Can I De-Tune my 03 RM250?

[EnduroDude]

Hey there,
I have a power problem...too much, well actually the overall power is nice but the midrange is just too brutal for tight woods. I up-shift to tone it down and the low end gets unacceptable real fast.

I run the Steahly 10 oz weight with an FMF Gnarly pipe and Q muffler. The bike is set up for woods but the motor still feels like its wants an MX course which it never sees.
My thought is to try the stock pipe which never seen use yet to tone down the midrange but I don't want to loose any low end.

For smoother power delivery can I change something else like a thicker head gasket or change timing?
I need smoother power off the bottom for rock crawling and such.

 

[placelast]

I was in the same boat when I moved from SoCa and began riding trails of the central coast; wanted something more manageable at lower RPMs as desert riding was a thing of the past, and slower, tighter conditions prevail here. Eric Gorr had previously mildly modded my stock cylinder + head for more power everwhere, and it worked very well for the desert and other go-fast conditions. Therefore I bought an extra cylinder and had him port it for low to mid.

I believe his excellent book says (effectively, in this order) lowering the transfer ports, reshaping the head's dome, and perhaps increasing compression will accomplish what you are looking for. (Rich R. will post exactly what the book states any mninute now, to follow!)

If it had a tachometer I could be more specific but what his porting did was shift the plateau and "ramp up" to the left, peaking at lower RPM. I'm very pleased with the outcome. Well worth the $ - he knows his stuff.

At elevated RPMs it still has more than needed so I'm wondering if a Dynoport pipe and S/A would help shift even more to the left; am musing if someone can confirm their claim - althought I have no reason to doubt, it would make it easier to sell the idea to my wife (gee: what's this shiney new part cost that just came in the mail? Is it really neceessary?)

 

[Erick82]

Stock pipe will make the mid hit stronger, try some v-force reeds, if it dosen't change it enough, then go for the porting.

 

[1984RM250]

You also could try reed spacers. These help shift the powerband to low revs.

 

[fuzzy]

Need a wide ratio trans :)  ala RMX

 

[placelast]

Originally posted by fuzzy
Need a wide ratio trans :)  ala RMX

Don't laugh: the RM and DRZ gears can be interchanged, according to the Zook off-road team; two of the Yosh DRZs had an RM trans.

 

[fuzzy]

Sweet! There is your solution my friend...You can bet your butt that Team Suzuki's GNCC bikes have this setup! Same w/ team yamaha's YZ250's having WR gears...

 

[placelast]

Originally posted by fuzzy
Sweet! There is your solution my friend...You can bet your butt that Team Suzuki's GNCC bikes have this setup! Same w/ team yamaha's YZ250's having WR gears...

Well, the Cycle News article mentioned the WORCS DRZs, not the GNCC RMs. Nonehtless you have the right idea.

 

[EnduroDude]

Thanks everyone for the advise, I may try the DRZ gears but as I spend more and more on the bike to get it to fit and feel like my old EXC I have to stop and ask - why should I keep changing and modifying this track bike to work like something its not intended for? Still it is fun to show up at the events with something other than orange, dang those bikes have taken Enduro's over!

 

[snb73]

EruoDude,

Try increasing the preload on your exaust valve spring. If you put more preload on the spring, the exhaust valves will open later (at higher rpm's). Keeping the exhaust valves closed longer (for more of the rpm range) should give you more usable torque and less of a hit.

This won't replace a porting job, but should give the bike more low end torque for more of the rpm range. Plus it's a free mod.

Hope this helps, Steve.

 

[Nevada Sixx]

what if you go down one tooth on rear sprocket so you wont have to shift up to tame the power, and go to a 16 oz flywheel weight? if you put a small dent on the underside of your pipe, next to where your cylinder almost touches your pipe, that will mellow it out,, at least it did on my yz.

 

[snb73]

EuroDude,

I forgot to add, try a smaller front sproket. Go down a tooth or two. This will mimick a larger rear sprocket for about 1/3 of the cost. It's another inexpensive way to change the gearing.

Although this will make the bike "rev" quicker (higher numerical final gear). You will be able to handle those tight sections at a more controled speed using the same gear in the same rpm range as before.

Steve

 

[EnduroDude]

Originally posted by snb73
EruoDude,

Try increasing the preload on your exaust valve spring. If you put more preload on the spring, the exhaust valves will open later (at higher rpm's). Keeping the exhaust valves closed longer (for more of the rpm range) should give you more usable torque and less of a hit.

This won't replace a porting job, but should give the bike more low end torque for more of the rpm range. Plus it's a free mod.

Hope this helps, Steve.

Hey Steve,
I see 4 springs in te Exhaust valve assembly, looks like the exhaust valve return spring can be preloaded with addition turns on the cap that holds it in. There is also a spring on the exhaust valve Governer that would require shims to preload it. Are you talking about the first or second one mentioned?

 

[snb73]

EuroDude,

I'm refering to the spring that is located on the top left side of the cylinder(#14). You should be able to loosen the two screws(#13) holding the exhaust valve retainner(#12). Turning the exhaust valve cap(#10) clockwise will increase the preload. I wouldn't mess with any internal componants.

Here's a diagram, yours should look simular.

http://houseofmotorcycles.bikebandit.com/partsbandit/showschematic.asp?dept_id=124027

Steve

 

[EnduroDude]

Originally posted by snb73
EuroDude,

I'm refering to the spring that is located on the top left side of the cylinder
Here's a diagram, yours should look simular.

http://houseofmotorcycles.bikebandit.com/partsbandit/showschematic.asp?dept_id=124027

Steve

Cool, thanks Steve- that is the one I was thinking about. Factory setting is 1/2 turn preload. I will try 1/4 turn at a time past and test it out.

 

[Vytas]

Actually you want to lessen the spring force on the exhaust valve.  This will result in the valve opening easier and sooner.  If the spring tension is increased then the valve will open when there is enough force generated to overcome this spring force.  The greater the spring tension the HARDER the midrange hit, lesser spring tension means MORE MANAGEABLE midrange hit. 

I have my adjuster turned back, CCW, from the stock position.  Also, Eric Gorr "Mo Better" porting, FMF SST pipe, and V-Force reeds.  Power delivery was completely different, more controllable.

 

[snb73]

EuroDude,

I'll be interested in your results. I would suggest using a "sharpie" permanent marker to mark your starting point. I'd keep a log and annotate what effect each position had, bad or good.

Vytas,

My reason for this suggestion is as follows:

I purchased my bike used and rode it for about a week before the top end rebuild. The bike ran fine with good low end.

While taking my top end apart for the rebuild I found my exhaust valve rod (#31) was not connected to the exhaust valve lever (#19). The rod was just "leaning" to one side. The upper exhaust valve rod clip (#32) was missing, although it was connected to lower exhaust valve actuator (#23).

I finished the rebuild, but the 99 cents clip (#32) was on back order. I'm divorsed and my son, who is 12, was coming for his christmas visit. He has a 1996 XR-100 and this was going to be our first time riding together. Something we both were been looking forward to.

I figured what would it hurt, I rode the bike with out connecting the exhaust valve. We had great weather the week of his visit. We put in about 4 hours of riding a day. The bike ran fine with good low end.

Wouldn't you know, the day after he left the 99 cents part came in. I was interested to see what effect connecting the exhaust valves would have. The bike had lots more top end, but lost some low end grunt.

My theory is this:

If the exhaust valves open later it will constrict the exhaust gas flow. This should allow more low end for longer of the rpm range. As was my experience with my valve not opening at all. Unfortunately, we've hit the low 30's in southern Maryland. Otherwise I'd be playing with my exhaust valve spring and recording the effects.

Let me know how things turn out, Steve.

 

[snb73]

Oops, I forgot to add the link to the diagram. Steve.

http://houseofmotorcycles.bikebandit.com/partsbandit/showschematic.asp?dept_id=124027

 

[joereitman]

Anyone consider a restrictive washer between the cylinder and the expansion chamber like in the JR50 and the PW80? These old bikes of my son's were docile with the washers in, hit harder once washer out. The washer can be drilled out if it is too restrictive. Any thoughts?

 

[EnduroDude]

Originally posted by snb73
EuroDude,

I'll be interested in your results. I would suggest using a "sharpie" permanent marker to mark your starting point. I'd keep a log and annotate what effect each position had, bad or good.

Excellent idea Steve,
I may even make some permanent notches there for future tuning. I am pulling the suspension for mods there so won't be able to ride it for a while and will post when I can.

 

[EnduroDude]

Originally posted by joereitman
Anyone consider a restrictive washer between the cylinder and the expansion chamber like in the JR50 and the PW80?

Thats an interesting thought but I bet that it would cut performance everywhere bottom to top. Midrange is too hard a hit for me where I ride but I would welcome even more bottom end down real low. My top end power is no good at all but I hardly ever ride it there so I don't even notice unless I wring it out on a road.

 

[EnduroDude]

Originally posted by Vytas
Actually you want to lessen the spring force on the exhaust valve.  This will result in the valve opening easier and sooner.  If the spring tension is increased then the valve will open when there is enough force generated to overcome this spring force.  The greater the spring tension the HARDER the midrange hit, lesser spring tension means MORE MANAGEABLE midrange hit. 

Thanks for the info Vytas,
How is your bike used- MX, Enduro, CC etc?

 

[Vytas]

MX :thumb:

 

[EnduroDude]

At Quicksilver I went to a 49 on the rear vs the 51 I had, plus made suspension changes for off road use vs motocross.
The combination kept the rear hooking up and tamed the motor just enough.
I may leave it alone...it worked real well. Thanks again everyone for the advise and info.

 

[motometal]

You could retard the timing, but this would probably take away low end (as well as midrange).

If the power valve opens late (more preload), by the time it opens you will be closer to the peak of the powerband, and it will hit harder.

Conversely, if it opens sooner, the tuning at that rpm will be less than optimal, softening the power delivery in that (mid) range. As the rpms rise, the system gradually comes into tune with the valve open the whole time.

You won't hurt anything by tinkering with the preload setting, give it a try. Don't overtighten the two screws holding it in place.