Tuning and modiying your ( actually my) two stroke MX 'er for four stroke like power

sosomechanic

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Jan 8, 2001
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Like a lot of guys here, I ride off road on a 250 MX bike that will never see a track. The light weight and quick handling are fun and less tiring in the sometimes tight and twisty woods stuff here in Alabama. The main gripe I have with the MX two strokes is the abrupt and peaky power delivery that is much harder to ride and manage than the four strokes in less than optimal traction conditions.. I love the four stroke engines ( last bike was an '01 YZ 426) but the extra weight and arm snapping torque sap enough extra energy to make longer rides less enjoyable for me, hence the quest to produce the most broad and tractable power I can from my light and flickable 250 two stroke.
My current ride is a 2003 RM 250, here's some very detailed info on what I've done so far to try to make it run like a four stroke :) So far my mod list is as follows: Started with a 10 ounce flywheel weight, ( still massive hit and wheelspin, might be enough if you're riding on a groomed MX track). Next made a 19 ounce flywheel weight ( heavy as a boat anchor, and the most I can fit under the stock stator cover using a 3/8" thick spacer plate). This made it less prone to stalling but still a motor with weak low end compared to the massive midrange hit and very good top end. Still very pipey and hard to control on the hardpack clay gas lines, gravel roads and steep slate hills we climb in a local strip pit. Next I decided to go more aggressive and did a home porting and head machining job for low/mid ( took 1.5mm from cylinder base, used epoxy to reduce size of rear transfer ducts and remove hook, widened squish band on head and removed material from dome to lower compression to 195 psi) . The porting job definitely bulked up the low end noticeably ( though it's still far from a four stroke) and smoothed the midrange hit somewhat while diminishing the overrev a bit. Not as much difference as I hoped for ( I used the spec's from Eric's book on the low mid port timing and squish band, thanks Eric!) . Tractability and controllability were significantly improved from stock at this point, but even after much jetting and power valve tension adjustment tinkering the improved ( but imbalanced compared with the strong mid range) low end ramped into a midrange surge that was difficult to control in the slick, loose and hardpack conditions I ride in. The porting made a significant difference but the power delivery is still somewhat tiring and at times frustrating( when the pipe comes in at the wrong time) :bang:
It seemed to me at this point that I might best concentrate my efforts in bulking up the low end torque so the ramp up into the mid range surge would be smoother.
I've seen the posts about the carb divider plates really bulking up the low end on the KDX's so I decided to make one. Made it from 18 gauge steel and profiled it to match the slide curvature. I put it in on the reed side of the slide like the RB designs plate. Initially the jetting was so rich after installing the divider plate that the bike would barely run. Went full lean on needle clip, and put in a 40 pilot ( stock was 50). Still way rich from idle all the way up until the main jet came in, even with the airscrew several turns out on the 40 pilot. This dramatic change ( perhaps improved fuel signal) made me really wonder how it would run if the jetting could be dialed in so I ordered a 35 pilot ( smallest available from Keihin for the PWK carb) and the largest straight diameter needle available ( two sizes up from stock needle) and pulled the divider plate until the parts came in.
While waiting for the jets to come in a V-force reed setup arrived that I'd bought on a well know auction site starting with E and ending with bay. The V'Force reeds made the engine feel slightly more responsive in the low end and definitely made the mid range surge come in quicker and at less throttle opening ( might not produce the same results on a stock cylinder). Nothing dramatic, but probably worth the $43 it cost me. The V-Force didn't like the stock 50 pilot and was way rich. Dropped the pilot to the new 35 and still had to go out on the air screw about 2.5 turns to get clean response at 1/8 throttle and below. It richened the low end jetting considerably, didn't affect the needle much..
Decided to try the divider plate again since I had the new jets in hand, started with the 35 pilot and stock needle set to full lean. Ran very well right off the bat and only needed the air screw richened to 1.5 turns out to be remarkably crisp and clean running all the way down to near idle speed lugging. Strangely , the V- force seemed to lean the jetting when combined with the divider plate, especially the needle..
With the divider plate in the low RPM, low throttle opening (1/8 and below) torque and snap were dramatically improved. There was no way to miss it if you lugged it around a gear high at low RPM' s and rolled the throttle open. I've spent a load of testing time the last several weeks evaluating the low end, low throttle opening feel of the bike and trying to improve it and the divider plate felt like it nearly doubled the torque and snap in that range ( mind you, this might only be adding a few ft. lbs. of torque). The reason for all the low end/ low throttle opening testing is that I find it easier and more effective in the loose and slick stuff to ride a gear higher on two strokes and torque the motor to prevent wheelspin ( assuming the bike has enough low end to do it without having to muscle up into the midrange surge). You can still go plenty fast in low traction conditions like this IF the bike has enough low end. The RM didn't have enough here stock to be quick when ridden this way, the divider plate helped a bunch here.
This was posted for people interested in getting the most useable and tractable four stroke-esque power from a lightweight and well suspended two stroke MX bike. This bike is still far from a four stroke but is much easier to ride on the low to moderate traction stuff now. Still not where I want it yet but getting there, that's the fun part. Feel free to add suggestions to the thread that are useful for making two stroke MX power less peaky and torquier.
Anyhow, I'm still looking for some port timing specs from enduro machines ( KDX, EXC etc.) to possibly tame my cylinder port time/ area more for a broader, flatter, and torquey power delivery. If you can help here I'd appreciate it. A tracing of the port windows on a piece of paper ( scanned or photo'ed and emailed) would be great if anyone has an enduro cylinder sitting around. I suspect the transfer port ducts are too large and the transfer port windows too large also for the kind of smooth and torquey power I'm ultimately planning to build. Any expert testimony here?
Thanks and have fun, Michael.
 

gwcrim

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Oct 3, 2002
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Wow... imagine if you had done all that work to a big bore. You'd have tons of low end. Wanna do my KX500?
 

WillyM

Member
May 18, 2004
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power

I have built alot of 250s of the green kind and if you lower
the compression it makes it more "peakey" on the dyno
and if you bump it up it makes more torque.
I think that if you went to a 38 mm carb it would help also
but it is a shot in the dark ,smaller carb more compression and a fly wheel weight should do a ton.
epoxy the transfer ports also make them smaller and that would also help.
 

sosomechanic

Member
Jan 8, 2001
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Here's the way I did the divider plate: I made the plate a little wider than the bore's max width and carefully pared down the width on a belt sander until it was a very snug fit in the throttle bore. Next I marked holes on the outside of the carb body a little above the throttle plate and drilled through and tapped for #10-32 set screws. These holes were drilled at about a 15 degree downward angle. Two of these on each side apply significant downward pressure, and since the bore gets narrower below the plate this locks it in place pretty securely. I used loctite to make sure the set screws stay in place and are sealed. Two of the screws are inside the reed manifold, so short set screws of 1/8" length were used on the forward two holes so they wouldn't interfere.
There are any number of ways this could be done, this was just the first idea I had that would work with stuff I had lying around. If you do it this way the accuracy of the holes is pretty important for a clean and sturdy install.
Regards, Michael.
 

sosomechanic

Member
Jan 8, 2001
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Response to WillyM on power reply above

Thanks for the contribution Willy. Lowering the compression was mandated by detonation. I didn't make it clear in the original post, but I actually raised the compression by 20 psi or more with the original porting and head mods, this detonated substantially on 93 octane pump gas so I bumped compression back down so it would live. The RM engine is extremely snappy or responsive with virtually no flywheel feel whatsoever in stock trim. Even with the 19 ounce flywheel weight the stock power delivery was difficult to control on low traction surfaces. This bike came stock with a 38 mm carb, but I have thought about going to a 36 ( maybe that's what you meant). After installing and dialing the throttle divider plate I'm not sure that a 36 would be any better. Perhaps a 36 with a divider plate would be an improvement.
 

jmitchell

Member
Oct 26, 2003
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I've ridden a WR400 and a YZ250 in the woods and I will definitely take my 2 smoke over the WR anyday. The fact that the bottom end doesn't have alot of torque is to your advantage. Also a 2 stroke does not stall nearly as easily as a four stroke. Spending more tme in the saddle will outshine anything you are doing mechanically. :ride:
 

cactusreid

Member
Jan 13, 2003
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soso: the bike you are trying to build is already in production!! It's called a GasGas EC 300! I've ridden a 04 RM 250 in the bush this summer. It doesn't hold a candle to my 300. The gasser 300 is nick named the 3 stroke for a reason. Get a ride on one if you can-because everything that your asking for ,is there. Make sure that your getting a EC model not a XC ,DE, or SE bike as the Ec's have a way bigger flywheel and a much smoother power delivery from what I have heard.
 

d36RM racer

Member
May 26, 2004
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jmitchell said:
I've ridden a WR400 and a YZ250 in the woods and I will definitely take my 2 smoke over the WR anyday. The fact that the bottom end doesn't have alot of torque is to your advantage. Also a 2 stroke does not stall nearly as easily as a four stroke. Spending more tme in the saddle will outshine anything you are doing mechanically. :ride:
I agree the gamboni ranch H.S. last year was a muddy mess it rained all day and I passed tons of 4strokes on my stock 03 RM250 all that torque just caused the tire to spin my rm climbed everything with ease but I also had the right tires dunlop 756's
 

Vic

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May 5, 2000
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Damn that torque!!!

:rotfl:
 

sosomechanic

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Jan 8, 2001
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Reality check time. A couple of the last few posts are plain ridiculous. Saddle time aint the primary factor in making an engine with minimal low end torque, a potent mid range burst and extremely quick throttle response effective and easier to ride on loose and slick terrain. The '03 RM 250 motor ( like most other years for the RM) is tuned from the factory to suck in conditions like my area. Very low flywheel mass and a strong midrange hit are simply nowhere near the most effective power traits for slick clay hardpack, loose gravel, pinestraw, mud or other low traction surfaces which dominate many off road areas including mine. I have a really hard time figuring out how the lack of low end torque on my RM 250 is to my advantage in the woods. If you want to go quick on bikes like this with a weak low end output then you have to bring it up into the midrange where you are constantly trying to keep the HIT from biting you. With sufficient low end , which it now has, I can lug it more and use the midrange burst less achieving better hookup and drive in tight and slick stuff. To advise someone riding a bike with power output that strongly mismatches their riding conditions to forego thoughtful mods ( like porting, flywheel weights and powervalve adjustment) for more saddle time as the best solution makes absolutely no sense.
On the other post, exactly what does it prove if you're flying by the four strokes in slick mud on a stock RM 250? Is that clear proof that the RM 250 was better suited to the conditions? I can think of at least one other possible explanation. How about this one, the four stroke guys you were passing just possibly might be slow ass riders :rotfl: I can't believe someone actually believes that a stock, late model RM 250 is superior in slick mud to four strokes. That is completely detached from reality. :bang:
 

RM_Dude

Member
Jul 16, 2001
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soso ..... knowing what you know now what would you have done differently?
What had the most impact, would you have just done the carb divider and went riding?
 

sosomechanic

Member
Jan 8, 2001
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The carb divider was done after the low/mid porting but I don't think it would have done a lot to tone down the harsh midrange hit, which was a primary objective. The power is very nice now, much easier to ride yet still quite fast when you get past half throttle. I think for the amount of low end increase I was looking for the porting and carb divider plates were a must. The bike is deceptively fast now, much less wheelspin than stock when you roll on the throttle but plenty of upper mid ponies and far more low end. Now if I can figure out what piston to use for a 300cc big bore kit I'll be in there. For a little bit anyway. Later, Michael.
 

Rcannon

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Nov 17, 2001
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SoSo, that last post was anythign but so-so. Great post!

I think the factories leave weight off of the flywheel in an effort to make the bikes feel faster. I am sure the revs will build quicker with less weight.

The 10oz weight I added to my YZ 250 made a huge difference.

I have two cylinders and heads for my bike. One is stock, the other has been pro circuit ported for extra mid and top. The bike is a rocket, but very difficult to use with the porting. Low end power was reduced a huge amount in exchange for more top.

I dont consider myself a top rider, but I have yet to let anyone ride my bike that did not prefer the cylinder with more low end. Most tracks in my are so short that the bikes never really top out before another turn or jump comes up. The low end powerbands are far superior for these conditions. The only place the ported cylinder is better is on flat ground. It is much more fun!
 

sosomechanic

Member
Jan 8, 2001
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You're right on Rcannon about the flywheel mass, suzuki seems especially prone to trying ot minimize flywheel mass so the engine is extremely quick to respond ( thus feeling more potent). This might actually work best for expert riders on well groomed MX tracks but is hardly ideal for mortals riding in less than perfect conditions.
I've had several MX bikes over the years that would cause a white knuckles death grip on the bar when they came up on the pipe. They hit so hard and fast and were so hard to control that they felt like riding a pissed bull. Thrilling and very fast in ideal conditions as you mentioned, but very frustrating when you try to make it work off road. The type of power like this (and your PC ported cylinder) is prone to get you in trouble in a hurry if traction is lacking.
Keep the rubber side down, Michael.
 

Tom Ludolff

Member
Oct 3, 2002
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It seems everyone puts these divider plates in horizontally. Can you put one in vertically. What about both...Like a "Plus Sign" +. What would that do???
 

Studboy

Thinks he can ride
Dec 2, 2001
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Tom, the vertical idea would not work because what they are trying to accomplish is a stronger signal and less turbulence under partial throttle conditions.
 

Lissa

"Am I lost again?"
Apr 28, 2002
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You don't have to go nuts with the engine mods. On most MXers used for off-road I find a simple final gearing change and a flywheel weight to be the most effective mods for tuning the power delivery. The suspension has loads to do with it as well. If your riding a bike that has suspension that is still valved for MX and are riding off-road it only will magnify the hit that much worse when the bike starts to deflect and kick around off of small objects like stumps, roots and rocks. Look into revalving the suspension for the terrian you ride on. I feel this is the single most important mod I do on any of my bikes. Another part of it is throttle control and using the clutch to help smooth the power delivery and find that traction point. I've ridden in some of the nastiest cr@p the ECEA East Coast Enduro series can throw at you. I've never felt that my bike ever lacked anywhere powerwise even on the side of mud incrusted mountains that were littered with rocks. Yes it has less bottom-end then a 450cc 4str, but It makes good useable power that can be tractable when I ride it that way. When I'm racing closer to home I use that healthy mid-range for what its worth since sand is forgiving. Part of it is bike set-up the other half is the rider and knowing how to use the bikes power most effectively.
 

2-Strokes 4-ever

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Feb 9, 2005
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Hey original thread guy. I know EXACTLY what you're talking about with your '03! I ride the nastiest, tightest, rockiest stuff I can find. (sometimes even get a glimpse of the back fender!) Click on my name above (2-Strokes 4-ever) and read my thread about my woods set-up. Got it just where I need it now, and it's awesome. Lots of big power when needed...but tons of torque to make it "lugable".
 

usgpru27

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Jan 15, 2005
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Guys.. What flywheel weight would you recomend for a stock 01 CR125 with a Bills pipe for trail riding? Any of them fit under the stock case cover??
 

XNAGEX

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Mar 8, 2005
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In response to the orignal thread , would it not have been easier to buy a thumper that you liked and started cuttin' weight on it?
 

Chris_S

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Nov 25, 2002
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XNAGEX said:
In response to the orignal thread , would it not have been easier to buy a thumper that you liked and started cuttin' weight on it?

Not necessarily, depending on which attributes are most important to you. First off, it would be tremendously expensive to lighten a thumper as much as a comparable 2 stroke. Even @ the same weight, a 2 stroke will feel lighter due to less reciprocating mass and lower cg. The 2 stroke is also much easier to maintain, and faster/easier to start.

I bought into the 4 stroke hype, now I've seen the light!
 
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