[marxd]

Hi,

just in the process of rebuilding the top end on a 2005 CRF250X.
This bike has a Athena big bore 280cc kit, with +2mm stroker Hot Rods crankshaft (done by previous owner)
The bike's cam chain tensioner/chain failed hence the rebuild.
We're trying to capitalize on the extra power this bike should have. Before the failure compared to other 4 stroke 250s this bike put out the least power/zip despite having the big bore kit in it. Although that may have been a symptom of a stretched cam chain?
Just wondering how to capitalize on the big bore kit for power and reliability?
Considering putting in a 250R cam? Has anyone done this mod? Is it worth it?
The exhaust is stock, the valves are kibble white and the head is going to be ported before install.
Any advice would be great thank you!

[riddler9]

I would put an '08 or '09 R head on it with Ti valves instead of SS. The Ti allows for more power because of less weight, just make sure you get the correct copper beryllium valve seats if you go with Ti. SS does not add power. People put them in expecting longer life - wringing the death out of an engine that is not making enough power only decreases life.

I can't say much about the cam with the BB. I can say that with a high comp standard standard bore piston and R head the '08 R cam makes plenty of power.

You may already have, but you need an exhaust ('05 R header + any decent slip-on), open airbox and no backfire screen. 100+ octane ethanol free with proper jetting will also go a long way.

Good luck with whichever path you take - big payoff if you get the correct combination.

[crfsonly]

We have built many 250X motors with the Athena Big Bore, 2007+ head with KW SS valves and a CRF250R cam. This combination works great. It's durable and has more power everywhere. I wouldn't put too much value on the comments about the Ti intakes adding significant power over the KW SS intakes. It's just not true. Yes, the stock Ti intake valves weigh less than the KW SS intake valves. However, it's not significant and KW offsets this weight difference with their Ti retainers and dual coil springs. Keep in mind the stock exhaust valves are already stainless steel and ALl of the stock retainers are steel.

Rather than port you X head I would recommend a 2007 or 2008 R head with KW SS valves and a 2008 R cam. You can then perform the pink wire mod to get the R ignition map. Again, this is a combination we've had excellent results with for both power and durability. Our new heads undergo our performance valve seat profiling and port bowl blending. Porting is also available.

Good luck!

Ken

[marxd]

Thanks for the response.
We went ahead with the R cam, SS kibble white valves. We left the head stock, it was in good condition and lots of $ already invested, probably the only thing left on this frankenbike that is stock.
So got it together and fired up on Friday. Took it out Saturday to break in the motor. On the second start up there is some piston slap or similar sound and a high pitched but fairly quite squeak, similar sound to when you just depress the button on a drill ever so slightly.
Let it warm up and all noises go away. This bike sounds and is running better than it ever had. So onto break in. Going smooth but progressively the bike becomes a bit harder to start. Then at the 1 hour mark (engine time) the bike stalls and seizes, no terrible sounds, nothing, the kickstarter is stuck solid. Tried rocking and a couple bump starts. Nope.

All fluid levels are the same, no coolant, oil or tranny leaks. Removed the spark plug which looked perfect, still frozen. Drained the fluids found some brass shavings (which points to the bottom end) and 1x 4mm long by 2mm wide (I'll attach a picture later tonight) of aluminum a mystery. So the engine is out and the tear down will start Tuesday. Any thoughts? We have a few guesses but until we tear into it, that all we got. Total bummer but hopefully the parts we touched are undamaged.