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For all the new crf250f owners, I found this old thread you may not have seen. I found it interesting. A good video about the differences of the 230 and 250 head. Click on CC for english captions. I'm still looking at upgrades and found this video {in link} about the head. Currently talking to Shane @ SW Airflow about porting and shaving the head. Will let TT know what the butt dyno reads when finished and of course any advice on upgrades is always appreciated.

 

Edited by Tbeasley

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10 hours ago, Tbeasley said:

For all the new crf250f owners, I found this old thread you may not have seen. I found it interesting. A good video about the differences of the 230 and 250 head. Click on CC for english captions. I'm still looking at upgrades and found this video {in link} about the head. Currently talking to Shane @ SW Airflow about porting and shaving the head. Will let TT know what the butt dyno reads when finished and of course any advice on upgrades is always appreci

 

Nice VID ,, We need to see more of the 250 head. I have been looking at these old XR heads for a while now !! How does this bearing setup assemble ??

CRF250.jpg.3db5ef6c63230db1ba43489e8a28404a.jpg

Edited by Scrammer

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Not sure about the bearing assembly. Maybe somebody will chime in for you. Looks like I'll  be the porting guinea pig. I think I'm gonna wait for a cam until somebody figures out which grind works best. Would love a bit more grunt on the bottom end. 

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15 minutes ago, Tbeasley said:

Not sure about the bearing assembly. Maybe somebody will chime in for you. Looks like I'll  be the porting guinea pig. I think I'm gonna wait for a cam until somebody figures out which grind works best. Would love a bit more grunt on the bottom end. 

More grunt on the bottom in the meantime while your waiting for a cam would be as simple as advancing the ignition timing with a modified flywheel key up to and including more advanced methods such as advancing the cam timing 2-3 degrees, or setting the squish tighter to raise the compression and make the engine more efficient. 
All above is mostly labor and basically just time involved. A Compression boost always improves off idle torque thats noticeable at all rpms. 

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3 hours ago, adnohguy said:

More grunt on the bottom in the meantime while your waiting for a cam would be as simple as advancing the ignition timing with a modified flywheel key up to and including more advanced methods such as advancing the cam timing 2-3 degrees, or setting the squish tighter to raise the compression and make the engine more efficient. 
All above is mostly labor and basically just time involved. A Compression boost always improves off idle torque thats noticeable at all rpms. 

 I have a service manual on the way. I'll see if that helps me. Not really a mechanic but know some. If its over my head I know a guy who knows a guy...… SW Air flow said he would shave .010 off and it would give me .1 to.2 more compression putting it at about 9.8:1 but mentioned any more I would have to advance timing. I haven't sent him the head yet  so I'll see if I can do the timing then ask about more compression. Thanks Adnohguy 

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6 hours ago, Tbeasley said:

 I have a service manual on the way. I'll see if that helps me. Not really a mechanic but know some. If its over my head I know a guy who knows a guy...… SW Air flow said he would shave .010 off and it would give me .1 to.2 more compression putting it at about 9.8:1 but mentioned any more I would have to advance timing. I haven't sent him the head yet  so I'll see if I can do the timing then ask about more compression. Thanks Adnohguy 

I don't know the power effects of shaving off .010 th's, but any engine with an OHC, it will slightly affect the valve/cam timing ,, slightly delaying openings !! Then "Cam Timing" chain slack may become a problem ...

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.010 off the deck surface is as far as I am comfortable removing before it starts to change cam timing. In the Honda car stuff removing .012 caused the cam timing to retard about 1 degree. In my experience it takes about 3 - 4 degrees before for you notice any difference in the power curve. Basically the dimenton change from surfacing causes the cam timing to retard which shifts the power curve up in the RPM range giving you more top end and less bottom end. So I feel the increased compression from removing .010 is well worth the under 1 degree of cam timing move.

Edited by SoCal Head Grinder
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The 250F head will benefit greatly from reshaping the sort turn because it is pretty much just a sharp edge. The guides also protrude a little too much into the port. The valves and ports are on the small side. They really made these heads for high velocity.

              CRF250F.     XR250R

Intake -    25mm.      28.5mm

Exhaust - 21.5mm.   23.5mm

It does have big valve seats that are really deep. You could safely cut them to accept a 2mm oversize intake valve and a 2.5mm oversize exhaust. There is a really bad step under the valve seats in both the intake and exhaust bowls. Cutting for bigger valves should help reduce this step.

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37 minutes ago, SoCal Head Grinder said:

The 250F head will benefit greatly from reshaping the sort turn because it is pretty much just a sharp edge. The guides also protrude a little too much into the port. The valves and ports are on the small side. They really made these heads for high velocity.

              CRF250F.     XR250R

Intake -    25mm.      28.5mm

Exhaust - 21.5mm.   23.5mm

It does have big valve seats that are really deep. You could safely cut them to accept a 2mm oversize intake valve and a 2.5mm oversize exhaust. There is a really bad step under the valve seats in both the intake and exhaust bowls. Cutting for bigger valves should help reduce this step.

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20200527_154152.jpg

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20200527_154850.jpg

Great info sir, thank you. 👍🏻
I don’t have as 250f yet, but I am planning on getting one sooner or later when I can find a used one for less than full retail. 

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4 minutes ago, adnohguy said:

Great info sir, thank you. 👍🏻
I don’t have as 250f yet, but I am planning on getting one sooner or later when I can find a used one for less than full retail. 

I like them so far and I think they will be really fun with some work.  The guy in South America is absolutely making them rip!  I do have concerns with the crank (unfounded, just a gut feeling) and the head shedding heat.  It's gonna get hot around the exhaust valves and between the seats when you lean them down to make power.

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20 minutes ago, SoCal Head Grinder said:

I like them so far and I think they will be really fun with some work.  The guy in South America is absolutely making them rip!  I do have concerns with the crank (unfounded, just a gut feeling) and the head shedding heat.  It's gonna get hot around the exhaust valves and between the seats when you lean them down to make power.

Shane (I believe) are you able to source oversize valves yet? How about over size pistons (big bore)? Aftermarket Cams? Do you know how large a piston will “bore in” and still leave .080 - .100 sleeve thickness? 
 

Happy that you posted intake port opening ID. Nice to know. 
 Thank you for your help sir. 

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@socal sounds really good. Thanks for the info. The head was delivered today. I can send it on to you if your ready for it. 

Edited by Tbeasley

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1 hour ago, adnohguy said:

Shane (I believe) are you able to source oversize valves yet? How about over size pistons (big bore)? Aftermarket Cams? Do you know how large a piston will “bore in” and still leave .080 - .100 sleeve thickness? 
 

Happy that you posted intake port opening ID. Nice to know. 
 Thank you for your help sir. 

Yeah, that's me.  I make my own oversized valves for heads like the 250F.  I already have 4 steel blanks and 2 stainless steel blanks to machine and test for the intake side.   The head will probably end up using 2mm over intakes and 1mm over exhausts but a bunch of testing needs to be done before I get it figured out.

 

I don't think anyone in the States has an off the shelf cam.  Web can make something pretty easy with what they think will work.  Check out http://www.luizracing.com.br/  It's in Portugese but they have a bunch of videos on Facebook.  They have a 350cc kit I believe.  In their early pictures it looks like the overbore pistons (280cc or 310cc) were out of something else and they renotched them for valve clearance.  They were also using oversize valve but with Google translate I couldn't find any info.  I messaged them with no response.  This hole shot is impressive!

 

 

16 minutes ago, Tbeasley said:

@socal sounds really good. Thanks for the info. The head was delivered today. I can send it on to you if your ready for it. 

Oh yes!

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@adnohguy. Got a 4 degree offset flywheel key ordered. I hope its same as the 230. Looks the same as mixer ordered on a different thread I found.  Looks easy enough to install. I'll find out 

@SoCal Head Grinder got the head shipped to you this morning. Really excited about it. 

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For anyone wanting to do the flywheel timing advance. I had to get a flywheel puller. My universal puller we used on a yamaha didn't work. I went ahead and ordered one from my Honda dealer.$27.79 with tax. Partszilla has them too. Not sure of an aftermarket fit.  4 degree Offset woodruff key was right at $18.00 with shipping. You might find both for less $$ if you shop around or machine the key yourself for nothing. Also got a service manual for under $60.00 shipped.  The job is easy if you have an impact with sockets.

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Got the timing advanced today. Not huge difference but I can now pull the front wheel up with just the throttle. Before this I was having to bounce and pull to wheelie. I've only done the advance timing and EJK fi controller on the motor but its pretty good so far. Well worth the effort and simple to do. Can't wait for the head to come back from porting, get it installed and some aftermarket cams to come out. Still looking at an exhaust system but....

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