CR250 exhaust flange?

Hi, so i have been reading up on these aftermarket boyesen exhaust flanges and they say that only the 04 did not line up but the 02/03 matched the port perfectly.

I understand that 02-04 CRs pipes go over the flange with the newer ones go in(05-07), as i have both. So since the 02-04 are all a 3 bolt pattern to put the flange on the cylinder. The 04 flange don't match the port perfectly effecting low end. the port is a D shape on its side while the flange is perfectly round causing almost 30% blockage(i read).

Since these years are so similar would it be possible to get an 02/03 OEM flange and put on the 04 cylinder to get rid of this "blockage" . Or is the bolt pattern or the angle different?

Thanks ahead of time for any responses!!

I have the Boyesen flange on my 03 CR250. I felt zero diference power wise. If I were to do it again I'd just get the Pro-circuit version. I still might seeing as the boyesen uses the tiny O-rings which don't last very long

05-07s go in? Is this just on the 250s? Because I have an 05 125 and my exhaust fits over the flange?

You probably felt no difference because even the stock flange met the seem of the port perfectly. in 04 the slock flange is round an the port is a D shape. there is a definite blockage i took my stock one off and checked.

i believe this is exclusive to the 250 in the last gen. CRs. i had an 07 125 and it went over. the 05 250s had a 8-9 bolt plate in the front that pulls out for you to clean your powervalve MUCH easier. my 04 is a pain.

I was just looking to see if it was possible to put the 02/03 flange on the 04. i dnt think the cylinder changed much if at all those years, so if the port lined up in 02 i want to find out if that falnge that matched up in 02 also matches up to the 04

I would like to know about this also. I have an 04 but I cannot find anything on an 04flange anywhere.

As far as I know the oem 02-04 exhaust flange is the same configured in a round shape like the 02 & 03 exhaust port on the cyl's. In 04 Honda redisgned the porting specs & exhaust port in the cyl. giving it that D shape form. Boyesen supposedly came out with a fix with there D shaped exhaust flange. I'm running one on my 03 cyl. but it's round with a raised lip on the bottom part of the inside of the flange. You would have to call Boyesen tomorrow to see if they have one specific to the 04 model with a D shape port. I agree with cmjb I didn't notice any difference in performance after bolting it on. In my case I purchased it before my oem one cracked as they're prone to that. The PC unit is a lot cheaper if your planning on upgrading yours with a better one.

According to this TT post Boyesen has one specific to the 04 model. I'd still call & confirm. 02-03 Cr250r part# BEF-02 / 04 CR250r part # BEF-02A http://www.google.co...iKQyi1g0dp6gCQw

Boyesen website good luck...

http://www.google.co...99lI6qZOT2QqAZw

Edited by H4L

I'm not to worried about it cracking. I'm just in the middle of building one from the ground up and read about it. I'm not to sure about buying one. I have noticed that if my 04 is jetted correctly with stock gearing it will pull harder from low to just above mid but has absolutely nothing up top compared to my 2005 which was the last years of "improvement". I also read that if your change when your power valve opens it could move it up top, but it should pull up top as long as your power valve is completely open when revving. Is this correct?

Also even if the port is minimally blocked does it effect it that much? And since the flange is round and port is d shaped, and the round flange cuts off supposedly 30% i would think that they couldn't expand the flange enough to stop blockage without making it to big for the pipe to fit.

So is this boyesen flange even better? Well better with eliminating blockage.

I'm not to worried about it cracking. I'm just in the middle of building one from the ground up and read about it. I'm not to sure about buying one. I have noticed that if my 04 is jetted correctly with stock gearing it will pull harder from low to just above mid but has absolutely nothing up top compared to my 2005 which was the last years of "improvement". I also read that if your change when your power valve opens it could move it up top, but it should pull up top as long as your power valve is completely open when revving. Is this correct?

Also even if the port is minimally blocked does it effect it that much? And since the flange is round and port is d shaped, and the round flange cuts off supposedly 30% i would think that they couldn't expand the flange enough to stop blockage without making it to big for the pipe to fit.

So is this boyesen flange even better? Well better with eliminating blockage.

I've owned 02,03 & (3) 07 models & ridden 05 / 06 bikes with exception of the 04. From my understanding the 02 - 04 bikes have similiar powerbands. The 02 / 03 bikes have more up top in comparison to the 05-07 bikes in stock form. It's possible your 04 bike doesn't have the powervalve cables adj. correctly. Another thing you can check is the RC motor & cables as a potential problem. When I did my first topend on the RC valve motor I had a similiar powerband as you described with good low to mid pull but flat up top. After doing some trouble shooting I found that the powervalve cables weren't adjusted correctly after the topend rebuild. The powervalves should operate between 3K - 8K rpm & at fully open should give you the topend power back.

Edited by H4L

I adjusted them according to the manual as much as that's worth. Did i possibly make the cables to tight? If anything they are to tight. It is also still jetted rich for the cooler spring temps (45-60 degrees) so i am definitely running rich. That's why I am building it up. Got it last spring and it was clapped so I got it to the point where it was raceable if need be but still not up to par as i had 3 other bikes to ride/race. Now I am actually going to do what i should have last spring.

As a side note, would the stock exhaust flange rob your top end power also or bottom end only? I read up to an extra hp with boyesen.

I figured since Im only 17 and only run B that extra hp isn't going to make a difference, I have it pretty dialed in the way of handling. Just don;t have the power i would like.

Any good mods besides porting, reed block, kiehin PWK airstryker and adjusting the squish? Any rarely known mods that really wake the CRs up?

Also has anyone had any experience with that new APT smart carb? They seem pretty awesome.

You men

I adjusted them according to the manual as much as that's worth. Did i possibly make the cables to tight? If anything they are to tight. It is also still jetted rich for the cooler spring temps (45-60 degrees) so i am definitely running rich. That's why I am building it up. Got it last spring and it was clapped so I got it to the point where it was raceable if need be but still not up to par as i had 3 other bikes to ride/race. Now I am actually going to do what i should have last spring.

As a side note, would the stock exhaust flange rob your top end power also or bottom end only? I read up to an extra hp with boyesen.

I figured since Im only 17 and only run B that extra hp isn't going to make a difference, I have it pretty dialed in the way of handling. Just don;t have the power i would like.

Any good mods besides porting, reed block, kiehin PWK airstryker and adjusting the squish? Any rarely known mods that really wake the CRs up?

Also has anyone had any experience with that new APT smart carb? They seem pretty awesome.

You mentioned some good upgrades with porting, tighter squishband & PWK Kehin carb that really wake up the motor. If your a fast rider then port work would be a good option to consider. When I first purchased my 02 all I did was install a 50t rear sprocket & adjusted the jetting & suspension clickers. I didn't have time to do anything else as I literally bought it that morning to race in the afternoon. The bike was competitive & was able to keep a good friend within sites on a 96 CR250 with engine & suspension mods. These bikes are very competitive once set up they're a different animal all together.

Get your motor running good once you establish that & feel it needs more power then consider doing some port work. It's possible your not jetted right & not getting the topend power. I'd still look into the RC valve motor & cables to elimainate it as a possibility. Make sure the powervalves are opening all the way at 8K rpm.

I have been riding Cr250s since i was 12, but mainly just practice as i raced a 125. I would consider myself a faster rider as i am top 5 in 250/open B almost all the time and won i think 4 this year.

Suspension on both Crs we have was done by pro-action sprung and revalved. I looked at my RC valve today and it was working fine.

I am possitive the jettings off. I never really tried to jet it perfect since i never raced it since i have a 250f and 450. Only got it decent enough for a practice bike.

Only really concerned with it now because next year when i turn 18 i want to have a solid 2 stroke under me and sell the 4 strokes to keep the cost of racing down.

After jetting, porting is definitely getting done then by Eleven Ten mods.(there shop is 30 mins from my house)

When i get it ported/squish adjusted, should I definitely run some kind of race gas to prevent predetonation?

Right now we usually just get 93 pump for them. Save the race gas for the built 250f (gets expensive).

Has anyone seen that APT smartcarb they seem absolutely amazing.

Also, do they make PHAT heads for these bike? If they do where can you find one?

oval exh port and round flange is like that intentionally. seems like the newer yamahas are similar. doubtful youll find any info about the theory behind it on this forum or any other mainstream forum.

Edited by harryhandshake

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