De-tuning a dirt bike

[rmak]

I may have brought this subject up before here. I've asked on several forums, but have not gotten a definitive answer. Or maybe I have and don't like it.
I've inherited my oldest's Suzuki RM125, which is quite a quick motocross style bike. I want to de-tune it and gear it to be more of a trials style bike that I can use to put around our field and woods.
When I've asked I get change sprocket (not a whole lot available from a dealer), do flywheel work (too much expense and too much hassle). One suggestion was to sell it and buy a trials bike, but what's the fun in that?
If there's technology to make these bikes faster, there must be a way to reverse the process. What effect would a smaller exhaust chamber have on it? Anything I could do to the carb? Any other gearing options? (maybe other sprockets that could be adapted?)
Anyhow, maybe there's new members with ideas or old members with new insights. Thanks!

[gumpt1oo]

I'm no expert so I may be barking up the wrong tree, but over here in England learner riders are restricted to 33bhp machines. I know that bigger cc 4 stroke machines are restricted to 33bhp by using a reducing washer in the exhaust header pipes, and I think the inlet is reduced also. Not sure whether this would work on a 2 stroke with a power band though.
The other option would be to go smaller on the rear sprocket if a bigger front sprocket is not available, or maybe a smaller carb?

[mrt202]

I would imagine that de-tuning a 125cc 2 stroke would leave you with a very disappointing and barely rideable bike. I doubt you can do enough to make it "like a trials bike". If you just want to be able to ride it slower without smoking the clutch prematurely, your best option IS re gearing. Go to the minimum possible number of teeth on the front sprocket, then start adding teeth to the rear til you get something close to what you're after. I've done that to an old KTM 250 and a Penton 250 to make them fun on tight, slow trails.

[davidc]

Yeah, I was thinking sprockets to be the only reasonable and afforable way to go. Don't see many ads for "We can slow your bike down!"

[rmak]

Thanks for the input, guys. As luck would have it, one of my buddies said he is interested in buying my Suzuki as is. That creates a new problem. Where could I find a good older trials bike?

[BluesRider]

I remember you asking this before,and I think my advise was something like"trade it for an XR200"Sounds like that's pretty much what you decided to do,albeit for a trials bike.

As you have no doubt figured out,gearing down is a bad idea.All that's going to do is make the power hit that much harder when it hits,and do nothing to move the power band down lower in the RPM range.Uncontrollable wheelies will not only be present in the first two gears,but now third,and probably fourth,too.

A heavier flywheel would help,but do nothing to widen the powerband.It would just soften the hit,to make it a bit more controllable.

Something cheap you could try is double stacking the head gasket,to lower the compression.That's an old enduro riders trick.I'm assuming it has a metallic head gasket(if it has a gasket),like every other high perf 2 smoker I've ever worked on.I wouldn't try this with a fibre gasket.A safer way is doulble stacking the cylinder base gasket(less chance of blow out),but that will also change port timing a bit,so could be better-could be worse.

Restricting the exhaust would probably work too,but I think it would play havoc with the jetting,and probably create overheating problems.

The only real way to get the type of power curve you are looking for is expensive,and you probably couldn't even find anyone with the skills to do it.A reshaped expansion chamber,a welded up and re-ported cylinder,and a smaller carb is what it would take.

As far as finding a trials bike,they're kind of rare beasts in the U.S.,and a bit spendy,if you ask me.Keep checking fleabay and craigslist,and put it out on the forums you frequent.A while back,one of the guys on my local RatPack forum was selling a Gas-Gas.I'll see if he still has it.

I'd look for a '70s vintage Bultaco,Montesa,or Ossa,if it were me.They're muy fria!