the big gun power. 

Strongest settings IME for small bores look to be 52p/200m and needle @1 clip. NE3W needle or JD red for the 113.5 tube. 

FYI, the latest version steps up the performance, as you can go with a huge 80 pilot where above intermediate idle it crosses over and feeds above the main jet. Vapor mix is sourced from the carbs intake bell.

Suzuki…translation in Japanese means…..” gears soft as butter”…