Slap/clunky sound from engine.

So I let my 20 yr old take the WR426, 2002 out for a 70 mile ride. I'm sure he I over revd it.

So I warmed it up sounded fine, took off and when I decelled to a stop sign I heard a slappidy clank, very suddle, but can be heard best if your hands are on the bars cause you can slightly feel it.

It's slight enough that it took a few stop signs to determine there is somthing funny sounding.

No performance problem, and no sound when riding. I Have narrowed it down,,, you can have it idling on stand, just give suddle throttle and as the throttle comes back to idle you can hear the slap.

What you think ?

My thoughts were, if it was piston slap I would hear it during other modes of riding.

Maybe that cam chain tensioner broke some teeth and pulled back,, loose cam chain spanking due to looseness ?

Again, starts and runs great, not slap at all while riding,,,, but if coming into turn hard with clutch pulled in and rpm comes close to idle before nailing it you will hear it.

Does sound like coming from top of jug.

When was the last time you rebuilt the top end, or replaced the automatic cam chain tensioner ?


Pretty hard to say what it is without pulling it apart.....but  if it is the tensioner failing, stop riding it and check it.

Unknown, I bought this 2002 used 3 years ago. Starts and runs fine still..

Yeah, it's time to do some maintenence.


You could be 99.9% of the way to a catistophic failure with the motor being 13 years old....Yamaha's don't exhibit any signs before going self destruct.........or not, but I would certainly plan on a top end while it's apart.....

I look on ebay for piston kit,,, they state,,, top end bearing not included in kit,,, what are they referring too ?

The wrist pin bearing.


Do not buy a no-name ebay kit for your motor.

I was going to get the Wisconsin piston kit from ebay.

Do you mean Weisco?


I would not buy anything until you pull the motor apart and look.


Your crank could be shot.

Yes Weisco... I will start tonight, after I get in and find out what's slapping in there,, if I'm in deep then I'll order the kit.

Hoping it's a broken cam chain adjuster.

What did you find? I'm having the same noise.

Yesterday I stripped allot off,,, tonight I will remove top cover, inspect, then take off head,,, will get jug off if I have time. Little bit daily.

Was not the tensioner

That's why I'm going into the motor I'm kind of expecting to find a broken lifter spring in a lifter is slow tapping the top of the piston.

Ok, got the jug off. Lifters were in spec,, down to low side but in spec. I find nothing severe,, crank has no play, rod has no up down on crank or piston. I do see some skuffs of jug wall, and on piston.

The bike was low on oil when my kid rode it hard,, I'm beginning to think this was the start of piston slap.

Bore is 95mm so stocker,,,,, I did find 2 gaskets stacked on head,, never seen that on a 2 stroke.... anyone know if that's normal ?

Anyway, at this point I only can find skuffing that muratic acid will remove, no gouge. I see hone marks in the skuffs so that's good.

Piston slap ? That's what I think it was.

Any comments ?

Probably.  Or the starter motor clutch system was not disengaging. Sometimes at really low rpms it can act strange.


You can re-cross hatch the cylinder if those marks are gone, if you own the tools, otherwise send it out.


I would do a valve train re-shiming, top end, hone the cylinder, and use OEM GASKETS only. 


Take the head and turn it upside down and pour gas in it and see if the valves are sealing.

I Already Ordered Weisco piston kit with gaskets.

This bike no elec. Start,, 2002.

I'm gonna apply muratic acid to the cylinder walls,, done this to all my 2 strokes,, and scotch Brite by hand the cylinder wall, takes glaze off, never had issue in past with my 2 stroke bikes and sleds. I know everyone says hone, and that's great cause it's a quick 30 second process,, but if you have scotch Brite sheets,, 2 min., and some angular elbow grease you get the same result.

I will do the gas trick on head to verify seal on lifters. I'm assuming all good since I was still in spec and all looks good.

I may be missing something ttreewlkerr, but your bike isn't a 2 stroke. I hope I'm reading your post wrong. Double gaskets is either the POs way of putting a bandaid on a leak, or someone convinced him that the extra gasket would improve performance by creating space above the cylinder. Either way, I'd go back to one gasket, and +1 on the OEM stuff. But, I'm gonna bet you'll find your issue once you pull the clutch cover.

Bikes done, yes 4 stroke,, it is a 3 piece gasket,, weisco came as 3 piece gasket too.

Check the rubber cushions on back of clutch basket. They wear over time and if stock basket is still in this bike,replace the cushions at least. I had these completely tear apart on my 98 and had to split cases just to clean all of the rubber pieces out. If the rubbers on yours are worn out, it could be the basket rattling that you are hearing.

I noticed today that sound does not alter when clutch pulled or not pulled, load or decell. It defiantly rpm dependent, just before co ing down to idle, gone at idle. In gear on recall just as I pull inclutch approching idle OR , in neutral.

Sure seems like piston slap symptoms,,,,, yet I put new pistonkit in weseco, samesound. Maybe its just the bikes noise and I'm listening to much.

If the basket is rattleing due to worn rubbers, it can make the noise weather you're using clutch or not. Put bike in gear on the stand and roll rear tire back and fourth a little. See if you can hear the basket rattling. Or lay bike on its side and pull clutch cover off and physically wiggle basket to check for looseness or play etc.

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