I have a 2005 yz125 and the bike runs well in low to mid power but when I open the throttle there is no power. It feels like the gas cuts off and the bike **** down. What do I need to do to clean the carb? Do I just pull it off and spray carb cleaner? Do I need to double check anything else? Thanks in advance for the help...
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Re: how to clean carb on yz125?
ALWAYS have a fire extinguisher on hand when working on carburetors. Remove the water trap bowl at the bottom of the petcock, (gas valve). Is there any water or trash in the bowl? Drain a cup of gas from the tank. Is there any water or trash in the cup? Dump it, clean it and re-mount it, ( not all bikes have a water trap bowl ). Drain the carburetor. There should be a large plug on the bottom of the carb float bowl. Remove the plug then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. If the bike doesn't start and run properly then shut off the gas and remove the carburetor from the engine.
Remove the float bowl and clean the entire carb with a spray carb cleaner from the auto parts store. Wear protective goggles to avoid getting spray in your eyes. Spray into all the little airways and fittings in the carb. Remove the idle screw and the air screw on the outside throat of the carb and spray into the screw holes as well. < < READ CLOSELY > > Be sure to put these two screws back in the same hole they came out of. IMPORTANT > do not tighten these two adjusters down. Only screw these in until they LIGHTLY seat. Now turn each adjuster one and one half turns outward. Put the rest of the carb back together, clean the air filter and install the carb. Let the float bowl fill then start the engine. This process should get you back on the road. Post a comment to get back to me if it doesn't. A “very helpful” rating for this answer?Thanks
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Most likely your slow (idle, pilot, whichever name you prefer) jets are plugged up in your carburetors. If the bike was stored improperly, or old/dirty gas was in the tank, these jets get plugged up pretty easily. One if not all are probably affected. The slow jet is where your bike pulls gas while it has a closed throttle position. Its the smallest jet in the carbs. When you twist the throttle, the needle (blocking the main jet from being used) opens up and allows gas to pass it and up into the vacuum of your carb and into your engine. If the idle jets are plugged you will usually notice that if you give the bike throttle it will run great in the higher RPM (3000+) but will die and be "glitchy" or boggy when going from no throttle to open throttle. You can try to cheat a carb clean by getting some carb cleaner and spraying that down the carbs, and using seafoam or equivalent to get those jets clean, but its always a better bet to see whats going on inside and clean those things so you get your throttle response back. Goodluck.
your carb: of course your carb has an idle and a mixture screw built-in. the idle is the knurled (usually gold) screw with the spring on it. and the mixture screw is the small one (usually gold too) set into the carb body. you'll know it because it's not hoding anything on. just sitting in the body. turn it only 1/4 turn at a time. your slide-needle should be mid to down if anything. c-clip up and needle down, that is. you want it to run as lean as possible w/o doing damage of course. almost all amateur's bikes run WAY too rich, all the time. if you ever look at the starting line of a pro race compared to a local one, you'll be amazed. there's no smoke coming from a pro's bike. let your bike run a little hotter and cleaner and change the piston and rings more often. you and your bike will be happier couple.
a good way to test how your bike is running is to get it hot then run it wide open in fourth gear and hit the kill switch (if it still has one) while you still have it pegged. take the plug out and check the color. it should be light tan to pretty clean. any darker and your losing power. (don't use a new plug when you do this). are you getting a good spark? use the best plug you can get for your bike too. if your carb has adjustment for low, mid, and high end, then you should do it three times. at those three throttle speeds. ATAC: are you sure you're going to remove the ATAC-valve system? if not then let me know. there are things you should know and do to make sure it's working right. it works very well if maintained and adjusted correctly.
as far as how it runs...it should absolutely rip. i was riding a cr 250 back then, but even a 30-year old 125 should scare you if it's running right. of course the piston and rings sounds like your major issue, but make sure it's breathing and flowing clean from airbox to silencer. are you replacing the silencer along woth the pipe?
Boyessen reeds are still your best bet. make sure your not sucking any air in through your intake manifold too. if it's old and cracked, then that could be why it was running too hot. of course make sure the surfaces on the manifold and the cylinder are in good shape and that no air can get sucked-in between them. a medium-depth scratch on the length of the seal surface can be bad.
i switched from kx's to yz's to cr's through the 80's and i loved the hondas. even managed a 4N (4th in the nation) plate on the cr. have fun and go fast! (don't ride over-your-head though) :) i know every screw, spoke, and gasket of your bike and how it works like the back of my hand. if you have any other questions,
plz feel free. i have lots of little factory-mechanic tips to help make your bike faster always stays in perfect shape, if you're interested.
Can you confirm if this has a 28mm Dell'Orto carb ?
I would think that you might be possibly running too rich in mid range between ¼ and ¾ throttle opening in which you could try to drop your needle (raise the clip)by two notches,and if this helps but not eliminates the problem,maybe a slightly smaller main jet.
You need to fix the fueling first - without fuel it won't run!
Your other problem sounds like an air leak - either the manifold or the carb. Check by spraying WD-40 or CRC at the manifold and carb join while the bike is running - if the revs change (either up or down) you have a leak.
Try the air filter first (make sure it's clean). The needle valve on your carb slider could be bent, or not seated correctly or just set wrong. If you have a power exhaust valve, it could be set wrong or malfunctioning as well. Good luck.