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How do I know if crank is bad

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I've been away for a few months and decided to fire up my 05 300 yesterday, I was just standing beside it letting the engine warm up and I noticed when I cracked the throttle open that the bike went up into high rpms smoothly but when I backed off the throttle an extreme vibration went through the whole bike. When the rpms lowered the vibration went away. I know 2 strokes vibrate alot but this seems so bad that i feel like something is loose somewhere. Before I start ripping it apart is there a way I can tell if its crank or crank bearings?

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A very noisey motor and it gets louder as it runs. That is if rust has taken out the main bearings and rod bearing. The only sure way is to take off the top end and inspect. If there is any up and down play in the rod or if there is rust on the crank it is time. Just cracking the throttle is not a good test. What it did could have been normal. You could pull the mag cover off and check for up and down play as well. If you keep water out off the motor I have seen 300 cranks go 16,000 dirt miles. I'm not kidding. Also if it is the lower rod bearing it will knock under load when you try to lug it.

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was the bike on a stand or the ground ? ive noticed they vibe more when the wheels are off the ground. cant say if your bearings are worn but you can never balance a 1 cylinder and thats why some rpm are smooth and some have more vibration.

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Thanks for the replies, I probably only have one, maybe two rides before calling it quits for the winter then I will pull the jug and inspect. Also, I don't know if this matters but I did have the head milled not to long ago.

I'm liking this bike I bought it last spring and I'm still working on bringing the maintenance up to snuff as well as setting it up the I like it.

thanks.

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A very noisey motor and it gets louder as it runs. That is if rust has taken out the main bearings and rod bearing. The only sure way is to take off the top end and inspect. If there is any up and down play in the rod or if there is rust on the crank it is time. Just cracking the throttle is not a good test. What it did could have been normal. You could pull the mag cover off and check for up and down play as well. If you keep water out off the motor I have seen 300 cranks go 16,000 dirt miles. I'm not kidding. Also if it is the lower rod bearing it will knock under load when you try to lug it.

Exactly what mine did, but it also knocked a bit when idling.

ACF3B2.jpg

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+1 to what GMOSS posted. I had a similar issue on my YZ125 where the engine would make some weird noises on deceleration and it turned out to be a worn out piston pin. Never figured out what the cause was though.

Easy enough to check though, remove the pipe and the flywheel cover and feel the piston as you move the flywheel back and forth. If you can feel a noticable knock, it is time to rebuild.

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+1 to what GMOSS posted. I had a similar issue on my YZ125 where the engine would make some weird noises on deceleration and it turned out to be a worn out piston pin. Never figured out what the cause was though.

Easy enough to check though, remove the pipe and the flywheel cover and feel the piston as you move the flywheel back and forth. If you can feel a noticable knock, it is time to rebuild.

that's NOT the piston pin, that is the con rod pin! :lol:

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that's NOT the piston pin, that is the con rod pin! :smirk:

:banana:

I guess you have big fingers :bonk:

Although, my yz125 did have that happen to the wrist pin and I would assume cause a very similar noise. I'm a bit shocked to see that on the crank pin though.

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