blown engines

S

Snafu

Member

May 22, 2002

13

0

• Dec 29, 2002

• #1

I've Had my bike(yz 125) for a year now and the motor has blown up twice now. The first time a skirt of the piston went to pecies all through the engine and the second time some peice of brass came up and sized the crank shaft. All this happend in a less then one year. I have a 1998 yz 125 with FmF pipe and silencer, Wiseco piston and hot rod kit,and Boyesen reeds.I also never fouled a plug. Could the carb. be running lean? I need to fix the problem, I don't wanna pay $700 again. Any advice would be helpful.

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Dec 29, 2002

• #2

How long are you going between top end services-a broken skirt normally is bacause the clearnace is too big between the piston and the cylinder-maybe you cylinder is in need of replating.

 

S

Snafu

Member

May 22, 2002

13

0

• Dec 30, 2002

• Thread starter
• #3

the top end was almost brand new it had about 3 hours on it, after the skirt broke we had the cylinder replated and of course a new top end and rod and the crankshaft turned.

 

marcusgunby

LIFETIME SPONSOR

Jan 9, 2000

6,450

2

• Dec 30, 2002

• #4

Strange-i would wonder where the bit of brass came from??next time you rebuild it you need to check the piston clearance-the cylinder may have been honed out too mucha nd still had excessive clearnace even with a new piston.

 

David Trustrum

~SPONSOR~

Jan 25, 2001

1,396

0

• Dec 30, 2002

• #5

The brass is the thrust washer. Overheating bigend, Lack of decent lubrication? or crank pressed too tight perhaps?

I would have someone measure the bore for taper & clearance. Perhaps the replate job was fine, perhaps not? Perhaps it was just a piston with a hairline crack or it got dropped in transit. Ah, you said skirt broke & then replated so maybe not a problem now.

 

S

Snafu

Member

May 22, 2002

13

0

• Dec 30, 2002

• Thread starter
• #6

thanks, i just checked the oil the day befor, do you think it wasn't getting enough oil from the carb running lean which I think I is?When I take the engine to a shop should I have the carb reset?

 

David Trustrum

~SPONSOR~

Jan 25, 2001

1,396

0

• Dec 30, 2002

• #7

Think of the leaness of the carb (if it is) as air to fuel mixture not fuel to oil.

Assume you run a decent oil at a reasonable ratio.

The bike doesn’t sound like it has seized the piston so what makes you think it is running lean? Detonation damage to the top of the piston may hint at this, but it sounds like mechanical damage.

 

S

Snafu

Member

May 22, 2002

13

0

• Dec 31, 2002

• Thread starter
• #8

when the engine blew up a friend of mine was there who is a mechanic and we took the head off the engine there was oil in the motor but the cylinder was too dry he said and suggested that there wasn't enough oil from the gas mixture getting into the engine. I run a 40:1 ratio and I've never checked the settings on the carb so he figured the carb might be running too lean.

 

W

WorldWideRacer

Member

Jul 12, 2000

78

0

• Jan 1, 2003

• #9

I suggest you run at least 32:1 in a 125, if not more. Also, check your airboot going into the carb and see if it shows any sign of dust. Dust can very quickly wear the piston an seize a rod bearing. You may have an air leak past the filter or are running poor/not enough filter oil.