I have an odd situation with my son's 300ex. We'd taken a ride and it was running just fine. (The top end has been a little noisy, but I'd hoped to get through the summer before dealing with that.)

After we rode, we parked the bike for 30 minutes or so and discovered that we couldn't restart it. After a couple of hours of messing with battery/starter, etc, it turns out that the engine appears to be seized.

Any ideas what would cause this?

I've seen engines seize from corrosion after sitting for months, but never after sitting for only a few minutes.

Unfortunately, we didn't have much time to do much more troubleshooting, but we're headed back up to our vacation spot where it's stored tomorrow. My thought is to put some penetrating oil into the cylinder and see if we can (gently) break it loose. I'm already planning to rebuild the top end over the winter, but if I can get it running and get a few more weekends of riding, my son will be very happy.

TIA for any words of wisdom.

 

not saying this is the case but how is your oil level in it? I once had a cycle didnt check the oil and ran it it got hot and when I shut it off it seized up piston rings got hot enough to stick to the wall of the cylender and it wouldnt turn over again

 

Nope, she's got oil. I'm fanatical about checking it every morning before we ride and every evening when we put it away.

 

Not sure if this can happen on a 300ex, but I have seen it on other engines. The float needle doesn't close, and because the tank is higher than the engine, gas can drian into the carb, and if the intake valve happens to have stopped in the open position, gas can fill the cylinder and you won't be able to turn it over. Easy check is to pull the plug.

 

Thanks. We had a similar problem with our old Suzuki with a vacuum petcock... it got left on "prime" and filled the cylinder. I was planning to pull the plug to squirt some oil into the bore, so I'll be sure to check that it's not just full of gas. I assume that the correct way to do that is with a match?

 

Yeah, a match or lighter should do it.

 

Instead of trying to turn it over with the starter, put it in 4th gear and rock it back and forth.
It's easier to break the piston free this way, if it will break free at all.

 

Thanks all. I was able to free the piston.... with a crunch and a grind.

Ended up opening up the engine and finding major carnage. The rings were twisted like a pretzel and there were shards of metal from the piston and the bottom of the cylinder. Looks like a total rebuild or engine replacement is in my future.