Yeah sounds like a valve issue, was going to buy a CRF and read about the valve problems "taking it CFR's fall into the same problem", and to make it reliable you really needed to replace the valves with stainless ones with new guides, springs etc, just thought stuff it and brought a WR instead, over 4000K's with valve clearance checks no problem. I'm talking 250cc, but from memory the 450's had issues too. I was able to score a XR650R, no problem with valves on that old girl , same deal, valves are staying within tolerance, bonus with the XR = no shims, you can manually adjust them. Due to the high RPM's the manufacturers wanted the CRF's, CFR's and WR's etc to have titanium valves.
Now watch me go for a ride now and drop a valve on the WR!!
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Now watch me go for a ride now and drop a valve on the WR!!
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