OzVMX Forum
Clubroom => Tech Talk => Topic started by: Nathan S on September 10, 2014, 09:24:37 pm
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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?
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Is it a power reed model or the full reed before suzuki went back to the power reed?
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Try it with no muffler.
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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
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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
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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!!
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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.
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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 ;-)
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It's the "full reed" model.
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Just drop it around and I will build you a pipe ;D
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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?
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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 ;D ;D ;D ;D
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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!
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Stop riding moderns Nathan, they are spoiling you.... ;D Suzuki won every World 125 championship from 75 to 84....they didn't make too many dud 125's.
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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.
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Whats the "stinger" diameter?
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Checked the compression?
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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 (http://yz125k.weebly.com/articles.html)
Cheers,
Alex
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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.
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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!!
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There will be combination of factors. Basicaly you have to spend the time tuning the engine.
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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.
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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.
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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.
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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.
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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 :o
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.
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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.