XR motor in a CRF/CR frame?

aj :clue:

Rich Rohrich said:

The TTR250 cylinder head is horrible and even with lots of work it will never flow enough air to adequately feed an engine that size. A $500 FCR carb won't change that. Cam selection is also fairly poor so you'd end up with a heavy bike fitted with an underpowered engine that stops making power way too early.

As cujet pointed out, building a WR290F or a WR305F is relatively cheap these days with the availability of new cast cylinders from Athena (290) and stroker cranks from Hot Cranks. You wouldn't have the simplicity of air cooling, but in all other respects you would have a vastly superior bike.

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I'm not surprised that it's a power limited head design. Evidently I should have thrown a new head into my little factory frankenstein mock-up. My idea was based on the premise that Yamaha could (should) have the easiest time whipping up a decent air-cooled playbike, not that any of the OEM's are ever going to offer such a beast in the future.

Interesting topic here. I agree, the dependability iof a XR (type) motor in a modern day frame would be a highly sought after machine. The major manufacturers are on the right track (CRF-X and WRF lines) but these are still just regeared/recamed racing motors that will need reworking way too soon. I'm very interested to hear where this topic goes. I actually have a 1997 XR400R and a 2001 CR250R, both in very good condition. How hard would it be to do the transplant? I haven't done any measuring, but just by eyeballing it, it doesn't look like the XR motor would fit, simply due to the air cooled jug. Please keep the info coming!!!!

The other benefit of these newer bikes is the offroad versions usually have e-start, in addition to already having pumper carbs, better ignitions, etc. Plus the manufacturers have really focused on trying to lower the weight in the frame for better handling. A tall and dated air cooled motor would not be for everyone...as Rich pointed out adding displacement and lowering the revs will give the majority of people what they really want and much more economically.

I believe you will find a few CRF250X rolling chassis pretty soon. When the costs of rebuilding the motor is more than the bike if worth! CRF400X??

So a stupid question. What is the point of putting an oil/air engine in the modern CR chassis for a woods bike? For a MX bike that needs a more rigid chassis with stronger suspension pieces I can see the CR frame being a great benifit but then you'd be better off leaving the engine that came in it in place if it was a MX bike. It's not difficult to take a modern USD fork and adapt it to something like an XR chassis. Then install a proper length rear shock and the old tech chassis is significantly better suspended which I believe is what you are really looking for in a woods bike. Honestly I think the biggest problem many folks have is that they are not honest about what they REALLY need out of a bike and what kind of riding they really are capible of. Remember Scott Summers rode pretty much stock chassis XR's for many years with great results. I don't know that many people that are faster than him. A few but not too many.

Just my $0.03

One advantage of the air coolers is that they don't pop radiators when you crash them on a rock. The XR's are top heavy bikes which can make them tricky to ride in the woods. This is one reason to swap the engine out to a MX frame, but the XR's are top heavy because of the engine and not the frame so you'll just make the MX top heavy as well.

The seating position / tank configuration on the XR for sure makes the bikes a bitt more tippy. The ALoop kit goes a long way toward making it feel more balanced and actually helps channel more air across the engine.