20 Most Recent 2004 Yamaha YZ 125 - Page 6 Questions & Answers

Change plug then use a small amount of starting fluid then kick it if it fires then dies your carb needs to be cleaned pull float bowl pull both small brass jet screws and clean they must be clear to see through when put up to a light reassemble and you should be good

It is normal for the powervalves to leak, but it does depend on where it is leaking from.

The hose on the left side is the power valve chamber vent - it is normal for it to leak thick black oil from this hose.

If the leak is on the right side, and the oil is transmission fluid, then the seal on the actuator shaft needs to be replaced.

Excessive oil from the powervalve hose is a possible sign that the compression is low. Low compression equals lower exhaust temperatures and this can lead to incomplete combustion.

Head Nuts: 20 ft/lb
Cylinder Nuts: 22 ft/lb

I believe you can get a copy of the complete manual from the Yamaha website:
http://www.yamahaownershandbook.com.au/lib/download.php?f=5MV-28199-30.pdf

A compression test will tell you your condition. (Throttle wide open while doing that) If that's good, your jetting is too rich, could be the issue.

Coil? how about the battery?

The pressure is 145 PSI from the factory. Hopefully you know to use Nitrogen only.

The manafacturer of the piston you are going to use has a specefic piston to clyinder fit. Use the service manual to determine the sizeing needed.have it measured and honed by a machine shop.

Heat the bike up, give it a ride for at least 15-20 minutes. in idle, try adjusting the little black knob attached to your carburetor. This will adjust your air/fuel mixture. Adjust it little bits at a time, and then wait for a second before adjusting further (to allow the mixture to change). Pay attention the idle, and how the bike sounds. Adjusting that carburetor should help.

for more info on 2t carbs

http://www.freeengineinfo.com/two-stroke-carb-tuning.htm

http://tech.flygsw.org/walbro_tuneup.htm

The std bore is 54mm and some available piston sizes are (Vertex Pistons)

53.93
53.94
53.95
53.96
53.97

Oil leaking from your powervalve hose could be caused by a few things mate. First thing to check is your air filter.A blocked(due to dirt)air filter is the most common cause of a rich running 2 stroke. If you have cleaned it recently, be sure you haven't over oiled it. if your not sure, after applying the oil wrap it in a clean cloth & squeeze it (don't wring it like you would clothing) this will remove any excess oil. I recomend a proper air filter oil(from a motorcycle shop) but don't recomend the "no toil brand" of airfilter oil unless you clean it out after every race( if you race your bike),grease the seal & replace the filter in the bike.If you don't have propper air filter oil you need to use a thick oil otherwise the oil will sink to the bottom of the filter.
Next suggestion is your oil fuel ratio. If you are using one of the ultra expensive synthetic or semi-synthetic oils try leaning its ratio to 40:1. Be sure to very regularly check your sparkplug colour after doing so. It should be a tan colour & not wet,black & oily.Please be very vigelant with you plug checking as if you go too lean a seizure can occur so only try this if the other checks prove ok.
Another cause could be that the powervalve seal(behind the cover) has popped out letting the oil/fuel out. Replace/reposition the seal & this should help.
Another suggestion is to check your gearbox oil. Is it dissapearing yet you can't find a leak? This symptom indicates you have a blown crank seal. If so it is allowing gearbox oil to enter the crankcase & is richening your oil fuel ratio due to this fault.
Finally it could be running just too rich ie delivering too much fuel/oil mixture. This is commonly expressed by the symptom of a very wet & oily muffler.To rectify this you need to reduce the size of the main jet in the carburettor. After diong so you must carry out regular plug checks like you did for adjusting the oil/fuel ratio.Care again must be taken as to not seize the engine.
Hope this helps regards
Andrew Porrelli
PS look for my tips on 2 stroke mx'ers comming in the next few weeks

See if you loose spark when it shuts off. If you have good compression (Do a compression test) then you need to verify which you are loosing, fuel or spark.
Also, what color is the spark plug, should be chocolate brown.

The parts are available. Go to the site below where you can see a parts diagram for your specific bike. You will select the actual brand, year, model, etc., once you go to the site. Part numbers and prices are also shown. You can order parts from this site. In the event no price is shown on a particular part and/or the notation "Not Available" is in the description, the part is not in stock. www.babbittsonline.com/pages/parts/viewbybrandand/parts.aspx

The springs help to hold the exhaust firmly to the cylinder and prevent any rattling noises.

Try adjusting the little black knob attached to your carburetor. This will adjust your air/fuel mixture. Adjust it little bits at a time, and then wait for a second before adjusting further (to allow the mixture to change). Pay attention the idle, and how the bike sounds. Adjusting that carburetor should help.

A few things to check:

Clean air filter?
Fresh fuel?
Replace the spark plug - they do burn out.

In addition to these items, it's likely that your compression is not up to the task - the 125 has a single ring on the piston and they simply don't last. In fact, racers will re-ring after just a few hours or less of riding. Your bike might benefit from a fresh ring.

Also - check to make sure your exhaust pipe is clear - if it has been stored it is possible that mice have nested inside. I know it sounds odd, but I have seen it more than once, so I thought I'd mention it.

Other than these things, we would need more info to troubleshoot further. I have been racing these bikes since 1981, so I am sure we can get you going again.

only reason would be your running too lean. which causes more heat, which causes pre-gnition and pinging. Also if timing is off you create more pressure which in turn causes more heat

The only way to remove a crankshaft bearing is to split the center case. Don't replace just one bearing, replace both while you are in there. Be sure to install new crank seals as well.

fill it until gear oil comes out the fill hole. Not sure of the exact amount, but I am guessing around 1 pint. Pour slowly because the opening isn't very big because of the ring gear being near the fill hole.

lack off fuel buddy on carb will be the mixture screw turn this out or just fiddle with it also run a b8eg plug in the engine

buddy is that the cdi unit the stator plate and the magneto ???

if you mean not to start they could. a reed valve is simply a one way valve that captures the air fuel mixture in a two stroke motor forcing to go into the combustion chamber rather than back out the carb