engine/chain noise in motor

[ady]

ive done the cb gear trick,also checked valve gaps,inspected cams t-chain etc,everything looks mint.i was wondering if mabee this noise could be piston slap,anybody know how many hours for a piston under cross country race conditions.

[Boit]

Piston slap has a distinct sound. Plus, if the engine is worn enough that the piston is "slapping" against the cylinder walls, I would expect it to burn oil and smoke due to blowby, not to mention loss of power due to lower compression....etc..etc.

[ady]

i read somewhere that due to the large piston dia and short skirt the skirt wear was high,i know on 2 strokes you can have a worn piston skirt that will slap with no noticable loss in power.

[Taffy]

ady

does it happen at tickover?

when you blip the tthrottle only?

or all the time?

Taffy

[Boit]

Yes, the 426 has a very short piston skirt. However, from listening to many of these engines, it's clear that piston slap would be abnormal. Could it be that what you are hearing is preignition knock from poor fuel quality? The amount of operating hours on this engine before cylinder/piston/ring wear becomes an issue is relative. I use mine strictly for MX but it would seem to me that cross country use might facilitate this wear since the engine is operated for extended periods in dusty conditions before and oil and filter change. If dirty air also gets into the intake, wear is accelerated even further. From all the different posts describing noises attributed to the counter balancer gear and one or two others sources, it would appear to be prudent to investigate and elminate these other possible causes before yanking off the topend. I think the next time I visit an auto parts store, or Sears, I'm gonna invest in one of those mechanic's stethoscopes for when I begin hearing unusual engine noises.

[Taffy]

boit

i very much doubt that the problem is wear of either the rings, bore or piston.

the tolerance levels are minute with nikasil compared to air cooled & a cast iron sleeve.

the reduction in friction is better & there is no glazing involved. it's an all wins & no loss situation. the only time you lose is at the wallet.

i used 2 sets of barrels over 13 seasons of racing with open carbs on my roadracer. i used to internally mic the barrels & the wear at 3/4 barrel height was .0005" at worst.

everyone is shitting themselves over 'how long before i rebuild my engine?' but i can see these things doing roadbike mileage if they are well looked after.

they would last even longer if we left them at WR timing. remember it's revs that kill!

Taffy

[ady]

the noise is at its worst at tick over,it sounds like a timing chain slapping around without a tensioner.as revs start to build it turns into a sort of wooshing noise before the exaust note takes over.a freind of mine has a 6 month old 426 that is just starting to make the same sound only quieter.Our bikes are highly maintained and oil changed every race but this noise is starting to p*** me off now,looks like another full rip down soon,i will let you all know what i find.

[tige]

do a search in the wr400 gen discussion forum for(chain like noise in motor) there you will find your answer. i have a 2000 wr400 and my friend has 2 99wr400 and they all make that noise when your slugging around in a high gear. by the way it has nothing to do with pistons or heads, your way off. do the search