Making a 2-stroke good on tight trails?

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#1
I recently bought a 91 RMX250 and it looks like I will be doing mostly tight, wooded trails. I rode earlier this week and man was this thing a bear! Most of the trail I never got out of 1st gear and found myself "feathering" the clutch way too much. Once I got into the powerband I was fine but there were a lot of tight sections where the motor just bogged out. I have heard the biggest improvement I can make to cure this is gearing. My local bike shop says since I have a new chain to just go down 1 tooth on the front. And if I still need more bottom end to go up at least 2 teeth on the rear which will require a new chain. Does this sound right to you guys? Also I have read about flywheel weights. Could someone help me out with the pros/cons?

Thanks!!!
 
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#2
Dropping a tooth ot two on the front sprocket sound like the best bet. A RMX is built for the bush/trails etc... so gearing should be no problem if stock.
 

RM_guy

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#3
One tooth down on the front is about the same as 3-4 teeth up on the back. A smaller sprocket on the front is hard on the chain so I would get a 2 tooth larger rear and splice in how ever many links you need to get it to fit. Having 2 masterlinks is better than a smaller front sprocket IMO.

In anycase try to get the wheel as far forward in the swingarm slot to shorten the wheelbase. That will help turning a bit.

A heavier flywheel would help too but I'd try the gearing first.
 

CaptainObvious

Formally known as RV6Junkie
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#4
If the bike has a low speed bog I would look at your jetting. An adjusted air screw and a changed clip on the needle is a free power mod!
 
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#5
With more riding experience in general and tight woods specifically - we don't know how much you have - your speed will pick up as you gain confidence, and as a consequence should be using less clutch.

Until then it would not hurt to lower the gearing one or two teeth in the rear; the front is already as small as one should go at 13T. The chain will need an extra link and master with the o-rings of the same brand/model; try to bum one off of a friend or shop.

A few years back I had a 52T on the back of my RMX, but found I spent too much time in the upper gears, and went back to stock. Last spring I tried it again but soon discovered it resulted in exiting tight corners/turns with too much RPM, lifted the front end and lessened control - stock was better as it forces you to use lower engine speed and hence less rotational mass effect for easier control.

Do raise your forks in the clamps until the caps almost touch the handlbars. If you are 5'-9" or taller then buy Ceet's tall foam so you can scoot forward in the cockpit. Taller bars do help, as does a new upper tripple clamp to move the perches forward, albeit the latter would be a difficult to impossible find these days new - try e-bay; the RM250 of the same years had similar dimensions/components.

An added flywheel weight would improve steady low-RPM plonking. If you do use Steahly's then buy an IMS/Moose/MSR shift lever because it needs to bent out to clear the ignition cover with the added spacer.

If the carb is a stock Mikuni and it runs cleanly, then congradulations: you're doing better than most.
 
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#6
Top clamp: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2432341909&category=35587
 
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#7
I guess I should have been a little more specific. I used to ride some when I was younger on a yz80 and then spent some time on a KX125. I mostly just rode on open farm land where you could run wide open. I'm nowhere near an expert but I'm certainly no a newbie either as far as riding is concerned. My riding buddy has a bone stock 88KDX200 which I rode some the other day on the same trails and its night and day difference between his bike and mine. I just like the way the KDX "lugs" over obstacles at low rpms and then takes off when you snap the throttle. If I lug my RMX it stalls and when I give it more throttle it just bogs out. If I can get in the powerband it runs great but there's just not much low end power. Maybe the carb does need some work or replacement. Any RMX experts out there?

The trail I was on has a lot of technical sections where I guess a 4-stroke would be nice so you could just idle along. Lots of creekbeds with a lot of rocks that you couldnt really just blast thru. There are some tight turns that as soon as you come out of you have steep hill. This is where it was hard to keep enough speed to keep from bogging out.
 

gwcrim

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#8
If the bike has a low speed bog I would look at your jetting. An adjusted air screw and a changed clip on the needle is a free power mod!
And a new pilot jet is less than $10. If the bike doesn't idle, the jetting could be off. The pilot jet and idle mix screw (air screw) regulate the a/f mixture at low throttle. It's crucial that these jets are correct if you want good low end power and throttle response.
 

lwsmithjr

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#9
The RMX has an adjustable preload on the powervalve. Could be yours is way off. If it is opening too quickly (or stuck open), the bike may exhibit some of the symptoms you describe. I had a '90 model and rode tight woods with no problem. The jetting was a little bit difficult, but if I remember correctly, I ran a 350 main with the clip in the 2nd notch (from the top) on the needle. Mine had aftermarket exhaust and cylinder base and head gaskets from the same year RM250 (thinner gaskets which raised compression). I do not remember the pilot jet I used, but I do remeber that it was on the lean side and would barely idle. One size up and it blubbered at low throttle. I decided it was better to deal with a bad idle than a blubber off the bottom. Gearing was 13/50.

It could also be time for a new top end?
 
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atc3434`

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#10
lwsmithjr>>> A new top end was my line of thought too... or another mechanical problem, such as the powervalve, or even a bad crank seal.. that'll give you a lean bog down low for sure. Just some stuff to check into.
 
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#11
Those were great bikes,I had one and loved it.Since then I've had a string of RM's I have used for Hare Scrambles and Enduros.You've been getting some good advice here.

First,I'd make sure the compression in the engine is within specs,then you're not chasing other problems that are really a worn top-end.If thats ok,then I'd make sure the powervalve is at the stock adjustment.Now it's time for jetting.Get someone experienced to help you,those bikes could be made to run very crisp,top to bottom.Just don't start chasing jetting without knowing your compression is good,your air filter clean,and premix fresh.The stock exhaust and muffler sucked,I'd run an FMF Gnarly or ProCircuit Platinum 2,and jet from there.

The gearing is a personal preference, but a couple years ago I noticed the GNCC Pro's would actually gear their bikes a litlle UP ! Sure,they're way faster than me,but with a properly tuned motor,their bikes already tuned for a lot of low end via pipes,jetting,etc., you can make the motor pull by short shifting and have a much less tiring experience.Too low can make your bike wheelie prone,and you'll not even use first and sometimes second,because the bike just wants to stand up.

For instance,stock RM gearing is 13/49,Team Suzuki Off-road usually runs 14/48 or 49.I went to 14/49 and completely love the way it works.Good luck with your bike,and while you're working on it check every nut and bolt,especially the engine mount bolts.Loose or gone engine mount bolt = cracked frame. Doug
 
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#12
P.S.-I love my flywheel weight,a Steahly 8oz. I wouldn't buy one though until you get the jetting and gearing where you want them first. Doug