Jetting yz250 for more top end

chump6784
Profile picture for user chump6784
Posts
1772
Joined
5/9/2011
Location
AU
chump6784
I have a 2018 yz250 which is completely stock except for a gytr by hgs pipe and gytr hgs shorty silencer. The bike pulls hard off the bottom and mid but seems to run flat up top. I know the shorty will have an effect on this but before I go and buy a new silencer I was wondering if going up on the main and lowering the clip on the needle would give me more fuel and therefore better top end?

As it runs the bike is very crisp and I don't want to lose the bottom end it has. I rode a mates 06 cr250 on the weekend, it was softer off the bottom than mine but once it started pulling it was a monster. If I could combine the two it would be unreal
|
Spudnut
Profile picture for user Spudnut
Posts
1943
Joined
6/25/2018
Location
USA
Spudnut 7/3/2019 8:46pm
7/3/2019 8:46pm
Jd jetting kit
2
Paul_Pitzonka
Profile picture for user Paul_Pitzonka
Posts
693
Joined
5/1/2018
Location
Grand Terrace, CA, USA
Paul_Pitzonka 7/3/2019 9:02pm
7/3/2019 9:02pm
It’d probably be easier to achieve what you’re looking for by retarding the timing a degree or two... there is a trade off you do lose some hit off the bottom, but if you sit there, adjust it, and make a notes of how it feels I’m sure you can find a compromise you’d be more than happy with...
1
stimpy
Profile picture for user stimpy
Posts
8
Joined
9/22/2018
Location
GB
stimpy 7/4/2019 1:03am
7/4/2019 1:03am
Really depends on what jetting you have. Personally I would experiment with the main jet and be looking to go slightly smaller. Lean is where the power as long as you have a margin of safety. 2nd clip on needle and 178 main is usually a good start. Then tweak from there. Altitude and humidity etc can make a big difference though.Retarding timing can give some over rev but it will probably reduce power throughout the range.
chump6784
Profile picture for user chump6784
Posts
1772
Joined
5/9/2011
Location
AU
chump6784 7/4/2019 1:44am
7/4/2019 1:44am
stimpy wrote:
Really depends on what jetting you have. Personally I would experiment with the main jet and be looking to go slightly smaller. Lean is where the...
Really depends on what jetting you have. Personally I would experiment with the main jet and be looking to go slightly smaller. Lean is where the power as long as you have a margin of safety. 2nd clip on needle and 178 main is usually a good start. Then tweak from there. Altitude and humidity etc can make a big difference though.Retarding timing can give some over rev but it will probably reduce power throughout the range.
It came with 180 main jet in it, and I'm pretty sure it's on the 3rd clip. I'll play around and see how i go. Thanks for that

The Shop

See All »
Paul_Pitzonka
Profile picture for user Paul_Pitzonka
Posts
693
Joined
5/1/2018
Location
Grand Terrace, CA, USA
Paul_Pitzonka 7/4/2019 4:31am
7/4/2019 4:31am Edited Date/Time 7/4/2019 4:32am
stimpy wrote:
Really depends on what jetting you have. Personally I would experiment with the main jet and be looking to go slightly smaller. Lean is where the...
Really depends on what jetting you have. Personally I would experiment with the main jet and be looking to go slightly smaller. Lean is where the power as long as you have a margin of safety. 2nd clip on needle and 178 main is usually a good start. Then tweak from there. Altitude and humidity etc can make a big difference though.Retarding timing can give some over rev but it will probably reduce power throughout the range.
Lean is most definitely not where the power is... (yes, leaning out his a/f mixture may possibly be the correct direction he needs to go on jetting as he may be overly rich). But an engine running on the slightly rich side produces more max power than a lean one... Only benefits he will see from a true lean engine is a more responsive/ efficient engine, but it will create higher combustion chamber temps which may lead to a detonation problem (something YZ engines are notorious for anyways)...
1
1
H4L
Profile picture for user H4L
Posts
2595
Joined
3/18/2016
Location
USA
H4L 7/4/2019 9:47am
7/4/2019 9:47am
The 99 to current engine is basically the same with some minor differences. Have owned 5 YZ's & they don't have the top end power the 02-07 CR2's have.
The YZ engine stock or modified makes its best power from low to mid..
The best option I found on the YZ back in 05 was having PC port the cyl. & modify the head for race gas on an 04 model.
It didn't have the power the CR has mid-top, but it is better than stock.
I don't think you'll achieve the top end power you're after unless you have the cyl. & head modified.
AJ565
Profile picture for user AJ565
Posts
2337
Joined
3/12/2012
Location
San Antonio, TX, USA
AJ565 7/4/2019 2:32pm
7/4/2019 2:32pm
Get rid of the shorty silencer and get a long one. Shorty = more bottom end, long = more top end.
1
chump6784
Profile picture for user chump6784
Posts
1772
Joined
5/9/2011
Location
AU
chump6784 7/4/2019 2:36pm
7/4/2019 2:36pm
H4L wrote:
The 99 to current engine is basically the same with some minor differences. Have owned 5 YZ's & they don't have the top end power the...
The 99 to current engine is basically the same with some minor differences. Have owned 5 YZ's & they don't have the top end power the 02-07 CR2's have.
The YZ engine stock or modified makes its best power from low to mid..
The best option I found on the YZ back in 05 was having PC port the cyl. & modify the head for race gas on an 04 model.
It didn't have the power the CR has mid-top, but it is better than stock.
I don't think you'll achieve the top end power you're after unless you have the cyl. & head modified.
Thanks for that. I've never ridden a cr250 before this so I didn't know what their powerband was like. Even though it had more top end than mine I was still arriving at corners quicker on my bike but man that hit was fun.
chump6784
Profile picture for user chump6784
Posts
1772
Joined
5/9/2011
Location
AU
chump6784 7/4/2019 2:37pm
7/4/2019 2:37pm
AJ565 wrote:
Get rid of the shorty silencer and get a long one. Shorty = more bottom end, long = more top end.
Yeah I know, I was just seeing if there was something else I could do before getting a new silencer, the shorty looks and sounds so good
Paw Paw 271
Profile picture for user Paw Paw 271
Posts
3632
Joined
4/3/2013
Location
Benton, LA, USA
Paw Paw 271 7/4/2019 8:09pm
7/4/2019 8:09pm
You will not change the top end power by rejetting.

Paw Paw
3
1
williamsmotowerx 7/5/2019 6:34am
7/5/2019 6:34am
YZ's are borderline safe with stock jetting and pump gas. Straight C12 (32:1 oil), don't dilute with pump gas... jet down 2 sizes on main and you will make more power.

Silencer is not killing top end. A head mod helps quite a bit, stock head modified. Porting too. These cylinders are just very outdated. They run good for what they are... but a few simple changes to the casting could make them stellar
1
stimpy
Profile picture for user stimpy
Posts
8
Joined
9/22/2018
Location
GB
stimpy 7/5/2019 8:55am
7/5/2019 8:55am
Paw Paw 271 wrote:
You will not change the top end power by rejetting.

Paw Paw
Proper jetting definetly changes the power. I've had mine on a dyno and messed with all sorts of things over the years. I was not suggesting that any motor should be run extremely lean so as to detonate or overheat. Just that running rich will lose power in my experience. I'm completely on board with williamsmotoworx suggestion. Second clip and a 178 main is worth a go.
Paul_Pitzonka
Profile picture for user Paul_Pitzonka
Posts
693
Joined
5/1/2018
Location
Grand Terrace, CA, USA
Paul_Pitzonka 7/5/2019 12:30pm
7/5/2019 12:30pm Edited Date/Time 7/5/2019 12:47pm
Paw Paw 271 wrote:
You will not change the top end power by rejetting.

Paw Paw
stimpy wrote:
Proper jetting definetly changes the power. I've had mine on a dyno and messed with all sorts of things over the years. I was not suggesting...
Proper jetting definetly changes the power. I've had mine on a dyno and messed with all sorts of things over the years. I was not suggesting that any motor should be run extremely lean so as to detonate or overheat. Just that running rich will lose power in my experience. I'm completely on board with williamsmotoworx suggestion. Second clip and a 178 main is worth a go.
I’m not disagreeing a properly jetted bike will definitely have minimal power gains over an improperly jetted bike... All I was saying was that an engine will almost always have more peak horsepower when it’s barely on the rich side of the stoichiometric ratio... I can almost guarantee your bike was still on the rich side of the ratio even though you were leaning out your jetting...
H4L
Profile picture for user H4L
Posts
2595
Joined
3/18/2016
Location
USA
H4L 7/5/2019 12:34pm
7/5/2019 12:34pm
chump6784 wrote:
Thanks for that. I've never ridden a cr250 before this so I didn't know what their powerband was like. Even though it had more top end...
Thanks for that. I've never ridden a cr250 before this so I didn't know what their powerband was like. Even though it had more top end than mine I was still arriving at corners quicker on my bike but man that hit was fun.
Agree with Paw Paw. The jetting will not change the topend power. That is not where the YZ2 powerband oem porting specs & head are designed to work. Correct / optimum jetting will help the YZ engine run cleaner in the low-mid where it is tailored to run. I'd have to think you are arriving at the corners quicker due to you being the better rider.

About 5 yrs. back I was at a local track & was pulling a YZF450 on the straights on an 03 CR2 with mods.. When I pulled off the track the rider on the YZF came over & asked if I was running an older HPP CR2 engine with an overbore. To his surprise it was just a standard bore 03 CR2 engine. The YZ2 engine is an outdated design as mentioned above, but can be tuned (porting & head reshape) to run decent with mods, not stellar though IMHO.
stimpy
Profile picture for user stimpy
Posts
8
Joined
9/22/2018
Location
GB
stimpy 7/5/2019 2:22pm
7/5/2019 2:22pm
chump6784 wrote:
Thanks for that. I've never ridden a cr250 before this so I didn't know what their powerband was like. Even though it had more top end...
Thanks for that. I've never ridden a cr250 before this so I didn't know what their powerband was like. Even though it had more top end than mine I was still arriving at corners quicker on my bike but man that hit was fun.
H4L wrote:
Agree with Paw Paw. The jetting will not change the topend power. That is not where the YZ2 powerband oem porting specs & head are designed...
Agree with Paw Paw. The jetting will not change the topend power. That is not where the YZ2 powerband oem porting specs & head are designed to work. Correct / optimum jetting will help the YZ engine run cleaner in the low-mid where it is tailored to run. I'd have to think you are arriving at the corners quicker due to you being the better rider.

About 5 yrs. back I was at a local track & was pulling a YZF450 on the straights on an 03 CR2 with mods.. When I pulled off the track the rider on the YZF came over & asked if I was running an older HPP CR2 engine with an overbore. To his surprise it was just a standard bore 03 CR2 engine. The YZ2 engine is an outdated design as mentioned above, but can be tuned (porting & head reshape) to run decent with mods, not stellar though IMHO.
Are you saying then that you can run any size jet you like and the top end power will stay the same ? On my 250 dropping the main jet a few sizes really cleaned up the top end producing more peak power and delaying the drop off after peak. My 265 build ended up running very different jetting and a custom ignition curve and lots of time to get it right. Far more than it was worth.
H4L
Profile picture for user H4L
Posts
2595
Joined
3/18/2016
Location
USA
H4L 7/5/2019 2:47pm
7/5/2019 2:47pm Edited Date/Time 7/5/2019 4:22pm
stimpy wrote:
Are you saying then that you can run any size jet you like and the top end power will stay the same ? On my 250...
Are you saying then that you can run any size jet you like and the top end power will stay the same ? On my 250 dropping the main jet a few sizes really cleaned up the top end producing more peak power and delaying the drop off after peak. My 265 build ended up running very different jetting and a custom ignition curve and lots of time to get it right. Far more than it was worth.
I didn't say you can run any size jet. You can reread my post. The stock YZ2 engine can run better with optimum jetting, but it won't necessarily pick up more power up top unless the cyl. port timing is changed combined with a head reshape. Some engine tuners will raise the exhaust port aprox. 1mm to achieve that depending on the engine. That will raise the peak power in the upper part of the powerband range.
Agree with you on the overbore of 265 as far as not being worth it. I've ridden on a couple & didn't feel it made that much of a difference considering the cost.
1
chump6784
Profile picture for user chump6784
Posts
1772
Joined
5/9/2011
Location
AU
chump6784 7/5/2019 3:00pm
7/5/2019 3:00pm
Thanks for the responses. I'm taking the day off on Wednedsay cos it's school holidays and the kids wanna go riding. I'll take some jet's and play around and see how I go
1
williamsmotowerx 7/5/2019 6:28pm
7/5/2019 6:28pm
Paul_Pitzonka wrote:
I’m not disagreeing a properly jetted bike will definitely have minimal power gains over an improperly jetted bike... All I was saying was that an engine...
I’m not disagreeing a properly jetted bike will definitely have minimal power gains over an improperly jetted bike... All I was saying was that an engine will almost always have more peak horsepower when it’s barely on the rich side of the stoichiometric ratio... I can almost guarantee your bike was still on the rich side of the ratio even though you were leaning out your jetting...
Stoich is just relationship of optimal fuel burn given the amount of air. It doesn't take into account what's really happening in a running engine being 4 or 2 stroke.

Leaner is almost always meaner. You just need fuel that's tolerable. A little rich down low keeps pipe cool for more bottom end 2 stroke... but it can't recover fast enough in mx situations.

Pit Row

See All »
Paul_Pitzonka
Profile picture for user Paul_Pitzonka
Posts
693
Joined
5/1/2018
Location
Grand Terrace, CA, USA
Paul_Pitzonka 7/5/2019 7:59pm
7/5/2019 7:59pm Edited Date/Time 7/5/2019 8:12pm
williamsmotowerx wrote:
Stoich is just relationship of optimal fuel burn given the amount of air. It doesn't take into account what's really happening in a running engine being...
Stoich is just relationship of optimal fuel burn given the amount of air. It doesn't take into account what's really happening in a running engine being 4 or 2 stroke.

Leaner is almost always meaner. You just need fuel that's tolerable. A little rich down low keeps pipe cool for more bottom end 2 stroke... but it can't recover fast enough in mx situations.
You are correct stoichiometric ratio alone isn’t enough info to explain what’s occurring inside an engine... I always took “leaner is meaner” as an adage referring to the more responsive characteristic of a lean engine and not the overall power produced... in my experiences an engine slightly on the rich side of the ratio tends to have a higher peak horsepower, but I may be wrong as I have been before... also re reading my post I see the confusion I did not mean an engine should continuously run in a rich environment there are benefits to both... I was implying at the top of his dyno pulls if he was seeing An increase in HP he was more than likely on the slightly rich side of the ratio
fullysicmate
Profile picture for user fullysicmate
Posts
1649
Joined
1/4/2013
Location
Queensland, AU
fullysicmate 7/5/2019 9:23pm
7/5/2019 9:23pm
The power on a yz 250 is flat up top, if you want a 250 2stroke with power up top get a cr.
r18b
Profile picture for user r18b
Posts
240
Joined
7/13/2017
Location
NL
r18b 7/5/2019 11:14pm
7/5/2019 11:14pm
What if you gonna raise the cylinder?
I'm riding a cr250 with a .8 basegasket instead of the .2

A friend of mine who had a yz250 liked the cr better in all ways. The time that i did ride that yz it felt heavy and in my opion way to much bottom.

So what if you gonna put the cylinder higher. It's not that you gonna lose power only you're changing it.
Don't know if it's working with a yz.
H4L
Profile picture for user H4L
Posts
2595
Joined
3/18/2016
Location
USA
H4L 7/5/2019 11:40pm
7/5/2019 11:40pm Edited Date/Time 7/5/2019 11:48pm
r18b wrote:
What if you gonna raise the cylinder? I'm riding a cr250 with a .8 basegasket instead of the .2 A friend of mine who had a...
What if you gonna raise the cylinder?
I'm riding a cr250 with a .8 basegasket instead of the .2

A friend of mine who had a yz250 liked the cr better in all ways. The time that i did ride that yz it felt heavy and in my opion way to much bottom.

So what if you gonna put the cylinder higher. It's not that you gonna lose power only you're changing it.
Don't know if it's working with a yz.
The YZ will feel noticeably heavier in comparison.

You can run into issues like the squish band in the head being too lg. by just raising the cyl.. The YZ & CR don't have what would be considered a good optimum squish band from the factory as it is.
r18b
Profile picture for user r18b
Posts
240
Joined
7/13/2017
Location
NL
r18b 7/6/2019 12:06am
7/6/2019 12:06am
r18b wrote:
What if you gonna raise the cylinder? I'm riding a cr250 with a .8 basegasket instead of the .2 A friend of mine who had a...
What if you gonna raise the cylinder?
I'm riding a cr250 with a .8 basegasket instead of the .2

A friend of mine who had a yz250 liked the cr better in all ways. The time that i did ride that yz it felt heavy and in my opion way to much bottom.

So what if you gonna put the cylinder higher. It's not that you gonna lose power only you're changing it.
Don't know if it's working with a yz.
H4L wrote:
The YZ will feel noticeably heavier in comparison. You can run into issues like the squish band in the head being too lg. by just raising...
The YZ will feel noticeably heavier in comparison.

You can run into issues like the squish band in the head being too lg. by just raising the cyl.. The YZ & CR don't have what would be considered a good optimum squish band from the factory as it is.
That's true about the squish. Well the Yamaha dealer was saying the same thing about the yz. He knows i'm riding with a large squish on my cr.

Well i tried it al from a 1.1 to .2 gasket. It all runs perfect with the right pipe but i prefer the .8 gasket. The standard gasket it runs like shit ,some like it , i don't like it how the bike rides

You don't know it if you not gonna try it.
cable
Profile picture for user cable
Posts
1253
Joined
6/11/2008
Location
Rockford, MI, USA
cable 7/9/2019 7:35pm
7/9/2019 7:35pm
Listen to williams. leaner main will help it rev out. go one leaner and ride it. check plug to see the color. go two leaner, and check plug. You will be suprised. retard the timing if you want even more rev. lots of free things to try

Post a reply to: Jetting yz250 for more top end