Question about 2003 Yamaha YZ 125

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I replaced the cylinder and piston and rings on my 98 yz 125

I replaced the cylinder and piston and rings on my 1998 yz 125. it started right up and i rode it for about ten minutes then it died and now it wont start back up. also my floats are sticking on my carb. any suggestions.

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You may have not tightened coil and coil wire screws enough. 2 strokers are notorious for vibrating their bolts loose. Bad earth will nullify spark. Check for clean connections on all leads from crank to coil and CDI unit. Is plug and cap 100%. Check top end gaskets if any sign of moisture on sparkplug.

The float pin may be worn and extending out past ends and jamming on carby housing. Take out and roll on flat surface and look for defects. Ensure float tab (bears on needle) is not notched from wear or bent too far. Clean seat and needle are essential, replace if suspect. Spray some carby cleaner thru fuel line entry whilst you have floatbowl off to clear any sediment or debris. (especially fuel scale or air filter foam)
Cheers.

Posted on May 04, 2010

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Just put in s new top end a yz 85 2008 , wouldnt start but could if it was bump started and ran bad. Opened her up again and there was marks all over the piston, help??


When fitting a new top end you may have broken the piston rings. On a Two Stroke engine the rings have a gap which must not enter any ports. To prevent this the slot for the rings has a metal peg to hold the ring gap in one place.
On Four Stroke engines there is no ring peg as there are no ports.
If a ring breaks whilst being fitted the broken parts will enter a port and then be dragged up and down the barrel causing damage to piston and barrel. The broken parts one having made their way to the top of the piston will be crushed between the cylinder head and the piston damaging both. The cyl head can be re used but the piston is most likely ruined.

Pete

Oct 03, 2013 | Motorcycles

1 Answer

When should you replace the piston rings on a yz125


Hi, Anonymous for this situation I would call my local dealer or reputable shop's service/parts department and inquire about any possible quick fix, answers, or parts inquiry. For more information about your issue and valuable "FREE" downloads that you will need please click on the links below. Good luck and have a nice day.
YZ125 piston life Yamaha 2 Stroke
YZ125 Part 15 2 Stroke Installing Piston Rings Cylinder and Cylinder Head
YAMAHA YZ125 Owner Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

Mar 17, 2012 | 2003 Yamaha YZ 125

2 Answers

Im putting my 05 yz 125 top end back together my question was what is the torque specs on the head and on the jug


G'day. The torque specs for the 8mm head bolts is 18ft/lbs.the jug?(cylinder or barrell) is just tightened down even & firm, as there is no acess for a torque wrench.
OK. other things to note.
Clean the cylinder first in solvent(kero or fuel).Then in warm soapy water in a bucket. Dry-(compressed air is best for this-wear eye protection).once dry lubricate all power valves with oil.
Using a clean white rag-soak the end in your 2 stroke oil & wipe the inside of the cylinder surface untill the rag comes back clean(you will need to do it a couple of times-turn the rag over & do it again).Lube the new piston just before assembly-Also pour a little oil down the main bearing oil holes & over the big end too.
Other things to be sure of are-
Piston on the right way round-arrow to the exhaust.
Rings right way up. Some 2stroke dirtbike rings will only go one way up.Look CLOSELY at the ring ends-on the upper or lower surface there may be an etched dot or "T" or "N" or other marking. the mark goes facing up(on both rings unless single ring piston).
rings positioned with end gaps over the locating pins(the ring grooves have a pin in them to stop the ring turning)
Piston pin clips in correctly.
Powervalve moves freely.
Again-watch when you fit the cylinder-it can be tricky to keep the ring end gaps over the locating pins as you feed the cylinder over the piston.
Hope this all helps
Regards(done this a thousand times)Andrew Porrelli

May 04, 2011 | 2003 Yamaha YZ 250

1 Answer

I have a 2000 yz 125. Melted a hole in the piston but didnt score cylinder. Replaced top end. Wont start. Has good fire. Crank case oil was a little grey. New plug,fluids,fuel.


I would be checking compression it may not be visible score on cylinder but it does not take much to loose compression on small bore motor with one piston ring and 11 years old

Mar 03, 2011 | 2003 Yamaha YZ 125

1 Answer

I have a 1999 yz 125. i just put new piston and rings in it an they say after a fresh rebuild you do whats called the break in period. an what i am asking is what do i do for the break in? how do you do...


hi, break in period is comonly known as running the bike in, if you have only replaced the piston and rings, and not had the cylinder rebored, then take it easy for a couple of tanks of fuel while the rings bed in, when the cylinder is rebored we run in the machine as the clearences between the piston and cylinder are much tighter, heat seizure can occure when the break in period is ignored. hope this helps , paul

Jan 18, 2011 | Yamaha YZ 125 Motorcycles

1 Answer

How do i change me piston and rings on a yz125 2002


If you have never stripped down an engine get expert help .Piston rings are very fragile and can be snapped very easily .They also fit in there correct place on the piston by a small pin in the ring slot and you will neen a piston ring shimming assembly kit to keep the pistons in the correct position while you slide them into the cylinders .

Mar 17, 2010 | 2003 Yamaha YZ 125

1 Answer

Cant start my 2004 80 ws raptor


buddy sounds like your piston rings have wore or broken strip the barrel off and replace rings making sure you put oil on them when you install

Sep 08, 2009 | 2004 Yamaha YZ 250 F

1 Answer

87 yz 125t. bored .30'' over need 56.75mm Piston


yea, for most bikes they only come in .020 increments now. if your looking to freshen up the top end, then youll have to go to a 56.00mm piston. there will be almost no change in displacement or jetting from 55 to 56mm. if your replacing the piston, its best to have your cylinder bored anyways. since the worn cylinder walls may have adverse effects on the new piston rings, and they may wear a bit prematurely. a bore only costs about 30-40 bucks most places, and its cheap insurance. plus you will have the peace of mind knowing that your bore is matched specifically for your piston. I would definitely recommend a fresh bore.

Jul 29, 2009 | Yamaha YZ 125 Motorcycles

2 Answers

SEIZED PISTON


excessive piston clearance.really needed bore job with oversized piston,not just honed and rings.measure piston to cylinder clearance,check spcifications and verify

Apr 05, 2009 | 2003 Yamaha YZ 125

1 Answer

While riding my yz 125 it just died and won,t kick up again


  • If a hole in the piston then install new piston and rings. Next, clean the carbon off the head and install a new head gasket.
  • If cylinder wall is scared the piston has seized up. Pull the cylinder off and take it to the dealer. Have them bore the cylinder and return it along with a new oversize piston and rings.
  • While at the parts counter, get a new stock spark plug and a supply of Yamalube Two Stroke Motor Oil. You need to add 4 ounces per gallon when filling up. This will be a 32/1 gas to oil ratio. NEVER ride the bike without at least the needed 32/1 mix.
Please rate the solution as a "FixYa" if able. Thanks!

Mar 23, 2009 | 2003 Yamaha YZ 125

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