OK. So I recently bought the my first 250 2-stroke to play with. It is an '03 in excellent original condition. Immediately after getting it, I added a Keihin PWK carb, a Doma pipe and a VF3 cage. After riding it a couple of weeks and comparing it to an '05 YZ250, I decided to see how it could be changed with porting. My choice of porting was a toss-up between Eric and Wes at Boyesen Precision Porting; both are highly recommended on this board. Wes had a better schedule at the time, so I went with him. I purchased a new head, cylinder and exhaust flap assembly from Service Honda and sent them to Wes. I sent Wes a fairly detailed description of my perceptions and what I wanted changed. I specifically requested:
1) A power band more like a YZ
2) Power to come on sooner, like a YZ
3) Maintain/improve overall power as much as possible
4) More over rev
5) Smooth out the hit in the middle
6) Race gas is an option if it provides noticeable improvement
Basically, something a 4-stroke rider could like. I received the parts back on the exact day I requested. It was obvious that Wes had run a porting tool over all the ports. When I installed the top engine mount, it became very obvious that the cylinder and head had been lowered (increased compression) as I had to oval out the engine side hole substantially. My jetting before the mods appeared to be spot on:
Main Jet: 165
Needle: DGL - 4th clip
Slow Jet: 38
Air Screw: ~ 2 out
Slide: 7
Fuel: 101 octane Trick unleaded, 32:1 HP2
Wes suggested that I go to a 175 main, but I only upped it to a 168 for the first ride. The first ride was pretty long and through sand. To tell the truth, I was a bit disappointed. But I noticed that the bike appeared to be lean; not enough for an outright bog, but lean. Interestingly, I never heard any detonation. After a 15 mile ride, the inside of the number plate was squeeky clean, so I decided to richen everything:
Main Jet: 175
Needle: DDJ - 2nd clip (1/2 clip and 2 diameters richer)
Slow Jet: 40
Air Screw: ~ 2 out
Slide: 7
Fuel: 101 octane Trick unleaded, 32:1 HP2
That made a real difference. The engine was much stronger. 3rd gear roll-on wheelies were a lot easier. At this point, I'm very happy with the results, but after 15 miles again, the rear number plate is still clean, so I think I'm still a bit lean.
Where the engine used to be pretty mellow down low, then really hit with power and roll off, it now builds much more predictably. As I wanted, there isn't any stong hit in the power band; just smooth useable power. I'll post again when I've had a chance to richen things out a bit more. So far, it's not a YZ (not sure that's possible), but it's a lot closer. I'll rate it as :aj: .
1) A power band more like a YZ
2) Power to come on sooner, like a YZ
3) Maintain/improve overall power as much as possible
4) More over rev
5) Smooth out the hit in the middle
6) Race gas is an option if it provides noticeable improvement
Basically, something a 4-stroke rider could like. I received the parts back on the exact day I requested. It was obvious that Wes had run a porting tool over all the ports. When I installed the top engine mount, it became very obvious that the cylinder and head had been lowered (increased compression) as I had to oval out the engine side hole substantially. My jetting before the mods appeared to be spot on:
Main Jet: 165
Needle: DGL - 4th clip
Slow Jet: 38
Air Screw: ~ 2 out
Slide: 7
Fuel: 101 octane Trick unleaded, 32:1 HP2
Wes suggested that I go to a 175 main, but I only upped it to a 168 for the first ride. The first ride was pretty long and through sand. To tell the truth, I was a bit disappointed. But I noticed that the bike appeared to be lean; not enough for an outright bog, but lean. Interestingly, I never heard any detonation. After a 15 mile ride, the inside of the number plate was squeeky clean, so I decided to richen everything:
Main Jet: 175
Needle: DDJ - 2nd clip (1/2 clip and 2 diameters richer)
Slow Jet: 40
Air Screw: ~ 2 out
Slide: 7
Fuel: 101 octane Trick unleaded, 32:1 HP2
That made a real difference. The engine was much stronger. 3rd gear roll-on wheelies were a lot easier. At this point, I'm very happy with the results, but after 15 miles again, the rear number plate is still clean, so I think I'm still a bit lean.
Where the engine used to be pretty mellow down low, then really hit with power and roll off, it now builds much more predictably. As I wanted, there isn't any stong hit in the power band; just smooth useable power. I'll post again when I've had a chance to richen things out a bit more. So far, it's not a YZ (not sure that's possible), but it's a lot closer. I'll rate it as :aj: .
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