Good morning folks, I picked up an old YZF750 supersport bike that was used to chase contingency back in the day. I was planning to shelve the motor to keep as a spare for my YZF vintage racer. I got the motor running, and compression was quite good, but when I pulled the cams to adjust the lash I discovered journal wear with grooves deep enough to catch a fingernail. What causes this? I imagine oil contamination or starvation to the bearing, but not having looked into many race motors, I don’t know if was just used for too many miles or if I should suspect an oil pump issue? My inclination was to repair the cams (snd likely replace the cam case), but am unsure if there may be a bigger problem. Appreciate any insight from those of you in the know.
I haven’t pulled the head yet. Do you mean the journal bearings in the cam case? Some are better than others Here are photos of the intake and exhaust caps for cylinder #1. Looks like some smearing. I had thought that crack on the one cap was just a casting flaw, but it looks pretty bad in this photo. There's no evidence of it on the top. Thinking more about these cams today, the PO had a spare motor with the head removed and another damaged head (cam bearings) that came along with spares. So, really, I don't know that these cams were actually damaged in this motor.
Parts got hot. If there is a head gasket to view, look to see if it has been stripped of a layer. I can't remember on this model, but sometimes the center section gets stripped and it contains a restriction for the cam oil supply that, when missing causes a pressure drop that will kill the cams eventually. The restriction is usually a brass insert in the center layer. Also, the cams are hollow, and if oil is allowed to get really dirty, the cams fill up with sludge. This can overheat them. When stripping a top end, always flush the cams.
Well, I finally got around to pulling the head on this engine. The headgasket was all three layers (the oil restriction holes on the gasket actually sit outside the head and are disused as there are external oil lines to the head). Compression was a little low on one cylinder, so I did a leak down test and discovered one of the intake valves on cylinder #4 was leaking terribly. So, I removed the cam case, removed the valve for inspection and discovered the spring was missing! Uh oh. I tore the motor down and found all four pieces in the sump. So maybe some of that metal got ground up and caused the bearing wear? The piston wrist pins were all badly scored (catch a fingernail). Yzf750 Rebuild by Michael Reed, on Flickr Yzf750 Rebuild by Michael Reed, on Flickr Untitled by Michael Reed, on Flickr
