Question about 2003 Yamaha TW 125
I don't know how many turns the air/fuel mixture screw needs on a Yamaha TW 125
Hi, Revasns2001 it should be noted that the "AIR FUEL" mixture screw adjustment "ONLY" manages your idle and has no effect on any other circuit also any intake leaks must be repaired before the A/F adjustment procedure can be performed otherwise you will never obtain a proper idle and you will waste a lot of time chasing the impossible. The A/F mixture screw's purpose is to fine tune the fuel charge entering the combustion chamber. The following applies to both 2 and 4 stroke engines:
1. The mixture screw may be sealed at the factory with a Welch Plug please review the following video for removal.
2. The mixture screw manages a range of 3 complete 360-degree counterclockwise turns from the bottom/closed position.
3. The mixture screw should have a spring and o-ring for tension and sealing integrity.
4. Turn the mixture screw clockwise until it gently bottoms out, this makes the fuel charge very lean and the engine should not idle if it does then the pilot/idle jet is too big and needs to be replaced with the next size smaller.
5. Turn the mixture screw 1 and 1/2 turns counterclockwise to establish a baseline for starting the engine.
6. To fine-tune the idle circuit, adjust the mixture screw 1/4 turn in or out to achieve maximum idle RPM, wait 15 seconds between each adjustment for the idle to settle.
7. Never go past 3 full turns out this will make the fuel charge rich, foul plugs, and produce black smoke out of the exhaust, if the engine RPM keeps increasing past 3 turns the pilot/idle jet is too small and needs to be replaced with the next size larger.
7. After achieving maximum idle back out the mixture screw another 1/8 of a turn then adjust the throttle cable idle stop screw to 950-1050 RPM.
8. This procedure works great on 99% of all engines, for the 1% that demand a more robust throttle response on aftermarket monster fuel delivery systems additional tweaking outside the box may be necessary.
For more information about your issue and valuable "FREE" downloads that you will need please click on the blue links below. Good luck and have a wonderful day.
Life On 2 Wheels
CV carburetor tuning part 2 Mixture screw adjustment
Yamaha TW125 1999 Service Manual
OEM parts for Yamaha
YAMAHA TW125 Owner Manual
Posted on Jun 03, 2017
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Posted on Jan 02, 2017
SOURCE: yamaha tw 125 2
ALWAYS have a fire extinguisher at 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 screw on the lower 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. Remove the idle screw and the air screw on the outside throat of the carb and spray into the screw holes as well.
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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. If the problem is still there, the float height needs to be reset. If possible take the carb to a dealer and let them put a float gage to it. Probably cost $15 but call and ask before you take the carb in.
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Posted on May 06, 2009
SOURCE: yamaha tw 125
The noise you hear is valve clatter. You should not adjust your timing chain. Instead get a manual and find the vlave clearances for that bike. (Ex. The exaughst valve may be .006" and the intake valve may be .004") Then you'll need a feeler gauge, and to do this you'll remove the valve covers and get the engine to TDC (Top Dead Center) This is where the piston is at the very top of it's stroke and both valves are shut. (compression stroke) You must then loosen the rocker arm nut to the point that you can insert the feeler gauge between the rocker arm and the top of the valve run the nut down tight to the feeler gauge and then pull gauge out and repeat on the other valve.
Posted on Aug 24, 2009
They are listed in the service manual.
You can download a free one at
Posted on Nov 21, 2009
Normally that is the sound of a loose chain. You may need to tighten the chain. If you are close to the maximum adjustment you will need to remove a link or two. If unfamiliar with removing links and half links, it would be best to have a dealer do the chain work.
Posted on Feb 18, 2010
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