Getting some torque out of a RM125D.

[Nathan S]

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?

[Lozza]

 Is it a power reed model or the full reed before suzuki went back to the power reed?

[Slakewell]

Try it with no muffler.

[Digga]

Not sure if they make one of these for that model or if it will help?

http://www.motorcycle-superstore.com.au/36020/i/moose-racing-torque-spacer-kit

[flyingdg]

It's all in your head......or your motorcycle's head that is. The RM's of that era had full hemi heads with no squish band at all. That will make a great road race motor (HIGH RPM) but sucks as a motocross motor. I would have it welded up and have a new combustion camber cut in it like RK Tek does. DG   

[OverTheHill]

Someone not been in there & over ported it [like me--bigger is better, woops that didn't work, wadda we do now], add some length to the header to try & reclaim it. Not helping sorry--umm, strobe it just to varify the ignition isn't stuck on retard or summit. Lozza's the man for all the right ideas!!

[OverTheHill]

i'm sure this isn't relevant but had a pe175 full floater 20 years ago that had blown the seal out on rh side of crank. Not that familiar with them now but think the seal came out on an angle at the top till the primary gear stopped it so didn't smoke terribly [or had drunk a lot of oil before it got to me] & just sucked air, ran terrible & gaspy low down so had an obvious fault not an obscure one like yours.

[John Orchard]

I remember testing the D & E models for Trail & Track back in the day, yeh they were screamers.

From memory they may have had flat-top pistons, might be interesting trying to get a squishband working correctly?  I didn't think small combustion chambers of a 125 were as critical about squishbands compared to big-bores?  I guess it made a huge difference between the RM250 Z & D models.

I think they were 'full reed' engines?  I wonder if you could find a 'domed' piston from another bike and get a squishband welded into the head?  It'd be an interesting experiment.

Much of the 'top-end' characteristics would have to come from the port-timing, although our tech guru 'Lozza' might say "it's all in the pipe"?

Maybe speak to Geoff Morris re a pipe to transform it, failing that .......... ride it like a 15 year old  ;-)

[Nathan S]

It's the "full reed" model.

[GMC]

Just drop it around and I will build you a pipe

[Slakewell]

Sometimes the jetting is so shit they dont run correctly until on the mains. So clean that up. check the timming etc
Maybe buy a new 30mm carb? Add 20mm in the header pipe. Will earlier model head and piston fit? 

[smed]

So... is there a particular reason why they're so hopeless?

Yeah, They are a pre powervalve 125cc  The best fix,Rev the shit out of it or get a 250  

[D project]

All the tests if have read from back in the day didn't rate the 83 !motor too good.
125's having any torque, is that possible. Although my 81 has more than my 84!

[Davey Crocket]

Stop riding moderns Nathan, they are spoiling you.... Suzuki won every World 125 championship from 75 to 84....they didn't make too many dud 125's.

[Nathan S]

My YZ125X has about 500% more pull down low, and probably still has 90% of the Suzuki's peak power. This isn't just "old 125 MXer", and it doesn't feel like there's anything particularly wrong with the bike.

Got a #30 pilot jet in it now. Figured the symptoms were more-or-less similar to a too rich pilot jet, so I tried a 25 and it was clearly the wrong thing to do: wasn't any better down low, and wouldn't come on song at all.