Question about Yamaha YZ 125 Motorcycles

3 Answers

I have a 1985 yz125 with a fresh top end and no compression

It is now bored 80/80 i bought new gaskets i had the cylender  honed i put it together exactly right and i cant get any compression out of it can somebody please help me out i'm clueless of what to do

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  • yz4life003 Jun 21, 2009

    my bike is a two stroke i dont have valves 

  • yz4life003 Jun 22, 2009

    how would i check the timing?

  • yz4life003 Jun 23, 2009

    i retarded and advanced my magneto and the compression didn't change at all, but there is a slight bit of air coming out of my magneto is that normal? 

  • yz4life003 Jun 28, 2009

    hello! is anyone there i really need help

  • yz4life003 Jun 29, 2009

    ok here is the deal my piston moves up and down smootly and yes i did put the rings on correctly.i dont have IM but you can e-mail me at yz4life003@yahoo.com thanks-matt

  • yz4life003 Jun 29, 2009

    to answer ur question yes i can turn the engine over 360 degrees without any resistance 

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  • Yamaha Master
  • 3,433 Answers

Your valve timing is incorrect

Posted on Jun 20, 2009

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  • Ned C Cook
    Ned C Cook Jun 21, 2009

    Ok, forget the valve part and fuel mixbut still ck the timing, :)

  • Ned C Cook
    Ned C Cook Jun 22, 2009

    piston at top dead center, your timing marks are on the magneto and the case. stating at about 10 defrees, spark plug out and plugged into plug wire and the plug grnded to eng. Rotate the magneto bring the piston up after backing up about an in.as you aproach TDC the spark plug will fire,spark at the plug) Look at your timing marks and see where there are aligned. That is you timing setting. Advance or ****** as desired by rotating the points plat behind the magneto.The magneto wheel may have to come off.. you will need to get timing and tune up specs.. Get those for me tomorrow and get back with me and I'll walk you through it. ok? _Ned.- Or for just ck'g. get the specs and follow the above

  • Ned C Cook
    Ned C Cook Jun 28, 2009

    Ok, I'm thinking on this for youWhy do you feel you aren't getting compression? Can you turn the engine over 360 dregrees without reasistance?

  • Ned C Cook
    Ned C Cook Jun 28, 2009

    Is there a way you can instant message so we can go through this a little smoother? I have yahoo instant messenger.

  • Ned C Cook
    Ned C Cook Jun 28, 2009

    Big question, no insult intended, Did you put the ring s on? Just making sure.

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Wrong size piston

Posted on May 07, 2013

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Check the crank seals.that is why the compresion is coming out by your magneto.

Posted on Jan 06, 2010

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1 Answer

What is the ring end gap for a small 3 horse Briggs and Stratton engine? should you hone a aluminum cylinder?


I'm adding to what has already been posted on this question.
You certainly can re-ring an aluminum cylinder B&S. A ball hone can be used to break the glaze on the cylinder wall. Install new B&S rings gapped at their recommendation or .0035 " per inch of cylinder bore.
Small engines are very resilient and will run in spite of numerous things being not up to spec such as bore taper (which is what you correct when you bore, in addition to removing scarring.)
If you are doing your own work, the cost will be fairly low providing you already have all the normal tools and are good with your hands. Rings, borrow a ball hone, gasket set.
As you get into the rebuild you will see why it is easier to buy a new engine.
It could be a fun project and a good learning experience.

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Low compression in 2 cylinders


Your head gasket is blown or motor has seized due too overheating or lack of oil

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Hi, Jarrett for this scenario you will need your service/owners manual if you can't find the first and best tool you ever bought for your Yamaha, despair not, for a mere $0 you can download another one. 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.
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I have oil in my throtle body 1993 toyota4runner 3.0L What could it be


IF YOU REPLACED THE HEADS WITH REBUILT/NEW HEADS:
A side effect of increasing compression when you bolt on fresh heads, is broken piston rings. It's beneficial to replace the rings & bearings and at least remove the ring ridge at the top of the cylinder wall if not re-bore when replacing heads due to the higher compression that will cause.


If you just put on a set of used heads.

There's also the chance that the new headgaskets didn't seal completely, (usually due to a problem with surface prep or a warped head/block), but if you've got high mileage, it's usually the rings.

A set of rings can be replaced, but you'll want to make sure they haven't cut into the bore so much as to cause blow-by once new rings are installed. Eliminating the ridge by carefully using a ridge reamer at the top of the cylinder to prevent breakage of the top ring from impact.
Hone the cylinder walls gently & reinstall pistons & new rings!
Good luck.

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Today my 1985 yamaha yz125 lost compression and cut off. i tore down the top end and everything was in great shape only had 19 hours on piston and rings, but the rings stuck because the power valve came...


Hi, Amynallen for this scenario you will need your service/owners manual if you can't find the first and best tool you ever bought for your Yamaha, despair not, for a mere $0 you can download another one. 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.
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The kick start on my blaster used to work now it just goes to the ground and you can use your hand to do it. are the idler gears bad or is the compression bad? and can a reed valve cause this


If you can turn your engine over by hand easily by pushing down the kick lever, and your not Arnold Schwarzenegger, You have a compression problem. Do a compression check. If your bike has under 100 PSI you need to disassemble the top end , bore or hone the cylinder (depending on condition) and install a new piston, rings, and gaskets. This should fix low compression. If the lever is going down but the motor is not turning over then you need to remove the side cover and check for broken kick gears. Reed valves have nothing to do with this problem.

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2 Answers

Not starting


Hi there
  • Low compression, and lots of blow-by forcing oil into the air filter via the the crankcase breather. Sounds like you need a top end rebuild. If you can get your hands on a compression tester, then check it. If it comes in under 120, then certainly time for a tear down and top end rebuild.

  • If you put a quick squirt of oil down the spark plug hole and if the compression pressure goes up on a second compression test, then rings/piston/bore are worn. If it does not come back up, then it may be leaky valves, or a blown head gasket.

  • do the compression test first. That will tell you where to start. Or, if you have the tools, tear the top end down and inspect. A motor re conditioner/machine shop will help to measure up and tell you if piston, and bore are ok. May just need new rings to freshen up.

Happy to answer any questions that you may have.
regards robotek

Apr 25, 2010 | 2005 Honda CRF 250 R

1 Answer

Bought a 2002 Rm125 about 2 months ago, with a fresh top end....First ride was great with impressive power throughout. About two weeks later I noticed that it was missing some about half throttle , and...


My first thought is low compression... which leads to a few questions...
What fuel to oil ratio have you used?
What do you mean by 'fresh top end'? To me a fresh top end includes piston and rings and cylinder bore, gaskets and crank seals. Anything less on a 2-stroke is an incomplete job and usually leads to premature failure.
Have you followed break-in procedure on your fresh top end? Double check the head bolt torque.

Do a compression check first then go from there.

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hi some had same size rings fit them with gaps 90 deg apart some had different width rings but they will only fit 1 way if you have stripped it put new rings in and run a cyl hone down the bores hope this helps merlin v12

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