Question about 2003 Yamaha YZ 125

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1997 yz 125 wont start, please help

I have a 1997 yz 125. the cylinder compression is between 115 and 120. plenty of gas, spark, and air, but wont start. it also backfires about every 8 to 10 kicks. I know the compression is a little low, but is it to low to make it fire?

Posted by tinner_2420 on

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Ernest Stalker

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2 stroke bike are very dependent on compression. and being a bike that is 12 years old my best educated guess is that the top end needs to be rebuilt. i usually have to have the top end done on my honda about every three years.

Posted on Jan 13, 2009

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Anonymous

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Check your reed cage assembly, if i back fires means the reeds arn't doing there job

Posted on Aug 18, 2009

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Anonymous

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Rebuild it, compression should be 155-160 at least. backfires and hard starting is a common problem with compression as low as yours.

Posted on Jun 02, 2009

Anonymous

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Have a look at your power valves my bikes power valve pins broke and the power valve went into the piston and smashed the piston and power valves

Posted on Apr 18, 2009

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No matter how new i think my plugs are in my 2 stroke bikes, i always replace them first in my troubleshooting sequence. never had much luck with NGK plugs. these would show a good spark (when grounded to the cylinder), but its just not enough for light it off. change the plug and see if that works.

also, if you try to start it and immediately pull the plugs, are they wet? if so, then gas mixture to the cylinder is good, just no spark to light it off.

good luck.

Posted on Jan 18, 2009

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Compression and spark but wont start


Hi, Anonymous are you getting fuel? 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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Hi, Anonymous and the usual suspects are:
1. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
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for kx125, that kind of problem i kind of remember my friend just like that too. so clean everything inside . so u have to changes the valves and also the lifter then do the piston pin too,.also chick the rod if in good! that's it for now then when u do that tell me what going on!

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KICK HARDER TRY PULL STARTING IT..IN SECOND GEAR..DOES IT HAVE ENOUGH COMPRESSION TO START?IT SHOULD START PULLING IT...START OFF WITH NO THROTTLE THAN IF NOT STARTING CRACK THE THROTTLE SO LIGHTLY AND SHE SHOULD FIRE OFF WHILE IN GEAR BEING PULLED OF COURSE BE CARE FUL NOT TO RUN INTO. WHO EVERS PULLING YOU..USE A LONG TOW ROPE..IF YOUR NOT A SEASONDED RIDER AND KNOW YOUR BIKE VERY WELL I WOULD NOT ATTEMPT THE PULL START...IF NOT CHECK YOUR COMPRESSION AND TIMING..MAJOR VACUUM LEAK MAYBE AT BASE GASKET..IS ENGINE SUCKING TO MUCH AIR? GOOD LUCK..

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1 Answer

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For your dirt bike to run you need 3 things:

1. Fuel (gas/air)
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3. Spark

Start trouble shooting through these. Make sure your gas is turned on, and that it flows through the carb; if you tilt the bike does fuel run through the overflow tube in the carb? Make sure you have good spark; take out the spark plug from the cylinder, still connected to the ignition wire touch the threaded end to somewhere metal on the engine and turn the bike over. You should see a spark. Finally, do you have good compression, how hard is it to kick over etc.

These are the first things to trouble shoot, I would look on youtube if you don't know what you're doing, there are plenty of helpful videos there. You can also try putting a thimble full of fuel into the cylinder and then kick it over to help it start if it has been sitting a while.

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1 Answer

I was riding my 2003 yz125 and it stalled since then i have not been able to kick start the bike i checked the plug and its getting plenty of spark i put a compression tester on it and its reading 150 psi...


Hi, David and the usual suspects are:
1. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
2. Failed alternator/generator and or voltage regulator.
3. Loose or corroded battery terminals and or cables especially the "NEGATIVE" cable, look for loose, corroded, or broken connectors inside the cable harness at "BOTH" ends.
4. Failed main circuit breaker or ignition switch, check for loose connections and continuity.
5. Failed system and or ignition relay, check for continuity.
6. Failed ignition coil, ignition/electronic module.
7. Failed CKP, CPS, CMP, MAP, TPS, or BAS sensor, corroded, loose or broken wire connector pins/sockets.
8. Throttle cables and or idle speed improperly adjusted hot idle speed should be 950 RPM to 1000 RPM.
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12. Restricted, blocked or kinked fuel line.
13. Fuel tank empty.
14. The gas cap is not venting properly or fuel tank venting system blocked.
15. A failed fuel pump, pressure regulator and or fuel injectors.
16. Vacuum line from intake manifold to petcock broken, cracked, or not attached, carburetor vent line plugged.
17. Needle and seat stuck closed in the float bowl.
For more information about your issue and valuable "FREE" downloads that you need please click on the links below. Good luck and have a nice day.
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OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

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Oct 24, 2012 | 2003 Yamaha YZ 125

1 Answer

My yz 125 made a loud popping noise when i tried to kick it up now it wont start any suggestions


They can do this from time to time. What can happen is the expansion chamber gets pumped with fuel vapor and then can be ignited, causing a large backfire. I hate to attempt to answer your question with a question, but how old is the piston and ring? A ring won't last very long in a 125. For racing, it's changed every race. For joy riding 15-20 hours of run time and it's time for a ring at least. Weak compression can make it impossible to start. If you have perfect compression the cause for the non-start could be something as simple as a burned up or fouled spark plug.

Start by re-ringing the motor (you can do this for about $25 inside of 2 hours time). Clean the carb out, clean the air filter, fill the tank with fresh fuel and replace the spark plug (NGK B9ES - from Autozone for $1.89). If the bottom end of the engine is good, the above steps should get it fired up - providing your ignition system is good, and based on the apparent backfire, I would say you have spark.

Scott

May 17, 2011 | 2003 Yamaha YZ 125

1 Answer

Loss of power, wont go over 40 mph, and takes a while getting there...


Check the "Basics" first. Clean Air Filter, Check the Spark Plug. When you kick it over, Is there resistance on the kick starter which means it has compression. Is it hard to start? When the motor gets tired, It will lose compression and be hard to start. I would start on the Air Filter and spark plug. Also, Does it have fresh gas in it?

Aug 10, 2010 | 2003 Yamaha YZ 125

1 Answer

I have a 1985 yamaha yz 125 wont start


By what you have written,I take it the motor still turns over.Take the spark plug out and see how much compression you have.If there is very little,it may have been the piston seizing.When they cool down,they can sometimes be moved again.This is the most likely cause of your troubles.If you have compression,take the s/plug out,put the bike in first gear and see if you can turnover the motor AND g/box.What you are looking for is a broken part in the g/box that locked up the driveline.Do this in each gear.Sometimes you can take the pipe off at the head and see into the combustion chamber through the exhaust port.Otherwise the head needs to come off.Hope this helps you

May 11, 2010 | 2003 Yamaha YZ 125

1 Answer

Loosing compression


Make sure...
re-check compression, what does it have
piston isn't in backwards
ring end didn't catch when putting on the cylinder

Dec 02, 2009 | 2003 Yamaha YZ 125

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