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?

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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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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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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

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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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1 Answer

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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1997 rm 125 won't start why should I do?


For your dirt bike to run you need 3 things:

1. Fuel (gas/air)
2. Compression
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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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.
9. Air/fuel mixture screw improperly adjusted.
10. Accelerator pump damaged or not working.
11. Water or dirt in the fuel system, carburetor or filter.
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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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

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

Yz 125 2001. starting ploblem after top end rebuild


Kinda sounds like your needle valve is sticking in your carb and it is giving it to much fuel

Jun 03, 2010 | 2003 Yamaha YZ 125

2 Answers

My yamaha yz 125


if your getting spark than i would take the carb off and clean it you could have a cloged jet.( just because your getting fuel from tank to carb dont mean its getting from carb to jug) i would try that first......yz-maniac.

May 14, 2010 | 2004 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

3 Answers

Yz 250 wont kickstart


Hi!
You obviously look after your machine and very sensible when laying up, however as you are probably aware 2 strokes rely heavily on compression, A. to produce excellent combustion and B. to draw fuel into the chamber. So when you try kick starting you can't get enough revolutions to draw the correct amount of fuel into the cylinder, where as you push start you constantly turn over the engine increasing compression slightly and bingo it fires.
Can you remember how when kick starting it before this problem the feel on resistance when kicking over? has it now become a lot easier? if so you possibly have worn or broken compression piston rings.
A compression test will reveal this, I hope this sorts your query?
Press the Blue button please for my FREE effort,
Thank You!
Paul 'W' U.K.

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