KX 250, After a Fresh Rebuild... it now Knocks?

[Alin99]

Hi,

I just performed a top end rebuild on my 2000 kx250 and while that solved the problem with lack of compression and the bike not starting, it now knocks very loudly. :yikes:

Any ideas? It did not knock before the rebuild when the bike ran. The piston, rings, cylinder head, and spark plug were all replaced during the rebuild.

I'm really afraid to keep it running with the less than a second repeating knock, in case it's wrecking something.

 

[macauleyc64]

is :whoa: it the pistion rings

 

[TWRT]

The knock may be a lean condition.

You could have an air leak (left side crank seal? or something around the reed cage?)
or
The pilot is too lean for the 'new' amount of compression?

Did you check the play in the connecting rod?

I'm just thinking out loud here.

dave

 

[_JOE_]

Why did you replace the cylinder head?

What end gap did set on the rings?

How much piston to cylinder clearance was there?

 

[whenfoxforks-ruled]

If its not detonation, maybe its the sound of your engine running better than ever before? Some pipes its like the sound and pressure wave converge/separate and it sounds odd? How is the silencer packing? And second the ring gap,piston skirt clearance and thrust washer clearance on the big end of the rod.

 

[rmc_olderthandirt]

Hi,

I just performed a top end rebuild on my 2000 kx250 and while that solved the problem with lack of compression and the bike not starting, it now knocks very loudly. :yikes:

Sounds like the rebuild was forced due to a seize. Did it lock up or did it just lose compression and stop running?

If it locked up while you were running there is a good chance that the rod bearing was damaged. Like TWRT said, did you check the rod play before you put it back together?

Another thought: Any chance you left out the wrist pin bearing?

When you took it apart, was the piston in one piece? Were all the pieces accounted for? If a chunk of piston is still down in the case the rod could be hitting it as it goes by.

If it is knocking you need to tear it back apart. Don't run it anymore, that will just damage otherwise good parts.

Rod

 

[Alin99]

Hi,

I'm going to try to quickly answer as many of the questions as I can, as this is my first bike and I'd need to do a bit more googling to be able to answer, and i'm at work so I can't.

What happened was buddy was riding it in 5th gear at a high speed and it simply died on him. We could not start it anymore.

The old rings don't look that bad (i can upload photo's when I get home). The piston is in one piece but has some uneven scortch marks on top, and a few vertical scratches in the back (i can upload as well). I showed the pieces around and it was suggested that maybe there's a tiny crack in the cylinder head, so I went and bought another one.

As for air leak around the reed cage, this is what I'm hoping it is, as I am missing a screw on it. This was also missing before the bike died.. but with higher compression, this could be it. I'm heading to Rona for one after work. :)

The connecting rod has a little bit of side to side play but not too much.

As for the ring gaps, I didn't pay too close attention to this, as I bought all the parts from a KW dealer and the parts are all KW for the 250. They fit pretty snug around this little raised dot thing on the piston. Is that bad that I don't know the clearance? One of the rings might have an issue, I hope not as we do have lots of compression, but when we put the top half of the engine back over the piston and rings, one of them got caught. It was bent it back into shape and placed in there.

The wrist pin bearing is in place.

 

[whenfoxforks-ruled]

Knock,you are just lucky it is still running! Was the cylinder cleaned up/honed, and the bridge relieved in the exhaust, you have to measure the thrust washer clearance with a feeler gauge, some rings are too tight and the gap must be filed to obtain proper clearance, and the bolt missing from the reed cage will cause a lean condition with a little dirt to boot. And the bent ring thing, lots a luck. A well worn motor can look great, actually measuring is not difficult, but does remove all doubt.

 

[Alin99]

No, the cylinder was not honed. As I'm just learning to ride the bike, and it's still above my skill level, I'm trying to avoid having an even bigger engine... for now. I've uploaded a few photo's in my profile under gallery photo's so you can see how the old parts looked.

As I don't know what you mean by 'was the.. bridge relieved in the exhaust' it probably wasn't done either.

Also, where is this thrust washer? Any idea what kind of clearance is needed? I've been using KW's Service Manual for a 2003 and while it hasn't let me down yet, I can't trust it's numbers. I thought the KW dealer would supply the right size rings for the bike..

I have obtained a new screw so the reed cage should now be fine. I haven't tried re-running it yet as I also want to do a gearbox oil change as there's probably some coolant in there since the rebuild..

 

[whenfoxforks-ruled]

The big end of the rod has thrust washers for clearance in between the rod and crank halve, hence, thrust washer clearance. It will tell you one part of possible wear. The other is the up and down movement, none. The axial is how much the top rocks from side to side, a little harder to measure. If you have been missing a reed cage bolt, and running it, it would be a good idea to remove it, clean it, new gaskets and sealer, properly torque it back together. Where did the debris come from that destroyed your engine? The pics are pretty blurred, looks like it ingested a lot of sand/dirt. On the exhaust side of the piston, is there a wear mark down the middle? So the new piston was not checked for skirt clearance, and the new ring gap was not checked also? And the foreign material that destroyed the top end, also went through your crank also.

 

[eli98]

If there was scoring on the cylinder, it may have needed honing, or a re-sleeve. Adding an oversized piston and ring set is not going to increase the amount of power all that much. It's not a small block chevy. I second the thought that you should tear the engine back down (I know it sucks) as you do need to determine the reason for the knock. When you're putting it back together, go very slowly, following the manuals every instruction. If something doesn't make sense to you, hop on this forum and ask, someone here will be willing to assist. Don't use power or air tools to take it apart, or put it back together, as they are unforgiving, and crankcase halves are frightfully expensive. Check your ring end gap. If it is within the recommended tolerance, ensure that the cylinder is smooth as glass, no verticle ridges. Honing should be done with extreme care (if you have a nickacyl cylinder, take it in to be done). Also, ensure that the exhaust port (and intake) doesn't feel as though it is protruding into the cylinder, use a finger nail and guess. If it at all feels rough, it will destroy your new rings. As you have stated that you are new to the game, you will likely not be able to guess on whether the piston is teh right size for the cylinder, so test the ring gap in several locations from the top to the bottom, as it may no longer be true. If you have any doubts on the sizes, take the cylinder, head, piston and rings in to a reputable repair shop, and have them measure everything.

 

[rmc_olderthandirt]

As for the ring gaps, I didn't pay too close attention to this, as I bought all the parts from a KW dealer and the parts are all KW for the 250.

You absolutely need to tear that top end back down. The knock for one thing but you skipped a very important step and it needs to be done.

Every top end I have done I have needed to file the ends of the rings to get the proper end gap. I just rebuilt the top end on my 250 two stroke. The required end gap is 0.022" and as supplied the rings were 0.008". It only took a few strokes with a file to get the ring gap how it should be.

This is the way it should be done. The cylinder has a certain tolerance on its size and you want the end gap on the rings to be as small as possible. The rings are intentionally provided oversize so you can fit them to your cylinder.

If you don't file the ring ends to get the required gap you have a high probability of seizing the engine the first time it gets to full operating temperature.

They fit pretty snug around this little raised dot thing on the piston. Is that bad that I don't know the clearance?

Yes, it is bad that you don't know the clearance. I am not sure what you mean about the raised dot on the piston.

The rings fit in a groove around the piston. There is a pin in the groove where the end gap of the ring is to be. This locks the ring in place away from the intake/exhaust ports. Speaking of which, the piston also has a front and back side, and needs to be install the correct way. They typically have an arrow on the top that needs to point towards the exhaust port.

Part of the trick of assembly is keeping the rings in place long enough to get the cylinder on. If the rings don't line up the cylinder probably won't go on and if you force it bad things can happen.

One of the rings might have an issue, I hope not as we do have lots of compression, but when we put the top half of the engine back over the piston and rings, one of them got caught. It was bent it back into shape and placed in there.

That ring is toast and needs to be replaced. As I mentioned above, if the ring isn't positioned so that the end gap is over the pin bad things will happen if you force the cylinder on. I am surprised it bent, I would have expected it to break.

Having been bent twice (out of shape and then back) it will be weakened. You do not want that ring breaking a short time down the road.

Bottom line is that the top end needs to come apart again so you can figure out what the knock is. Then replace the rings, gap them to the cylinder, install them with the gaps over the pins, install the cylinder carefully.

Rod

 

[Deaselevo]

Rebuild

Hi,

I just performed a top end rebuild on my 2000 kx250 and while that solved the problem with lack of compression and the bike not starting, it now knocks very loudly. :yikes:

Any ideas? It did not knock before the rebuild when the bike ran. The piston, rings, cylinder head, and spark plug were all replaced during the rebuild.

I'm really afraid to keep it running with the less than a second repeating knock, in case it's wrecking something.

 

[Deaselevo]

hey I did the same thing on my 2003 KX 250 and the knocking was annoying and loud. I've rebuild my motor countless times. I found out that its the crank berring that is gone. the knocking went away once the crank berrings were replaced.