Odd Low End Sound

[john_c]

Hello everyone....

I've got a 94 KDX 250.....

The thing runs great. As far as I can tell, there is no loss of power anywhere in the curve.

Over the past couple of rides, I've noticed an odd sound, that probably isn't all that out of character for a two stroke.

It seems to occur especially in the low end of the power curve, and *only* when I load the throttle.

It's a sound similar to a loose valve in a car. It's a bit more pronounced rattle than what I think sounds like a normal two stroke.

I was wondering if this is indicative of a common problem / adjustment or if I'm just being paranoid.

BTW, it's got about 1400 miles on the motor, has been ridden relatively lightly, and has never been rebuilt. I'm running 32:1 ratio, and 10w/30 golden spectro.

Thinking about it, I haven't adjusted the clutch cable at all lately.....could this be related?

Thanks for any tips!

John C.

 

[mmmbeergood]

Sounds like your normal kips rattle that comes from a tiny amount of play in the main powervalve. Most kdxs have it, or get it. It doesn't cause any problems, it's just there. Even after cleaning my kips and installing a new piston, mine still rattled.
Gordo

 

[CaptainObvious]

>>"It seems to occur especially in the low end of the power curve, and
>> *only* when I load the throttle."

Could it be your chain slipping on your counter-shaft sprocket? How does the sprocket look? How is your chain tension?

 

[DaveRS575]

Mine has exactly the same symptom. I intend on pulling it apart to inspect, Mine is VERY pronounced during low RPM hits. If you do yours first, I'd love to hear what you find. Otherwise, I'll post and let you know what we find.

Dave

 

[john_c]

Thanks for the replies all......

Good to hear that it's something others are experiencing.......like I said, it doesn't seem all that bad, but it does concern me a bit.

The chain is a bit on the loose side, but not much. I can just get three stacked fingers between the rear of the top chain slap guard and the chain. The sprockets don't look all that bad. I'm surprised how long they've held up.

I plan on trying to at least ride it out the rest of this season, until it gets too hot here in Tucson, and then rebuilding the motor this summer.

If I figure it out, I'll let you all know....and you guys do the same.

If anyone has any more ideas, I'd love to hear 'em!

John C.

 

[jpm200]

I had the same sound it was like a tapping at low revs under load.
It was due to the crank pin and big end bearing starting to fail.
The surface on the crank pin had started to spall.

 

[Roost165]

Heh ride a honda sometime you get all sorts of wierd sounds. Especially that wierd whirling sound. I kinda miss it.

 

[DaveRS575]

Ok...Pulled the whole top end apart last night, and inspected / cleaned everything. Lots of black sticky crud in the powervalves, and everywhere else for that matter. Cylinder looked fine, minimal scoring, piston had a bunch of black crud baked onto the top but scraped right off with my fingernail and some simple green, rings seated nicely, plenty of flex left, After cleaning, new gaskets, and reassembly, it started on the third kick, idled better / smoother than ever, warmed up nicely and seemed totally fine. Better in fact.

No rapping to report yet, but it really didn't do it before until it was hot, and under a heavy low RPM load, so ....just figured I'd give an update. Nothing was broken, just really gummy. Get plenty of brake cleaner, soap and hot water, rags, gasket kit, and go for it! It took me and a buddy 3 hours to do everything. It helps to have another peron there to get the piston back in, but not necessary. I like the fact that you can do most everything with just 8mm and 10mm 12mm 1/4" sockets!! Except the head bolts, a little more tricky to torque properly without that funky 12mm torque wrench adapter tool and a calculator! :thumb:

I'm ready for spring!!! :aj:

 

[atc3434`]

Another thought, before considering it could possiblly be all these mechanical problems, it could simply be detonation. Most bikes will do this if you load the throttle hard enough at a low enough RPM. Run a higher grade gas, if possible, to see if it goes away some. Only when hot, and when lugging, leads me to think thats all it is. The KDX220 I rode would do that all the time, was never anything more than a little spark knock. Just my 2 cents.

 

[DaveRS575]

I'm pretty sure that wasn;t the problem but it's good advice anyway. I always run Sunoco Ultra and Amsoil, and the problem occured after some time of using that mix... It does sound like a "knock" though... Guess time will tell... Thanks!

 

[TVRider]

What ratio do you run the Amsoil at? If it is the recommended (not by me, i'm looking for answers) 80:1, how does the black sticky crud get in the power valves unless the bike is too running rich. Or is a bike that is jetted correctly still going to produce that gunk?

 

[CaptainObvious]

Hey Dave:

Just keeping on the knock subject...

When you had your top-end off did you happen to check the free play of your crank bearing? What was the condition of your pin?

If you were experiencing any detonation you would have noticed evidence in the top of the piston. Did it look smooth? True pre-ignition will cause havoc with the inside of the cylinder. Something usually gives.

Just some miscellaneous ramblings.

 

[DaveRS575]

Hey Gary,

I did not check the crank bearing per se, We did give the piston a thorough check for any wobble, and full motion however and it was all good, no side to side movement whatsoever, and smooth flow of movement when turned over, no friction, grinding, or difficulty, so I made fair judgement that it was all ok. The top of the piston was covered with burned on black fuel, indicating to me that the guy who was running this before me (or me I for that matter) May have been running a richer mix than necessary. I go around 40:1 with the Amsoil Pre mix. I don't recall anyone recommending I run amsoil at 80:1 though?? (previous post by TVRIDER) That would be a mighty lean mix I'd think... right?

The cylinder was very clean, very very minimal scoring, and the top of the piston, once cleaned of all carbonized fuel/oil mix, was smooth and new looking. It was pretty well crudded up though. The valves were a bit tacky too, but once the gears and everything were cleaned thoroughly, they timed in nicely, and rolled smoothly open and closed. The gears all looked "good" taking slight wear, but fine good for at least one more season I'd say. I did not get a chance this weekend to get her really heated up, but hopefully very soon, so I can tell you if the noise is gone or still there. I can't wait! :aj: