20 Most Recent 2004 Yamaha YZ 125 - Page 5 Questions & Answers

The A stamping is not an oversize, oversizing is in 10thou\0.25mm increments, so it wouldnt even fit in the barrel.or if it was undersize it would just rattle around.
But you need to check for proper clearance.
you can check for proper clearance, with a feeler gauge between the piston skirt and the barrel, it will be around 2 thou(check your manual), as long as the clearance is within tollerance it will be ok.
check it in a few places to make sure the barrel is not out of round or tapered, an inside micrometer would help, but the feeler gauge will be pretty good if youre carefull

You can flush it out with ordinary kerosene. No need to spend lots of money.

since you are rebuilding this engine ... you should hone the cylinder for the best fit for the new rings.

not very much. I suggest draining the old oil into a container and measuring it. I say about 120ml?

change ur battery and genorator

THis is a simple job for some people. The trick is that you need both hands to do this. (1) pump your brake till it gets stiff. (2) Loosen the bleader valve and let out the flouid. (3) tighten the valve and repeat a few times. Check the brake flouid from time to time.

The volume is less than about 1/2 quart - it's usually not measured going in since you fill the shock completely - there's no need to measure. Some guys assemble the entire shock, then fill with oil and cycle the shaft, then install the bladder cap last. My method is to install the shaft and seal head and the bladder, then put about 2 PSI in the bladder, then bleed through the compression cap. My method is a bit more complicated to explain, but it's effective at eliminating ALL the air from the oil and it keeps the bladder from being installed in a collapsed state.

As far as the nitrogen charge - 145PSI is stock. 130-160PSI is typical.

Higher octane fuel is needed from what you describe. Without knowing where you are located and how hard you ride the bike, it's impossible for anyone to offer any jetting advice. Higher octane fuel will likely help, but if you (or a previous owner perhaps) have shaved the head for higher compression, you might have to run higher octane racing fuel.

Also - check to make sure the pipe is sealed well at the head pipe connection - if not, it will **** air at high RPM and can cook your piston.

Check also for leaking crank seal on the flywheel side (left) which can also **** air but will normally be accompanied by a very irradic idle.

water in fuel...drain carb & tank...also drain reserve side too !! most forget the reserve side which is lower in the fuel tank, and water goes to the bottom...

Never heard of black smoke from a two stroke? It is a 125 YZ two stroke I persume. If you are getting black oily residue out of the end of the exhaust, you are too rich on your jetting, assuming the rings are good and the air filter is clean.

your carburetor might need cleaning. The low air needle might be clogged. Try this take the air filter off, get a carburetor cleaner, start the motorcycle and right there at the air manifold(where the filter u take out)spray it with the carb cleaner and rev it but not too much. Carefully use protection for your eyes. Might be lucky and u don't have to get out the carb to cleaned it

You'll need to make a visit to you're local bike shop and use fork oil specifically. Don't try cutting any corners here spend the extra cash and use fork oil.

volt ac or dc?? why did you put the top end on?? i have seen pick ups go bad and the mag. let my know about the top end. on the coil i can check the specs so you can check it. you can give me a call at the shop or i can send it to you hear.
C.C. Motorsports (361) 885-7787
242 westchester dr.
Corpus Christi TX 78408
Mark

a company called JDjetting.
They sell kits that seem to work well.

They make a puller for these, not horribly expensive, but it's the only way to remove it. Available at any motorcycle shop.

Time for carb cleaning.
There seems to be some kind of obstruction to the flow of gas to the float chamber.
Clean the carb and blow out the jets (main & primary) with compressed air, not forgetting to blow out the intake of gas where the float pin sits.You may thn need to reset the float to get correct fuel level in the float chamber.
Best would also be to install 'inline' gas filter, inbetween the petcock and the carburator.
Hope this helps!

Be sure the clutch lever is adjusted for 1/8" to1/4" free play at the outside end of the lever. If the oil is extremely dirty, the grit can cause the clutch to drag. Change the oil, ride for 30 minutes, then change the oil again to flush out the grit. The only other thing that normally messes up a clutch is using synthetic lubricants or oils with "Special" additives. They can swell the cork on the fiber friction plates causing the clutch to drag. Usually the plates need to be replaced when this happens. NEVER use synthetics in your bike. 10w40 motor oil is what should be in the engine. Please rate my answer. Thanks.

what is it that your doing??

Two things could be at fault here. First - it's a good possibility you would benefit from a new top end. Top ends go away fast in these because they are single ringers. Under hard riding, the compression starts to fall off after just 2 hours. Poor compression means lower combustion temperatures which will lead the bike to run rich.

The other possibility is that your jetting is off. If you have had this bike for a while and it's always run well then slowly started to act up, its likely poor compression, but if you recently got this bike and do not know the history of it, then it could be a simple jetting issue - not at all uncommon for 2-strokes.

Also - no need to run the BR8EG. This is a resistor plug and is a higher performance plug. If you are a top Pro rider, you could feel the difference but for 99% of riders they could not tell the difference between a BR8EG and a B8ES. The B8ES is readily available from auto parts stores (NGK stock #2411) for $1.89 instead of $12+ for a EG plug form a bike shop. This info alone could save you a bunch of money. Keep in mind the stock plug is BR9EG, so you are running one step hotter which is fine.

Also keep in mind there is a lot of information that I do not know, nor do I know the overall condition of the bike. Internet diagnosis is always tricky, but I thought I would point these things out. I hope this has helped, but if not - we need more detailed info to properly diagnose the problem.

The major reason for this problem is generally it is in need of a tune up. It sounds as if your timing might be a little out. Go over it a bit at a time, clean battery terminals, charged battery, clean and correctly set plugs, and then check the timing. Without it going this is going to be difficult, but you can have a look at the timing marks and make sure they line up. You really need a manual, there are none for your bike at the freebie site, but if you Google it and hunt around, you should find one, I have for every model I have searched for. You could try to start it by running it down a hill and if that fails, and your search fails, then it is time for the professional.