Engine Knocking Or Piston Slap?

[burnleytazz]

Hi all bad day for me went to my local quarry today all fine then my engine started knocking and losing power on full throttle so brought it home.

Took barrell off piston absolutly fine, barrell fine. Noticed piston rings are worn (will this cause more side to side movement)

Took piston off and when i removed top end needle bearing all pins fell out of casing lookely i had placed a rag around the con rod

Will these parts be causing the engine knock or slap?

Also i replaced the original nahl piston with a s3 piston kit about 3 months ago. but it never come with a new bearing will i need a s3 bearing or does this not matter

Edited August 18, 2013 by burnleytazz

[lewis cosgrove]

If the small end bearing fell to pieces in your hand it would have most defiantly have been causing a knock. (Not sure if the bearings the same, probably will be though)

If the rings are worn change them while your in there.

You said you changed the piston to s3, when re-installing did you set the squish correctly?

Also check the con rod has no up and down movement, its alright if it can move slightly sideways.

Edited August 18, 2013 by lewis cosgrove

[burnleytazz]

If the small end bearing fell to pieces in your hand it would have most defiantly have been causing a knock. (Not sure if the bearings the same, probably will be

If the rings are worn change them while your in there.

You said you changed the piston to s3, when re-installing did you set the squish correctly?

Also check the con rod has no up and down movement, its alright if it can move slightly sideways.

Hi lewis yeah gonna get new rings aswell

when i got the piston kit i did get a base gasket 0.8mm but never used it the old one was ok (least thats one part i dont have to order)wot is squish and how do you set it?

Edited August 18, 2013 by burnleytazz

[lewis cosgrove]

The "squish" can be altered by using less or more base gaskets.

This simply increases and decreases the clearance between the piston and head of the cylinder.

I have a Sherco 290, with the squish set to 1.2mm which is stock.

For a Gas Gas i not sure what the exact "squish" should be, check the web and other forums on TC for an answer from someone that has a Gasser.

0.8mm could be to low but this knock most likely was the worn bearing.

Also with the correct "squish" set be sure to tighten the cylinder base/head nuts to factory torque settings.

Edited August 19, 2013 by lewis cosgrove

[zerorev3rev4]

piston kit 3 months ago and now the rings are already worn ?? how did bearing look then ? also check little end journal in conrod for any defects in finish ,

[burnleytazz]

piston kit 3 months ago and now the rings are already worn ?? how did bearing look then ? also check little end journal in conrod for any defects in finish ,

The bearing seemed fine bk then but if it had slight play would this not wear the rings down faster?

Also its been riden every sat and sun since then but it has also had regular maitenaince oil.plugs.seals and so on

Also there is very slight wear marks on the conrod itself

[0007]

Oooof, I hate pulling a motor apart looking for a noise

Seen this a hundred times where you cannot see the source of the sound you heard

At the shop I'm at we try to find a cause before we tear down, tear down is to verify the suspected cause, not the sole source of diagnosis

Any way, apply Ochams Razor (Wikipedia that)

The simplest most obvious conclusion is usually the correct one, if your top end bearing was not intended to be cageless then you likely have found the cause of the noise, however, it could be a result/symptom of the root cause

Detonation is normally the cause of small and big end rod brg failure, the bearing is not the cause

So, during diag you need to see if you have detonation or you will be repairing a result and leaving the cause alone to come back and break your bike again

Like replacing tire tubes over and over without looking to see if there is a nail in the tire

Edited August 19, 2013 by 0007

[tltel]

Hi, I'm with 0007 here, bad fuel or bad fueling, or a faulty plug can cause heavy knocking, Too tight a squish will also make a horrible knock. could be a combination of one or two of these, obviously change the bearing, make sure you set your squish clearance (there is loads of info on this site ) clean carb, new plug, air filter, new fuel and check fuel flow. My bike always Knocks as its running out of fuel.

TLTEL

[burnleytazz]

Cheers for the replies lads

Ive owned the bike since january bought it as a novice anyway when i bought the bike it kept over heating constantly checked everything air leaks fuel mixture. Went threw o rings like mad did a water pump replacment new piston

Then i was reading a thread on here about ignition systems and the rectifier and i went striaght out to garage to check my wiring and omg no rectifier

So went threw wiring and it had been bodged up by previous owner.

Next day picked a rectifier up from trials uk fitted it run perfect till now

So that will av been detonating

I owned and ran for 3 months without a rectifiet

[burnleytazz]

thanks everyone who replied received my parts today piston rings, top end bearing.

but just checked conrod for play and there is slight play up and down not much less than 1mm

will i need to split casing and have a look.

also what part will need replacing is it the con rod itself that is worn or do they have a bearing?

or is it the crank bearings?

all your help is appreciated never done a bottom end rebuild

[dadof2]

Near 1 mm up and down play in conrod almost certainly means big end bearing has gone. Whatever ha caused this probably means main bearings are on their way out as well.

What fuel and oil do you use and what mix ratio?

[burnleytazz]

I run 80-1 was using atf oil but it swelled my clutch pack so i ended up bk on light gear oil

[jse]

Near 1 mm up and down play in conrod almost certainly means big end bearing has gone. Whatever ha caused this probably means main bearings are on their way out as well.

What fuel and oil do you use and what mix ratio?

ANY up/down movement of con-rod is cause to replace. With this extensive engine damage, I'd guess dirt sucked past the air filter or foam filter not oiled.

Jon