sosomechanic
Member
- Jan 8, 2001
- 24
- 0
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.
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.