sosomechanic
Member
- Jan 8, 2001
- 24
- 0
Low/mid home porting job report for "03 RM 250, also need big bore info from Eric
First off, thank you for your excellent book Eric. I've really enjoyed using the recommendations in your off road and MX performance books to dial in a number of bikes with very satisfying results. Great information and it's rare and genuinely selfless to make your hard gained experience available so freely. Mega Dittos! :)
Using your reccomendations I've made some pretty big changes to my '03 RM 250 tuning. This has made a major improvement in how well it suits my terrain and riding style.. The main goal has been to 4'strokerize it with emphasis on a much stronger, torquey, chuggable low end with a more linear and broad power delivery.
I mainly trail ride on (gas lines and power lines and fire/log roads) and climb big rutted, rocky hills in surface mining strip pits. The stock power delivery was very hard to control and keep a straight drive on loose or bumpy hills or anywhere traction was not excellent. It was very responsive and would jump onto the pipe TOO quick. Hard to get control something that is so responsive yet lacking in solid bottom end. The quick power burst was not very usueable and could get you out of shape really quick on loose, slick or rutty terrain.
I started with a home made 11 ounce flywheel weight which helped a bit, then made a 20 ounce boat anchor that helped a bit more but it was still just too flat off idle and jumped way too quick up onto the pipe with a frenzy of wheel spin... Bad stuff for slow to moderate speed slick, loose and knotty terrain.
The flywheel weights and much tinkering with the flywheel weights weren't nearly enough for what I wanted so Friday I ported the cylinder for low/ mid. The cylinder base was turned down by 1.5mm, raised transfer ports a bit as their stock timing was pretty conservative relative to the exhaust timing ( 85 degrees main exhaust, 86 degrees sub exhaust, 120 degrees transfers on my stock cylinder), and epoxied the rear transfers to remove the hook and increase primary compression a bit). Also turned the combustion chamber to widen and blend the squish band. All in all this made a dramatic difference in power delivery, much more low end ( perhaps 50% more at low rpms with small throttle openings), much smoother and more controllable ramp up into the midrange (largely because of the beefier low end) and noticeably flatter yet adequate topend pull ( which I rarely get to use anyway and it had an abundance here stock).
The power delivery is far more useable now and required careful power valve tension adjustment to minimize wheelspin and retain decent topend. It wound up a little over 3/4 turn (stock is 1/2 turn). This pulled the exhaust valve in smoothly enough for only moderate and controllable wheel spin ( much more wheelspin at 1/2 and even 5/8 turn) and minimal dramatics while letting it open fully for decent top end considering the port timing now.
Despite much tinkering with power valve timing with the stock porting, the low end was too weak to allow a very useable power delivery for my conditions. I love to wheelie too and stock it was just a bit too scary to really get it straight up using any powervalve setting. The low mid porting and dialing the pValve adjustment made wheelying far easier. :laugh:
This has been pretty detailed, but I had a hard time finding this kind of first hand info on the forums and a ton of people would benefit from this sort of tuning ( people who don't primarily ride on MX tracks).
From here I'd really like to bump the low end torque up a lot more and maintain or even improve the linear nature of the power delivery (flatten and broaden the torque curve more). I wouldn't consider it worth the trouble or expense to gain 15 cc's with a big bore kit, but if I could bump it to 295 or especially 310 I'd do it in a heartbeat, even if some advanced machining/welding is required....
So Eric or others who KNOW, what displacement can be achieved reliably considering this isn't a high rpm race engine? I am an experienced machinist with a mill and lathe in my garage. I'm not scared to weld in a new sleeve and relocate the inner O ring for the head if needed. It looks like the subexhaust power valve stuff might get into the way of a BIG overbore. If needed I guess I could remove them or perhaps relocate them farther from the bore's centerline. I would also be interested in info about how much the stroke can be increased on this engine, a stroker crank with a smaller big bore might the best approach to get the desired displacement increase. One other area I'm curious about is installing a smaller carb ( or better yet putting a restrictor sleeve in the stock carb) and how apparent the difference will be in low to mid rpm/ low to mid throttle opening power. Any insight is much appreciated, Michael from Alabama. :worship:
First off, thank you for your excellent book Eric. I've really enjoyed using the recommendations in your off road and MX performance books to dial in a number of bikes with very satisfying results. Great information and it's rare and genuinely selfless to make your hard gained experience available so freely. Mega Dittos! :)
Using your reccomendations I've made some pretty big changes to my '03 RM 250 tuning. This has made a major improvement in how well it suits my terrain and riding style.. The main goal has been to 4'strokerize it with emphasis on a much stronger, torquey, chuggable low end with a more linear and broad power delivery.
I mainly trail ride on (gas lines and power lines and fire/log roads) and climb big rutted, rocky hills in surface mining strip pits. The stock power delivery was very hard to control and keep a straight drive on loose or bumpy hills or anywhere traction was not excellent. It was very responsive and would jump onto the pipe TOO quick. Hard to get control something that is so responsive yet lacking in solid bottom end. The quick power burst was not very usueable and could get you out of shape really quick on loose, slick or rutty terrain.
I started with a home made 11 ounce flywheel weight which helped a bit, then made a 20 ounce boat anchor that helped a bit more but it was still just too flat off idle and jumped way too quick up onto the pipe with a frenzy of wheel spin... Bad stuff for slow to moderate speed slick, loose and knotty terrain.
The flywheel weights and much tinkering with the flywheel weights weren't nearly enough for what I wanted so Friday I ported the cylinder for low/ mid. The cylinder base was turned down by 1.5mm, raised transfer ports a bit as their stock timing was pretty conservative relative to the exhaust timing ( 85 degrees main exhaust, 86 degrees sub exhaust, 120 degrees transfers on my stock cylinder), and epoxied the rear transfers to remove the hook and increase primary compression a bit). Also turned the combustion chamber to widen and blend the squish band. All in all this made a dramatic difference in power delivery, much more low end ( perhaps 50% more at low rpms with small throttle openings), much smoother and more controllable ramp up into the midrange (largely because of the beefier low end) and noticeably flatter yet adequate topend pull ( which I rarely get to use anyway and it had an abundance here stock).
The power delivery is far more useable now and required careful power valve tension adjustment to minimize wheelspin and retain decent topend. It wound up a little over 3/4 turn (stock is 1/2 turn). This pulled the exhaust valve in smoothly enough for only moderate and controllable wheel spin ( much more wheelspin at 1/2 and even 5/8 turn) and minimal dramatics while letting it open fully for decent top end considering the port timing now.
Despite much tinkering with power valve timing with the stock porting, the low end was too weak to allow a very useable power delivery for my conditions. I love to wheelie too and stock it was just a bit too scary to really get it straight up using any powervalve setting. The low mid porting and dialing the pValve adjustment made wheelying far easier. :laugh:
This has been pretty detailed, but I had a hard time finding this kind of first hand info on the forums and a ton of people would benefit from this sort of tuning ( people who don't primarily ride on MX tracks).
From here I'd really like to bump the low end torque up a lot more and maintain or even improve the linear nature of the power delivery (flatten and broaden the torque curve more). I wouldn't consider it worth the trouble or expense to gain 15 cc's with a big bore kit, but if I could bump it to 295 or especially 310 I'd do it in a heartbeat, even if some advanced machining/welding is required....
So Eric or others who KNOW, what displacement can be achieved reliably considering this isn't a high rpm race engine? I am an experienced machinist with a mill and lathe in my garage. I'm not scared to weld in a new sleeve and relocate the inner O ring for the head if needed. It looks like the subexhaust power valve stuff might get into the way of a BIG overbore. If needed I guess I could remove them or perhaps relocate them farther from the bore's centerline. I would also be interested in info about how much the stroke can be increased on this engine, a stroker crank with a smaller big bore might the best approach to get the desired displacement increase. One other area I'm curious about is installing a smaller carb ( or better yet putting a restrictor sleeve in the stock carb) and how apparent the difference will be in low to mid rpm/ low to mid throttle opening power. Any insight is much appreciated, Michael from Alabama. :worship:
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