I've noticed that when porting and cutting, you end up losing something but gaining something. The art is to have the gains outweigh the losses. My 250R porting provided amazing top end results, and I mean amazing...but the bottom end suffered a little. The top end benifits were way better than the loss of the bottom end suffer. If someone can show me all around gains I'm all for it. Another great video and excellent explanation from Chuck, great show!
Keep in mind that the more material you remove from the transfer port tunnels, cylinder skirting and the bottom end in general, the more you increase the volume of the lower end, and the engine loses pumping energy. The larger volume of the bottom end (the primary compression) will open the reeds later, (reeds like tight primary compression), and may even result in less charge flowing into the cylinder from the transfer ports, with slower velocities.
Please do a video on how well it runs and what differences you feel actually riding the bike. I’d really appreciate that! I have a yz250 too that I’m going to restore at some point but have a few bikes ahead of that one so just doing research on everything at this point. Interesting video! 👍
Perhaps it's time for you to build a backyard dynamometer to quantify your testing. Are you familiar with the Two Stroke Stuffing guy? He built on out of a bunch of junkyard parts. Alternately a friend with a stop watch can be helpful in that sense .I've played with port timing on a older GG 250 and at one point in my experimentation the end result yielded a power band about as wide as the edge of a razor blade.
It’s not practical enough for me to put effort into. The fact is that modern engines are excellent for 99.9% of riders. Modifying them increases the risk of failure & gains are minimal even in the best cases.
@@ChuckfromTrueTech If you can find a dyno somewhere, would make for some really nice content, especially as you have a stock cylinder ready to go so we could really measure the effects of the porting.
Keen to see how this goes Chuck! Port timing always seems to set the character for the whole bike and how it performs for different riding styles. I'm a big fan of low-end tractor power, currently on a Beta 300 for this reason.
Don't forget to do the head... both together is where you see the most benefits.... or just get a head from an 03 YZ250 and the sand or down to get the correct squish you're looking for. The 03 had higher compression... and with all this porting and potential head work, run C-12 at 40:1 and tune it. It's likely you'll get detonation and you'll get the most out of your work.
I got some slight detonation from a stock 2019 YZ250, which had very rich jetting when stock, (which has lower compression since 2010 I believe. I have the heads. .0085" difference is what I measured at the head gasket surface to the edge of the squish band.), using 90 octane REC (alky free) fuel from a local gas pump (My area does not sell 93 octane REC gas). The detonation was at the intake side of the cylinder, where the cylinder meets the head. The main culprit? The stock reed cage, specifically the reed stop plate that blocks off the 5th transfer port tunnel, making that port, that cleans the top of the cylinder from hot exhaust gas there, almost useless. It was those hot exhaust gasses, stuck in that corner, that caused the detonation, so it looks to me anyway.
The bulk of transfer flow volume flows through the top 2/3 of the transfers. Matching the transfer floor to the piston crown edge usually causes vortices 10deg BBDC to 1deg ABDC which --- hampers total transfer flow.
Hi just subscribed looks good and you obviously seen the way the barrel height is way off one standard base gasket and the ports are covered badly at BDC, what do you think of this.
@@ChuckfromTrueTech He's confused. Having the piston crown above the transfer floors at BDC creates a boundary layer that promotes smoother flow at the upper 2/3 of the transfers.
Curious what dremel tool your running. Ive waited forever to get one and now im ready to pull the trigger on one. Im thinking flexable shaft unit and one thst can take standard size burrs and be able to runn the wired little bits as well.
There’s only one industry standard and there are many good reasons for it. Foredom tool. Extremely robust and very handy. Unlimited tooling and replaceable and reasonably priced parts.
The current YZ250 is highly developed for it's intended purpose. It is unlikely that you'll make much, if any improvements at all. Perfecting the combustion chamber for the fuel you are running and the RPM range you'll be using will produce the best gains. The ducting in the crankcase and bottom of cylinder is shaped to push the charge back towards the secondary transfers, it gives the smaller secondaries more velocity while still allowing low velocity charge good access to the primaries. It also is the start of the "loop" of the charge to begin before even entering the transfers, making it more effective at exhaust evacuation. To truly understand 2-stroke porting you must look at the entire loop system. The current YZ250 is flow, loop, AND pulse tuned. Not something you'll easily experiment with without truly in-depth knowledge and tools.
This is why I experiment on my own stuff. I don’t have any intention of doing this for customers, just my own curiosity. The viability of a business based on enhancing performance (besides bolt-ons) is questionable considering the bikes are as good as they are.
I do port 2-stroke engines for a living, (small 30cc engines approx), and after 18 years off a bike (Am 61 years old now), I picked up a 2019 YZ250 2-stroke of course. I never had to do more modifications for any bike I had before this (last was a 97 KX250,--loved that engine and working on it a bit). Man, the bike was tall, and the engine is a feeble high rpm power engine that will never keep up with a 450 4-stroke that typically has 15 more hp. But that is what the YZ250 is designed for. I wanted more bottom end power (not doing MX anymore, and glad I am done because it is too fast today because of the faster 4-stroke), to make it a good bike for anything. :) I got the lower rpm power I wanted. Here is what i did. (I do not remove a lot of material that can reduce primary compression, because reeds like tight primary compression.). I did not change any port window dimensions in the cylinder. 1) FMF Gnarley pipe for a lower rpm peak target. 2) V-force reeds, and minimal cutting in the intake area to flow into the 5th transfer port tunnel, (stock reed stop plate blocks off the transfer port tunnel, so much for being highly developed there. Yamaha is getting lazy.), with no stock spacer plate, to slightly increase primary compression. Flow to the 5th transfer port tunnel is much better. Those bolt on's are for more lower rpm power, so I did some cylinder work to match. 1) Lowered the cylinder by .020" I machined .010" off the cylinder base, and use a .010" Cometic gasket, to replace the .020" thick stock gasket. I first removed the stock gasket and used a gasket sealer, but it started leaking. This lowers the exhaust port time to better match the lower rpm target of the FMF pipe, and raises compression (I run 100 octane now). Head squish clearance in now .048". 2) I roughed up the port tunnels (not sure if anybody still does that), opened up the 5th transfer port tunnel mouth a bit. and removed a little material from the boost transfer port tunnels to help feed the 5th port tunnel better, (got rid of those ledges). 3) I added some epoxy to the rear transfer port tunnel (hooks) to help fill the rear of the cylinder better at lower rpm's (A Eric Gorr mod). 4) still finding the best jetting and will use at least 2 needles leaner (not grooves, needles), to get the pig from spitting oil all over the backs of my jerseys, (Yes, stock was that rich at 32:1). So far, I have leaned out every jet, and it is still rich in the middle. I blame the "power jet". I don't like them. The bike still revs pretty good, (I failed to put a tach on it when stock), about as much as stock I "feel", (but not as strong of a pull to it), and the bottom end power is great now. Me at over 200lbs with stock gearing still, short shift all i want. Still pretty smooth too. Not an explosive hit. I love the engine now. Huge improvements. Of course, I also modified the suspension (lowered a bit, the seat lower also, the bike is too damn tall), and got rid of the batman Plastic, and even shortened the FMF turbine core spark arrested silencer by almost 4 inches. Now---it is a great bike, just like the old days. :) Question for you, if you know. The YZ250 vibrates a lot and can numb the hands, (aluminum frame makes it worse I believe). What do you think of Tom Morgan balancing/truing the crank mods to make the engine run smoother? Or any other anti-vibration tips? Doug in Michigan
@@EarthSurferUSA Yes, many OEM's have moved the reeds closer to the cylinder wall - this increases signal to the reeds while compromising open area around the boost port. Boost ports have become less prioritized as the bulk of the primary and secondary port flows have been optimized to the rear of the cylinder, the bulk flow requires only a "push" from upward-fired boost to really clean the high, rearward portion of the cylinder now. Smaller, higher-floored exhaust ports rely heavily on pipe effect and retained velocity for scavenging and stuffing. Vibration is inherent to 2t singles. The ones I tune all have counter-balancers so I may not be the best to answer but I will try. A true(under .001"TIR) crankshaft that has a dynamic balance job and a very light piston will be the best you can do for a 2t single without a counter-balancer. Maybe a KTM would have been a better choice, they have a balancer and you'd have like the engine better from a "state-of-tune" perspective.
I've noticed that when porting and cutting, you end up losing something but gaining something. The art is to have the gains outweigh the losses. My 250R porting provided amazing top end results, and I mean amazing...but the bottom end suffered a little. The top end benifits were way better than the loss of the bottom end suffer. If someone can show me all around gains I'm all for it. Another great video and excellent explanation from Chuck, great show!
Love it… I would be scared to do it , can’t wait to see the results. Fantastic content Chuck!
Keep in mind that the more material you remove from the transfer port tunnels, cylinder skirting and the bottom end in general, the more you increase the volume of the lower end, and the engine loses pumping energy. The larger volume of the bottom end (the primary compression) will open the reeds later, (reeds like tight primary compression), and may even result in less charge flowing into the cylinder from the transfer ports, with slower velocities.
That was a lot of cutting will be interesting to see the results
Looks like how the micro sprints boost ports are ported out
How did this work out
This is exciting. Thanks man. I can’t wait to hear the results.
Great review, be interesting to see what difference it makes ?
Please do a video on how well it runs and what differences you feel actually riding the bike. I’d really appreciate that! I have a yz250 too that I’m going to restore at some point but have a few bikes ahead of that one so just doing research on everything at this point. Interesting video! 👍
I posted the first test video a couple days ago.
@@ChuckfromTrueTech I’ll look for it 🤙
Perhaps it's time for you to build a backyard dynamometer to quantify your testing. Are you familiar with the Two Stroke Stuffing guy? He built on out of a bunch of junkyard parts. Alternately a friend with a stop watch can be helpful in that sense .I've played with port timing on a older GG 250 and at one point in my experimentation the end result yielded a power band about as wide as the edge of a razor blade.
It’s not practical enough for me to put effort into. The fact is that modern engines are excellent for 99.9% of riders. Modifying them increases the risk of failure & gains are minimal even in the best cases.
@@ChuckfromTrueTech If you can find a dyno somewhere, would make for some really nice content, especially as you have a stock cylinder ready to go so we could really measure the effects of the porting.
Keen to see how this goes Chuck! Port timing always seems to set the character for the whole bike and how it performs for different riding styles. I'm a big fan of low-end tractor power, currently on a Beta 300 for this reason.
You should get a dragy, you canesrn SO MUCH being able to track your ETA before an after modifications.
Thanks for doing this & filming it.
Great experiment
I'm more of a trail/woods rider I liked where the power was in the older 2 strokes low & mid
Don't forget to do the head... both together is where you see the most benefits.... or just get a head from an 03 YZ250 and the sand or down to get the correct squish you're looking for. The 03 had higher compression... and with all this porting and potential head work, run C-12 at 40:1 and tune it. It's likely you'll get detonation and you'll get the most out of your work.
I got some slight detonation from a stock 2019 YZ250, which had very rich jetting when stock, (which has lower compression since 2010 I believe. I have the heads. .0085" difference is what I measured at the head gasket surface to the edge of the squish band.), using 90 octane REC (alky free) fuel from a local gas pump (My area does not sell 93 octane REC gas). The detonation was at the intake side of the cylinder, where the cylinder meets the head. The main culprit? The stock reed cage, specifically the reed stop plate that blocks off the 5th transfer port tunnel, making that port, that cleans the top of the cylinder from hot exhaust gas there, almost useless.
It was those hot exhaust gasses, stuck in that corner, that caused the detonation, so it looks to me anyway.
Yes at BDC the ports are not fully open all round by about 2mm or so. Barrel needs to be lifted and head changed to suit.
The deck height is good though. I’m really not concerned if the piston doesn’t drop to the very bottom of the exhaust port.
The bulk of transfer flow volume flows through the top 2/3 of the transfers. Matching the transfer floor to the piston crown edge usually causes vortices 10deg BBDC to 1deg ABDC which --- hampers total transfer flow.
Fun. Subscribed Look forward to the reports
that looked really nice
Great video!
Hi just subscribed looks good and you obviously seen the way the barrel height is way off one standard base gasket and the ports are covered badly at BDC, what do you think of this.
Are you referring to the deck height? I haven’t seen any deck height issues with YZ250s.
@@ChuckfromTrueTech He's confused. Having the piston crown above the transfer floors at BDC creates a boundary layer that promotes smoother flow at the upper 2/3 of the transfers.
Awesome! Cant wait!
Curious what dremel tool your running. Ive waited forever to get one and now im ready to pull the trigger on one. Im thinking flexable shaft unit and one thst can take standard size burrs and be able to runn the wired little bits as well.
There’s only one industry standard and there are many good reasons for it. Foredom tool. Extremely robust and very handy. Unlimited tooling and replaceable and reasonably priced parts.
The current YZ250 is highly developed for it's intended purpose. It is unlikely that you'll make much, if any improvements at all. Perfecting the combustion chamber for the fuel you are running and the RPM range you'll be using will produce the best gains. The ducting in the crankcase and bottom of cylinder is shaped to push the charge back towards the secondary transfers, it gives the smaller secondaries more velocity while still allowing low velocity charge good access to the primaries. It also is the start of the "loop" of the charge to begin before even entering the transfers, making it more effective at exhaust evacuation.
To truly understand 2-stroke porting you must look at the entire loop system. The current YZ250 is flow, loop, AND pulse tuned. Not something you'll easily experiment with without truly in-depth knowledge and tools.
This is why I experiment on my own stuff. I don’t have any intention of doing this for customers, just my own curiosity. The viability of a business based on enhancing performance (besides bolt-ons) is questionable considering the bikes are as good as they are.
I do port 2-stroke engines for a living, (small 30cc engines approx), and after 18 years off a bike (Am 61 years old now), I picked up a 2019 YZ250 2-stroke of course. I never had to do more modifications for any bike I had before this (last was a 97 KX250,--loved that engine and working on it a bit). Man, the bike was tall, and the engine is a feeble high rpm power engine that will never keep up with a 450 4-stroke that typically has 15 more hp. But that is what the YZ250 is designed for. I wanted more bottom end power (not doing MX anymore, and glad I am done because it is too fast today because of the faster 4-stroke), to make it a good bike for anything. :)
I got the lower rpm power I wanted. Here is what i did. (I do not remove a lot of material that can reduce primary compression, because reeds like tight primary compression.). I did not change any port window dimensions in the cylinder.
1) FMF Gnarley pipe for a lower rpm peak target.
2) V-force reeds, and minimal cutting in the intake area to flow into the 5th transfer port tunnel, (stock reed stop plate blocks off the transfer port tunnel, so much for being highly developed there. Yamaha is getting lazy.), with no stock spacer plate, to slightly increase primary compression. Flow to the 5th transfer port tunnel is much better.
Those bolt on's are for more lower rpm power, so I did some cylinder work to match.
1) Lowered the cylinder by .020" I machined .010" off the cylinder base, and use a .010" Cometic gasket, to replace the .020" thick stock gasket. I first removed the stock gasket and used a gasket sealer, but it started leaking. This lowers the exhaust port time to better match the lower rpm target of the FMF pipe, and raises compression (I run 100 octane now). Head squish clearance in now .048".
2) I roughed up the port tunnels (not sure if anybody still does that), opened up the 5th transfer port tunnel mouth a bit. and removed a little material from the boost transfer port tunnels to help feed the 5th port tunnel better, (got rid of those ledges).
3) I added some epoxy to the rear transfer port tunnel (hooks) to help fill the rear of the cylinder better at lower rpm's (A Eric Gorr mod).
4) still finding the best jetting and will use at least 2 needles leaner (not grooves, needles), to get the pig from spitting oil all over the backs of my jerseys, (Yes, stock was that rich at 32:1). So far, I have leaned out every jet, and it is still rich in the middle. I blame the "power jet". I don't like them.
The bike still revs pretty good, (I failed to put a tach on it when stock), about as much as stock I "feel", (but not as strong of a pull to it), and the bottom end power is great now. Me at over 200lbs with stock gearing still, short shift all i want. Still pretty smooth too. Not an explosive hit.
I love the engine now. Huge improvements.
Of course, I also modified the suspension (lowered a bit, the seat lower also, the bike is too damn tall), and got rid of the batman Plastic, and even shortened the FMF turbine core spark arrested silencer by almost 4 inches.
Now---it is a great bike, just like the old days. :)
Question for you, if you know. The YZ250 vibrates a lot and can numb the hands, (aluminum frame makes it worse I believe). What do you think of Tom Morgan balancing/truing the crank mods to make the engine run smoother? Or any other anti-vibration tips?
Doug in Michigan
@@EarthSurferUSA Yes, many OEM's have moved the reeds closer to the cylinder wall - this increases signal to the reeds while compromising open area around the boost port. Boost ports have become less prioritized as the bulk of the primary and secondary port flows have been optimized to the rear of the cylinder, the bulk flow requires only a "push" from upward-fired boost to really clean the high, rearward portion of the cylinder now. Smaller, higher-floored exhaust ports rely heavily on pipe effect and retained velocity for scavenging and stuffing.
Vibration is inherent to 2t singles. The ones I tune all have counter-balancers so I may not be the best to answer but I will try. A true(under .001"TIR) crankshaft that has a dynamic balance job and a very light piston will be the best you can do for a 2t single without a counter-balancer. Maybe a KTM would have been a better choice, they have a balancer and you'd have like the engine better from a "state-of-tune" perspective.