After some expert help!

[ricky0115]

Okay so I just tried kicking it up and on the third kick I heard a puff of air like a blow off valve letting out pressure but I'm unsure where it came from it sounded like it was somewhere near the carb or air filter

[Calum]

@ricky0115 Probably tried to fire up but failed.

You haven't blown a hole through the piston have you? It does start right?

[ricky0115]

@Calum It will only start from bumping it, I'm going to take it to the garage on thursday so I will update with the issue.

[Mightyman]

A couple of my bikes used to occasionally make that sound when I was kicking them. I'm sure it only happens on an engine that isn't perfectly set up / running little bit lean or little bit rich? Either way the bikes ran okay.

[declan]

@mightyman I've only known 4 strokes to make that sound

[andrewj1680]

I'd say piston rings at a guess maybe just a cracked ring would cause issues

[ricky0115]

@andrewj1680 thanks

[ricky0115]

@declan I think it's drawing in way too much air, everything is standard air intake wise. How can I find the piston size I know it's been bored out but I'm unsure of the size

[oldman]

@ricky0115 markings on top of the piston should give you the size or measure the bore with calipers, either way it's the head off, not a big deal. Personally think your problem lies elsewhere but expect to be proved wrong, see what the garage says, hopefully something simple. Good luck

[Mightyman]

Just had a quick Google and first couple of results suggest worn rings / not correctly sized piston, which makes perfect sense because the sound will be the air squeezing through the ring seal when compression is happening.

I suppose this means my bikes could have been a bit down on power not having a correctly sized piston

[Nottsbiker]

I've had this before on a high mileage DT125R and due to being poor I left it with dire consequences. Basically the piston tried to make its way out via the exhaust port. I went from a standard bore to 2.00mm oversize in one go which made me cry when I realised.

[ricky0115]

Yeah I just tried to ride to work and the engine died doing 60 the back wheel locked up I have never been so scared in my life! I think it's ready for a new top end! Actually anyone selling a wr200 engine?

Cheers guys

[Mightyman]

Unlucky man, glad your unharmed though!

[Calum]

It's like, the signs were there. It didn't fail for no reason.

You see it all the time where people drive their engines to literal death when the bike was telling them it needed investigating.

[ricky0115]

@mightyman dt125r puff of air from top end didn't find any matches for me

[ricky0115]

It's not seized, managed to get it in the garage there's a hole in the piston and some scrapes on the barrel, they advised a full rebuild so I've gone with it. At least it will be done properly and will be under parts warranty!

There goes my new fat bars & koso digital instruments till next month

[Calum]

@ricky0115 And I refer you to my initial prognosis of the problem...

Who have you taken it to to rebuild it?

[ricky0115]

@calum yep you know your stuff for sure, kudos!

It's gone to CMC, Yamaha dealer.

[Calum]

@ricky0115 Yeah nice. I only ask because these engines MUST (and I can't stress that enough) be built using either genuine or Athena head gasket.

If they use anything else then it absolutely will blow the head gasket.

But if they're a genuine Yamaha Dealer then they should be using genuine gasket.

What piston are you going for?

A hole being burnt through the piston is a big sign of the bike being incorrectly jetted.

You've effective welded through the piston.

[ricky0115]

I have gone for the wossner? It's was about £70, it's a genuine Yamaha gasket, so the bore will be standard again I guess?

I have only had the bike about 2 weeks and I'm new to 2 strokes so I wouldn't have been aware of any jetting issues so I'll make sure it's up to scratch when I get it back! I know it has a 240 main jet but that's about it.

Thanks for your help!