Question about Yamaha YZ 125 Motorcycles

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Ive got a 1986 yz125, have just rebuilt engine, runs great untill you open throttle wide open then it just dies out, like you have hit the kill switch. any ideas would be very welcome. thanks john.

Posted by Anonymous on

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  • Yamaha Master
  • 41,663 Answers

Hi, Anonymous and the usual suspects are:
1. Severely drained or faulty battery.
2. Faulty regulator/rectifier or high output coil.
3. Faulty power valve, solenoid or installation.
4. Clogged air filter or exhaust.
5. Cracked or incorrect float level.
6. Float needle is not seating.
7. Faulty reed valve.
For more information about your question and valuable "FREE" downloads that you will need please click on the links below. Good luck and have a nice day.
Dirt Rider Magazine
02 yz 125 stall at full throttle
Yamaha YZ125N1 Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

Posted on Jul 12, 2016

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Posted on Jan 02, 2017

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  • 3567 Answers

SOURCE: honda 1300c backfire through carb at wide open

Try balancing ans syncronising the carbs.
BALANCE THE CARBS
Turn the throttle screw on the LEFT carb one half turn inward. Now start the engine and get it warmed up. With the engine running, remove the RIGHT spark plug wire. Adjust the idle speed on the left carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Now turn the throttle screw on the RIGHT carb one half turn inward. Restart and rev the engine. Next, remove the LEFT spark plug wire. Adjust the idle speed on the right carb to the point where the engine just can't quite stay running and dies. Now put the loose spark plug wire back into place.

Turn the idle screw on each carb EXACTLY ONE QUARTER turn outward and restart the engine. The engine will be probably be idling very fast. Adjust both the idle screws equally from this point to get to the desired idle speed.

SYNCHRONIZE THE CARBS
Remove the air filter and rubber fittings to allow you to see into the throat of both carbs. You may need a mirror to see inside. You need to be able to see the slides go up and down when you twist the throttle. Now turn the fitting where the cable goes into each carb such that there is just a bit of slack in the cable, 1/16" is plenty. Lock ONE of the cable adjusters down tight. All further adjustments will be made on the other carb.

With the motor turned off twist the throttle very slowly while looking at the slides. Both slides need to begin lifting at the exact same moment. If the slides don't raise at the exact same time then slowly twist the throttle until the locked adjuster slide just barely starts to move. Hold the throttle still and turn the adjuster on the other carb so that the slide on that carb just barely starts to move also. Now recheck the slide movement timing. Do this process until the slides on both carbs begin to raise at the EXACT same moment. Lock down the loose adjuster and re-check the slides.

Please rate this solution. Thanks!

Posted on Mar 30, 2009

  • 286 Answers

SOURCE: motor bogs when you give it gas

It is a fuel problem, it can be filter or rubber membrane,they are placed in the valve for fuel.
lp

Posted on Apr 23, 2009

  • 3567 Answers

SOURCE: 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

Posted on Jun 16, 2009

  • 3567 Answers

SOURCE: my 2008 kfx450r wont stay running. when i finally

ALWAYS have a fire extinguisher on hand when working on carburetors.
Open the petcock, (gas valve), and drain 1/2 cup of gas from the tank. Did the fuel flow freely? Is there any water or trash in the cup?

Drain the carburetor. There should be a screw on the lower right side of the carb float bowl. Remove the screw then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. Install a new stock NGK spark plug and try to start the engine. 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. Check the float height. The fuel level in the bowl may be low. Bend the tang (in the center of the float) downward to raise the fuel level. EASY, a little bend goes along way.
Put the carb back together, clean the air filter and install the carb. Let the float bowl fill then start the engine.

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Posted on Jul 06, 2009

yeskimoe663
  • 224 Answers

SOURCE: 2006 Suzuki Boulevard C50 engine dies

Even though you probably don't have that many miles on the bike, or if you do have a lot riding time on it, it sounds to me like you need a valve adjustment. That's a high reving bike for that engine that works pretty hard so you might have 1 - 3 loose valves.

Posted on Dec 10, 2009

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I get it started it runs as long as you have it wide open but you let of the gas it dies


This might require adjustment of the hi speed carb screw, the lo speed, and the idling screw as well.
The engine needs to be good and warm, you need to be certain you have a fresh fuel mixture in the tank, and you should check the air filter because it too affects the way the saw will run at low speed and idle.

I would start with the hi speed adjustment screw since wi does start. Fire it up and run it a bit to make sure you have it warmed up.

You may need someone else to help you hold the saw while you make these adjustments. With the throttle wide open, turn the hi speed screw slowly, in increments of 1/4 turn, to the left. It may be just a bit off. The objective here is to turn one way until it sputters or runs a little rough. Then turn it the other way until the engine smooths out. Continue to turn in the same direction until it again runs rough. Then turn back about halfway between those two points.

It should run well at wide open throttle. Place your screwdriver in the low speed adjustment screw and turn to the left, in small 1/4 increments like you did before. The objective here is to adjust to the point where the saw will throttle up quickly and smoothly, and more and more from an idling position, or as close as you can get.

If you find you don't get that, and the ramp up is choppy and unsmooth, adjust in the other direction. Eventually you should get to a point where you have quick response to your throttle. You should get to a point where it runs without being wide open.

If it fires up easily and throttles up smoothly but won't idle, the you need to adjust the idling screw. Adjust it until the chain begins to move on the bar when you let the throttle go. Once you get to the point where it idles with the chain moving, back off the adjustment until the chain just stops moving.

Make sure the air cleaner is clean. You may have to clean the carb with spray carb cleaner while it is running at full throttle. If you do, spray a tiny bit at a time, as it will almost kill the engine.

Finally, after all this work, invest in a new spark plug, even if the one in it isn't that old.

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