A knocking noise in my engine...

[YZ250]

I was riding at a sand hill and did a nice wall ride but i crashed at the end of it and my bike started to make a clunking/ knocking noise in the engine. at first i thought it was the exhaust pipe banging on the frame but when i listened closer i heard internal noises so i know it has something to do with the piston. i just want to be sure...

[YZ250]

I was riding at a sand hill and did a nice wall ride but i crashed at the end of it and my bike started to make a clunking/ knocking noise in the engine. at first i thought it was the exhaust pipe banging on the frame but when i listened closer i heard internal noises so i know it has something to do with the piston. i just want to be sure...

I took my piston head cap off and the spark plug and they were in excellent condition! but what i don't get is when i looked at my piston, it looked fine, but when i would push the kick starter down, the piston would go up but not down. I can push it down with my hand, but it won't go down on it's own. something is preventing it from going down but i don't know what. i'm gonna take the middle/ sleeve/ cylinder or whatever you call it off and see if there is anything broken. it can't be the connecting rod or the camshaft could it?

[DangisMX]

It could be the bearing that goes between the crankshaft and the connecting rod. It happened to me once on a 150 sx. First a knocking noise and later it broke. I couldn't move the kick starter at all, as if the piston was seized. The repair was not super expensive, but it was costly.

[JETZcorp]

One would like to hope it's not the camshaft.  After all, this is not the forum for people who worry about their camshafts.  Now, the Crankshaft, that's a different matter, although I certainly wouldn't want to see one of those damaged, either.

[ford832]

Most likely you've broken a connecting rod.It will still push the piston up(until it gets jammed)but if not attached ,can't pull it back down.

[JohnN]

Most likely you've broken a connecting rod.It will still push the piston up(until it gets jammed)but if not attached ,can't pull it back down.

I agree...

Remove the cylinder and it will all be clear to you..

[SachsGS]

I assume your bike is a YZ250 twostroke - hence no camshaft.When troubleshooting always start with the most basic ,check the rear skirt of your piston (pull the reedcage to view it),it has probaly broken off and fallen into your crankcase preventing full rotation of the crankshaft.Pull your magneto side cover off and try to rotate the engine by hand by moving the flywheel, if you feel tightness or play your conrod big end bearing is gone.If the problem is intermittant you may have a crankshaft main bearing problem, I've seen the ballbearing spacers fail and cause all kinds of strange problems.If the engine rotates freely by hand your problem lies in the primary drive/gearbox/kickstart area.

[YZ250]

Thanks for all the advice, i'm pretty sure it has something to do with the connecting rod or piston skirt. I almost have the cylinder off so i'll be able to see then.

[YZ250]

It could be the bearing that goes between the crankshaft and the connecting rod. It happened to me once on a 150 sx. First a knocking noise and later it broke. I couldn't move the kick starter at all, as if the piston was seized. The repair was not super expensive, but it was costly.

how much did you pay for all the neccesary parts and what are all parts that you needed to replace?

[YZ250]

I got my cylinder off and i took a look at the piston and it was fine. but then i pushed the kick starter down but it won't go down. so i took my magneto cover off and spun the flywheel by hand but the piston would stop in the same spot as with the kick starter. so i spun the flywheel backwards and it did almost a full rotation before i felt resistance. right before the piston would go almost completely up, it would get resistance, but only in a certain area, then it would rotate freely until it reached the previous position. I can see the connecting rod and it looks fine but would it be something with the bearing then? 

[SachsGS]

On the YZ250 the bigend of the conrod should be slotted and you should be able to see the individual needle bearings.If they are blue or "slanted" your rod is defective.Pay attention to the thrust washers on either side of the needle bearing,if they are "all chewed up" you know you have problems.If all this checks out O.K. pull your complete clutch assembly,including drive gear, off the motor to isolate the crank from the gearbox. Again try to rotate the crankshaft assy.,if it is still tight the you have a crankshaft mainbearing problem and the cases will have to be split. Were you riding at the Oregon sand dunes? I've done a few motors from this riding area and what seems to happen is that the crank mainbearings seize,welding themselves to the crankshaft and spinning in the aluminum cases.It's an ugly job to repair.

[YZ250]

So your saying that the bearing connecting the con-rod to the crankshaft might have melted together? if so, how could i be able to move the piston up and down at all? also, right where the con-rod connects at the bottom, there is circular steel circles on each side of it and i think they connect to the con-rod, but anyways, on those steel circles directly parrallel with the con-rod are some heat/burn marks (blue,purple,yellow all mixed) and they are only in that one spot. it's almost as if part of the explosion from the piston up top get's down to the con-rod. or it might be from the bearing getting over heated and causing the connecting cylinder attached to the con-rod to burn. that makes sense.....

[SachsGS]

No,in your YZ engine, the crankshaft rotates in the cases in two roller (ball) bearings - one on either side. These are your crankshaft mainbearings.I'm starting to suspect this may be where your problem is.I've seen cases where the spacers that space the ball bearings in the bearing assy. have failed and the balls move randomly and jam.I've also worked on YZ 250s where the mainbearings have actually failed and siezed. The heat marks on your crankshaft can be from either the machining process to bore the crank halve for the conrod bigend pin or you've got a bigend bearing that is siezing.A careful examination of your big end needle bearing will reveal which is true.

[JETZcorp]

I think pictures would be helpful.  It's difficult to diagnose a problem just based on some text, so I'm sure SachsGS could help you out some more if he could examine the parts himself from pictures.  That's what I'd do, anyway.