1989 KX250 slight knocking noise....

Micahdawg · Feb 23, 2005

My KX just started to develop a very slight knocking noise at low rpm. I'm wondering if it could be from the spark plug....maybe I need hotter or colder?

The bike is a 1989 KX250, standard bore, I split the case and put new crank bearings/seals in. I reused the crank/rod assembly which felt pretty tight. I also installed a used piston/wrist pin with new rings (piston was in good shape and felt tight too).

I ride at 500ft over sea level and jetted the carb to FMF spec (52 pilot, 174 main). Before this ride I had the head off and rebuilt the KIPS with two new subport valves.

The bike was hard to start when I got it out in the morning, but ran for about 2 hours all day perfectly fine. No boggin, no hard starting, no loading up. Pulled great down low and on top just as it has since the rebuild. Compression is at 155.

The bike doesn't seem to make a noise at idle (idles kinda high), but when rolling in 1st with load on the bike and giving it about 1/4 throttle it makes a weird little knocking noise that goes away as throttle increases.

Could a spark plug make this kind of noise? It's not very loud like a mechanical BANGING noise....and I can't imagine it's the rod bearing as it would probably not go away with rpms.

Oh, I use Klotz at 32:1 and 92 octane also. Any help or suggested plug would be nice. I think I've got a B8ES or BR8ES in there now.

Micah

RM_guy · Feb 23, 2005

Could be bad gas. Pump gas can vary a lot. Try a freash batch and buy it from a station that gets lots of traffic. It's more likely to have fresh gas. You may also want to try 93 octane if that doesn't resolve it.

Micahdawg · Feb 23, 2005

This gas is kinda old. I think it's been mixed for about 3 months. I just shook it up good and went with it. Hopefully that's all it is.

Micah

Maverik · Feb 24, 2005

Could very well be slight piston slap as well. You mentioned putting in a used piston, did you happen to mic the front to back of the piston, and the inside of the cylinder? Dont mean to alarm you, but if there is even just slightly more then stock clearance, you are asking for serious trouble. Saving the cost of new vs used pistons could end up costing big time. Just my opinion, but pistons are like bearings, if you have it apart, just replace them. Its not worth risking the damage that will be caused when that piston skirt catches an edge in the cylinder.

Micahdawg · Feb 24, 2005

I assume I would see piston slap as some scored markes on the cylinder and piston?

When i got the bike, i wasn't sure it could be salvaged as it needed the cases split, new top end, subframe, fork rebuild, tires, bars, controls, seat, etc.... you get the picture. So a $16 good used piston definitely helped out in the rebuild. I typically wouldn't do this, but I got a lot of parts for very cheap and did all the work myself and still have almost $1400 in a friggen 89 KX.

Once the bike was done I put about 15 hours on it before the KIPS subport valve broke. So I did a compression test before yanking the jug (it pulled 155) and decided that I would not be redoing the top end.

The cylinder looked just as it did previously. Aside from some decent carbon build up on the piston dome it looked fine as well.

Would piston slap be audible at all rpms or just low rpms?

Micah

Maverik · Feb 24, 2005

You wouldnt notice the piston slap noise at higher RPMS, it would blend with all the other noise. At least thats been my experience. I had a 91 KX 250 that slapped, it wasnt bad but i knew it would need some attention in the near future. As far as seeing the marks, check around the exhaust ports in the front of the cylinder and on the front and back side towards the bottom of the piston, Im sure there are normal wear marks in those 2 areas. Mic front to back where the wear marks are, then mic front to back a little further up the piston, those 2 measurements should be pretty close. You really wouldnt see any distinctive slap marks, since the piston is in motion it would just slide. You would only hear that initial contact. I know what you mean about saving a few bucks where you can, Im with ya on that, just not on a select few parts. Im merely speaking from experience, so please dont take my word as the gospel. IM sure there are peeps on this forum that have much more knowledge that I do about this issue. I just wanted to chime in and try to help and share some experience.

I didnt think about it before, but is there some sort of timing positioning on the older KIPS powervalve? This could also be an issue.

Micahdawg · Feb 24, 2005

Do you think there is any risk of significant damage if this is piston slap. My goal was to just buy another wiseco top end kit when the time comes, but I didn't want to buy a $100 kit if this bike wasn't going to come together.

The KIPS valves do have alignment marks. I think it's just to make sure the valves all open in the right positions.

I'm still wondering if this can't be a spark plug issue. The bike calls for a B10EG and I'm almost positive that I have a B8ES in there. Someone told me the lower the number the hotter the plug?

I don't know man. All I know is I need a stack of cylinder base gaskets if I keep yanking this jug after every ride :)

Micah

BradFrost · Feb 24, 2005

The moment I read that you put a used piston in I gave you fifty- fifty on the possibility of slap. That gas is kinda old though, especially if it was crapolla to start with. Don't go thinking I'm any saint...... I run my piston and rings way over their limits but when I do em' I save up and do the lot or it aint worth gettin dirty over. All the best with the testing. If you've got a bash plate you could always just tell yourself its a rock trapped in there somewhere and keep her in the high rpms :)

Micahdawg · Feb 24, 2005

Some more info....I used to run yamalube 32:1 all the time. Someone told me that Klotz protects better so i gave it a shot. I've ridden twice now with Klotz and didn't notice the knock the first time I rode (KIPS valve was broke so the top end bog had my attention). This is the second time on Klotz and I do notice it.

As for the piston....I think this was a piston used for racing and on the 3rd rebuild someone just pulled it and put it back in the box. It was in a Kawasaki factory box with a 1988 date written on it (marker) and lots of dust on the box. Looked like it was barely used. I did put new rings on it though.

Maverik · Feb 24, 2005

Its nothing that needs to be changed right away if its piston slap, just be aware that it will need it before long and start planning. If you are not riding balls to the walls everyday, chances are, you may get this riding season out of it. Now if it starts knocking real bad, so bad you can almost feel it, then definatly get it done and over with. I wouldnt go pulling the jug again just to check. I would just plan on spending what you need for a new piston and rings when you put it away for the winter. In my opinion, if it was plug related, you would have other symtoms, not just spark knock (pinging). It would be loading up, popping, running real hot, not wanting to start when warm, etc...

lol, i feel for ya man. Nothing more frustrating is there?

Micahdawg said:
I don't know man. All I know is I need a stack of cylinder base gaskets if I keep yanking this jug after every ride :)

Micah

Maverik · Feb 24, 2005

New rings are always good, but that will only keep the top of the piston snug, next time you tear it down, try to measure what i mentioned before. You will get it, just takes a little patience. I run Klotz, 40:1 in the KTM, It really is good stuff in my opinion.

Micahdawg said:
Some more info....I used to run yamalube 32:1 all the time. Someone told me that Klotz protects better so i gave it a shot. I've ridden twice now with Klotz and didn't notice the knock the first time I rode (KIPS valve was broke so the top end bog had my attention). This is the second time on Klotz and I do notice it.

As for the piston....I think this was a piston used for racing and on the 3rd rebuild someone just pulled it and put it back in the box. It was in a Kawasaki factory box with a 1988 date written on it (marker) and lots of dust on the box. Looked like it was barely used. I did put new rings on it though.

Micahdawg · Feb 24, 2005

Forgive my questions....but how exactly does the skirt of a piston slap the cylinder. If the piston and cylinder bore wear out with use...the bore increases and the piston's overall size decreases. Wouldn't this increased tolerance reduce any chance of the piston slapping the cylinder? Or is it the play that developes in the wrist pin that allows the piston to contact the cylinder?

The bike does run a little hot...but the left radiator ain't the greatest. Never seemed to pop or experience hard starts.

I also decreased the pilot jet from 55 to 52. FMF suggested it and I wasn't sure if that was a problem when chacing down my bog.

Micah

Maverik · Feb 24, 2005

When your piston travels up and down, at each point, top and bottom, it has an up and over or down and under type of movement it has to contend with from the crank, because the connecting rod doesnt move straight up and down. With increased clearance the bottom end of your piston has room to move or rock and will slap the cylinder. The less room to move, the less slap. Its not hard to explain, but for some reason i cant think of a good way to word what Im trying to say. Im probably confusing you more then helping, sorry Micah. Basically if there is room to move, it will.

When you put it back together Im sure you would have noticed excessive slop in the wrist pin. If you take it apart again, you can try grabbing the piston and while holding the rod, try pulling up and pushing down on the piston to see if it moves. On the wrist pin side to side play is normal, up and down shouldnt exist. And since you have it apart, you can try the same thing with the connecting rod bearing at the crank. slight side to side play is ok, up and down is not a good thing.

BradFrost · Feb 24, 2005

I use Motul Factory Line 800. Only time will tell you the right oil for your work. Even though I've been guilty of running 200 plus hours on two-strokes before when doing the top or bottom I'd always choose a new piston. It's the guts of the motor and the rings and piston tolerances really do matter and bed into each other. BTW I only got 200 + hours outta the bike coz I chose my fluid and filter maintenance intervals carefully and stuck to the schedule with premium products.
Another idea for you might be to jack you bike up ...securely.... and then kick it over and try and find the rev band that the noise starts in while having a good listen from somewhere other than the seat. Rest the handle of a large flat head screw driver on your ear and touch various sections all over the motor and try and narrow down where it's comming from. You might wanna park the arse of the bike out garage and partially shut the door on it to cut down the exhaust noise while you play doctor. A Gclamp will hold the throttle in postion if you've got Barkbusters or whatever you call em.

RM_guy · Feb 24, 2005

I account piston slap to a worn piston or cylinder bore. The piston will rock (pivot) on the wrist pin and the skirt of the piston will slam against the cylinder wall...and make a knocking sound. The forces on the top of the piston will act on one side of the piston to cause the rotation.

When you rebuilt it did you measure the cylinder bore?

Maverik · Feb 24, 2005

Thats great advice, you could also clamp something like a hose over the silencer (metal flex hose, found at any auto store) to move the exhaust noise away while you are searching.

BradFrost said:
I use Motul Factory Line 800. Only time will tell you the right oil for your work. Even though I've been guilty of running 200 plus hours on two-strokes before when doing the top or bottom I'd always choose a new piston. It's the guts of the motor and the rings and piston tolerances really do matter and bed into each other. BTW I only got 200 + hours outta the bike coz I chose my fluid and filter maintenance intervals carefully and stuck to the schedule with premium products.
Another idea for you might be to jack you bike up ...securely.... and then kick it over and try and find the rev band that the noise starts in while having a good listen from somewhere other than the seat. Rest the handle of a large flat head screw driver on your ear and touch various sections all over the motor and try and narrow down where it's comming from. You might wanna park the arse of the bike out garage and partially shut the door on it to cut down the exhaust noise while you play doctor. A Gclamp will hold the throttle in postion if you've got Barkbusters or whatever you call em.

Micahdawg · Feb 24, 2005

Thanks for the tips guys. You know, amazingly enough, I never did measure the cylinder bore. The guy I bought it from said he sent the jug to be stripped and replated so I assumed it was the standard bore. I has to be if a standard 67.4mm piston fits this good and has had no other problems.

What if this were NOT piston slap....what else could make a low end knocking noise.

Also, just revving the motor does not produce any noticable noise. It gets noticable sometimes under load. Imagine taking off from a standstil on a 30 degree grade going up hill. Slip the clutch out slowly and gas it a little. You'll hear a very slight knocking on the low end that goes away as you increase throttle. I don't think it is the motor noise drowing it out because....moving the throttle from a "little" to a "little more" doesn't make much more engine noise. It's already hard to hear anyway....I was trying to point it out to a friend and it took him a little while to hear it himself. It is definitely an issue under load though.

Wouldn't piston slap be occuring at idle and slight throttle as the bike is on the stand?

Micah

BradFrost · Feb 24, 2005

Depends on the wear but you'll need a screw driver stethascope to hear it if at all. Hmmm under load ..... you need a dyno :)

Micahdawg · Feb 24, 2005

I've got a burn barrel to prop the back tire on and a fancy "seat-o-the-pants" meter.

Micah

BradFrost · Feb 24, 2005

:laugh:
Nice one. If things go wrong you can rip the tyres off the rims and go for a spin on the road at night .... fire works and ice biking all at once.

uts · Feb 24, 2005

A knock could be your big end crank bearing. You said it felt tight, but why would you go to all the trouble to split the cases and not replace the crank assembly?. Or at least measure the specs on it.This is the only reason I would split cases, cause the main bearings will last a lot longer than the rod bearing. Piston slap sounds more like a rattle.

UTS

RM_guy · Feb 25, 2005

Ya know, I still think it's just detonation, especially when you mentioned that it got worse under a load. Have you tried fresh or a higher octane gas yet?

Micahdawg · Feb 28, 2005

OK, I had a BR8ES in the bike with old gas. This weekend I swapped toa BR9EG with fresh 93 octane, Klotz premix and 32:1 ratio. The knock sounded worse.

I'm starting to wonder if it's not piston slap. I've also got the worlds wierdest idle surge. Sometimes it's really solid, other times it really irradic. For instance, I was riding the bike hard through the mud, it was very hot and the idle was chopping like it was running out of gas?

As for those of you wondering why I would cheap out on the rod............The only thing worse than having $1400 in a 1989 KX is having $2400 in a 1989 KX.and it still not running right. At the time of the engine rebuild, I wasn't sure the bike would even turn out ridable. It was a salvage bike that I saved from the junk yard.

Micah

1989 KX250 slight knocking noise....

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