Was out riding my bike today, having no real issues. Had to get up to 100km/h so I took it up to around 15k RPM and it started to struggle, from there it started to create a bit of a knocking noise that slowly got louder. I'm guessing it might have been running a bit lean, as the engine feels quite hot after letting it sit for a while. I had about 10km riding to get home, so I limped it, and it started to feel like it was down on power, as the noise began to increase. By the time I was home, it didn't really want to move and the noise was quite obvious. Using a screwdriver to the ear to find the noise, I believe it is in the top end somewhere. I'm thinking head gasket due to the possible lean conditions, or else something to do with the cam chain. It should be worth noting, that the noise increased and decreased with the revs, however it seemed to disappear if I backed off the throttle. What I mean is I could rev it to 10k RPM or so, then let go of the throttle and the noise would disappear, until it began to idle again and it would be back. Any thoughts of what could have gone wrong would be appreciated.
I'm sorry to say, I have to agree - sounds like you spun a conrod bearing. <!-- s:-? --><img src="{SMILIES_PATH}/icon_Eyecrazy.gif" alt=":-?" title="Eye Crazy" /><!-- s:-? -->
Unfortunately you appear to be right. Stripped it down today and cylinder 3 piston is loose and the bearing looks melted at some points. Must have been too lean I assume. This is the second time this has happened. First time it happened a year or two ago so I replaced the bottom end with another. Kept the same carbies though. Is this caused by running too lean? It happened to a different cylinder than last time. I still have the old block too, with 3 pistons and bearings that are fine. I am thinking of just grabbing a piston from that and swapping over. I assume I would need new headgaskets too like last time? Is there anything I can do to protect against this? Is it due to an oil issue, or just from leaning out and getting hot?
The main problem would be insufficient oil delivery through the crankshaft at high revs. There was a topic about it a year or two ago. There were mods that could be done to the crank to alleviate it. Running lean won't cook your conrod bearings. It might damage the valves and pistons.
Here is the post on the crank problem <!-- m --><a class="postlink" href="http://fzr250.com/viewtopic.php?f=21&t=3345">http://fzr250.com/viewtopic.php?f=21&t=3345</a><!-- m -->
I'm looking at the bottom end of my spare 3LN engine, and can't work out where the oil restrictor is. Anyone got a picture of what I need to be looking for? Does the case need to be split to get to it?
I would say that you will find it in the crankcase under the main bearing shells once the crankshaft is removed, where the oil is fed into the main bearings.
