2003 Yamaha YZ 125 - Page 5 - Answered Questions & Fixed issues

Lack of power is usually bad valves. Clean the air cleaner and do a compression check. Could you take a moment and rate my answer? Thanks!

Well, the first thing that I suspect is that the clutch plates were installed improperly or something else to do with the recent clutch change was overlooked (fluid contamination, forgotten seals, etc..).

There are usually marks on the tension bolt brackets. Adjust each side until they are even and the chain has about a 1/4 inch to a 1/2 inch of slack in either direction at the chains longest span. Tighten the axle bolt securely and take it for a ride. Thanks for checking out Fixya.com Scott. I hope this helps you. Love them dirt bikes.

Most likely water blew back into the carbi and or the pilot jet in the carbi is blocked either way I first of all would remove the carbi and air filter and clean thoroughly, compressed air clean every thing - flush with fuel or carbi cleaner everything - remove pilot jet and pay special attention to cleaning it and the hole it comes out of with compressed air
if you get it going don't forget to change your gear oil after first test ride as no doubt there will be water in it also from this
good luck mate regards Jamie

I do not know which country you are in but in Australia trail bike such as your YZ125 do not have plates, they have the frame numbers stamped onto the frame, usually at the steering neck.

It sounds like you have an air leak. It's possible the crank seals dried out or became ineffective during storage - more than likely the magneto side seal. You might need to do a leak down test to see if it's the crank seal. It's running way too lean if it's reving as you describe - this is not good for the piston and ring, so I would not run it until you get the issue nailed down as it is damaging the cylinder and crankshaft from lack of lubrication.

I would suggest checking your carburetor almost sounds like you arent getting enough fuel possibly a problem with the float since it is effecting the bike both at idle and rolling throttle. Btw did you change out the fuel filter in line from the tank ?

Is the throtle cable stuck? if not look at a good carb clean, it could have enough gas to start but be too lean that would make it rev up. Two stokes also do this right before they run out of gas

• Main Jet: 410
• Main Air Jet: 2.5
• Jet Needle: 6BFY43-74
• Needle Jet: S-1
• Pilot Air Jet 1: #40
• Pilot Outlet: 0.7
• Pilot Jet: 40

pistons or valves broken or stuck

There are many possibilities for this problem:---bad piston rings, clogged air filter, carb problems, wrong plugs.
First check the spark plug condition, when this problem occurs:-----1) If your spark plug is dry and burnt brown its normal.
2) If your spark plug is whitish it means its to lean.
3) If your spark plug it fouled with black oil over it that means your bike is running to rich. ------------- Try playing with the idle screw or air fuel mixture screw on the side of the carb.
The idle adjustments are easy...........

1.) Adjust your throttle cable free play.
2.) Start bike and warm up.
3.) Once warm, turn idle up a little high at the adjustment and then adjust low speed pilot screw to achieve the highest possible idle w/o turning any other screw.
4.) Adjust the idle screw down to an acceptable level. ------------ There should be the highest idle. That is where the pilot circuit is most efficient and gives the best off idle performance. ---------- click this link below for more help:--- http://technoanswers.blogspot.com/2011/05/spark-plugs-keeps-fouling-on-2003.html ------------- This should help.thanks.helpmech.

The best I can offer is the following website. There you can see an exploded view if the entire engine. www.babbittsonline.com

engine leak down is crucial on two stroke just as much as cylinder compression is engine compression fix the leak first and check for others - spray soapy water over to spot any leaks and kick over with plug disconnected you will see bubbles appear

change fuel mix either up or down on ration to find the best mix then play with fuel mix setting on carbi to fine tune you will be roosting flat out in no time
PS stick a new spark plug in each test with fuel mix just to eliminate fouled plug from the equasion

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

Two strokes just leak oil especially where the pipe meets the motor you could try new oring but seriously only be worried if you dont have oil leaking from exhaust on a two stroke is rule of thumb

Mix to 32:1 and jet to get the plug to burn chocolate brown...
Golden spectro is good
Maxima
Belray
Yamaha makes an oil that's not bad
Honda oil also is good
Klotz
(All of these are specific two stroke mix oil)

melting spark plug is too lean. exhaust gas high temps caused by too much air. your oil to fuel ratio is correct, but your air to fuel ratio is not. adjust your needle clip position to add more fuel to the carb by lowering the C clip. this will raise the needle, giving you more fuel for the same amount of air. also check air filter element for tears.

Run a B9ES if you don't have a problem with fouling. If you do have a problem fouling, run a B8ES. Both of these plugs are available at auto parts stores for a few bucks each.

The stock plug was a BR9** - the R indicates a resistor plug which is not needed.

Scott

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