Getting some torque out of a RM125D.

[vandy010]

Whats the "stinger" diameter?

[vmxrider]

Checked the compression? 

[alexbrown64]

I'd read lots of things about how they have sod-all bottom end power, but this is way worse than I expected!

The bike seems to be in decent mechanical condition, and the jetting is fairly well right. Running 12/50 gearing, so it's not like its over-geared.

So... is there a particular reason why they're so hopeless? Any easy/known fixes, or do I need to work through the usual things?

This is floating around a few places and i have put it on my website as i'm slowly doing up the rival Yamaha YZ125K from 83.  The last article on the page gives you all the hop ups and stuff for your D
http://yz125k.weebly.com/articles.html
Cheers,
Alex

[YZAT252]

I had the same problem with my RM125B, I bought a new carby with correct jetting off Mikunioz, what a difference! Pulls way harder out of the corners now, without having to fan the clutch, best money I could have spent. In saying that though, I have no idea if the previous carb was jetted correct.

[DR500]

My 125D has very little torque below 75 % rev, but that's how a 125 is. But when she's on the power band it goes like the wind. Full revs and change gear as fast as you can!!

[Lozza]

There will be combination of factors. Basicaly you have to spend the time tuning the engine.

[Nathan S]

There will be combination of factors. Basicaly you have to spend the time tuning the engine.

With the Flux Capacitor?

Stinger diameter and combustion chamber shape both stand out as "of course!" moments.

[Lozza]

Stinger diameter will just condense or expand the power curve. After testing many many head shapes (on the dyno and track) I can confidently say it won't be head shape. As always it is a combination of jetting , ignition and compression then having the pipe to match that.

[Slakewell]

Again IMO one of the best ways to get your VMX back to being sharp is a new carb. Why we expect some old worn out carb to still function as new is beyond me. For around $150 it is great bang for buck. A nice new 30/32 would do wonders no doubt.
Nathan I did buy some V force reeds of one of the guys here and found his service very good. Nothing will make your bike suck more than like an old worn out carb as much as stuffed reeds.
Be a cool project to follow how you resolve this.

[MaxPower]

I had a 81 RM. loved it
Got an 82. Slow as balls. Had it ported. Meh. All season it sucked and I accepted the blame. 2 races before the end of the season the stator stopped making spark. I got a new stator and holeshotted the final motos. That thing was a rocket. All season I rode a bike that made half the power because I just though that's the way it was. And it was a defective ignition.

[80-85 husky]

mate had to PE 175's an N and a T. he pulled the restrictors out of them and the airbox lid and rode them like that. I bought a 250 X, got it timed, jetted and the muffler sorted and after he rode it he was in shock

He took his T in and got it sorted and was kicking himself he had spent 2 years riding dogs when he had all the real power sitting there.

[OverTheHill]

back in the PE400 days i seen to recal them having a restrictor at the start of the tail pipe [or something] We were Yamaha so only heard rumours--unless it was the std RM400 pipe. Someone will know if i'm talking crap 'again'. Was just wondering if something like that in a 125 [original pipe]. Can't beat new rings & a good bore. If in doubt, bore it & start again--then it's as sharp as it's going to be in that department--then go looking elseware if not happy.