Question about 2003 Yamaha YZ 250

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Jetting on 1994 yz 250. the main jet is a 330 the pilot is a 40 and the needle is a 61. we mix the oil to 50:1 but it loads up onan idle and in the middle. we are at 5000' elevation.

Posted by Anonymous on

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  • Yamaha Master
  • 43,120 Answers

Hi, Anonymous it should be noted that your performance issues are not electrical ie. faulty charging system, battery condition, and connections etc. If your fuel system (gas tank, filters, fuel valve and carburetor) is contaminated with ethanol sludge, varnish, rust, dirt, water etc. or your bike has been sitting for months or years without running these components must be "PROPERLY" cleaned and reassembled "CORRECTLY" before any adjustments can be made. Tuning your carburetor is fairly simple once you understand the basic principals. You engine is a simple airbox sucking air in and blowing it out, it is finely tuned at the factory for maximum performance once you upset that delicate balance by changing air filters, camshafts or exhaust systems your performance may go down the and the engine may run poorly, you need to compensate the air-fuel mixture in the carburetor in order for the engine to run smoothly and at peak performance. If you are running multi carburetors you need to sync them first and make sure your air cleaner element is clean and dry for paper elements or lightly oiled for foam and meshed elements and properly installed. Here is how and where you compensate trouble: "TIP" if your engine "BOGS" your not getting enough fuel.
1. Closed to 1/8 throttle is managed by the air screw and pilot/slow jet.
2. 1/8 to 1/4 throttle is managed by the air-screw, pilot/slow jet, and throttle slide.
3. 1/4 to 1/2 throttle is managed by the throttle slide and jet needle.
4. 1/2 to 3/4 throttle is managed by the jet needle, needle jet, main jet, and air jet.
5. 3/4 to wide open throttle is managed by the main jet and air jet.
6. Wide open throttle is managed by the main jet.
If you are running lean, spark plug electrode color is white, engine runs hot and feels like it is starving for fuel you need to go up on the jet size or move the c-clip down one notch. If you are running rich, spark plug color is black or dark gray, engine runs cool, and bogs down when accelerating you need to go down on jet size or move the c-clip up one notch. When your carburetor is properly tuned for maximum performance your spark plug electrode will be a light tan color like coffee with cream. If you prefer fuel economy over performance you can go down on main jet sizes until a satisfactory level of lower performance is acceptable versus MPH, your spark plug color will be whiter and your engine will run warmer. These tuning adjustments will only make improvements if your intake and exhaust system have no air leaks or sealing issues and the entire electrical system is in proper working order and you have no mechanical issues. For more information about your issue and valuable "FREE" downloads that you will need please click on the websites below. Good luck and have nice a day.
Two stroke Carburetor jetting How to performance tune your two stroke carb...
YZ250 Forward Motion Test Ride Carb Issues and Carburetor Tuning Part 26...
YAMAHA YZ250 LC Owner Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

Posted on Jul 21, 2016

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6 Suggested Answers

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Posted on Jan 02, 2017

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  • 109 Answers

SOURCE: tune FZR 250

should run then we had a fzr 600r and did work to it so i know from experience..

Posted on Dec 05, 2008

  • 3567 Answers

SOURCE: yamaha yz 125 1994

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Can I get a “very helpful” rating on this answer? Thanks!

Posted on Jul 06, 2009

voodoomox
  • 1689 Answers

SOURCE: what is a good carb set up on a 2005 yz125 with

this will depend on what gas oil mixture you have

check this topic out.

http://www.dirtrider.net/forums3/showthread.php?threadid=117838

http://forums.dirtrider.com/70/7640379/yamaha-dirt-bikes/yz125-carb-settings/index.html

regards,

voodoo

Posted on Jul 08, 2009

ckoira57
  • 267 Answers

SOURCE: need spects for carburetor main jet

http://www.cmsnl.com/yamaha-yz125r-2003_model11245/partslist/A-12.html

Site above has all the parts and specs for your bike. Jet ranges are large for a YZ as many mod's are available and riding conditions contribute largely to a peaky 2 banger. Start with #40 pilot, and #420 main. Trial and error from there. Cheers.

Posted on Apr 27, 2010

  • 9 Answers

SOURCE: First I replaced the pipes on my 06 650 yamaha v

FIRST, YOU DIDN'T NEED TO REJET THE CARBS. I WOULD PUT THE STOCK JETS BACK IN IT WILL RUN JUST FINE. YOU NEED TO HAVE IT RUN ON AN EGA MACHINE TO TUNE IT PROPERLY.

Posted on Nov 10, 2010

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1 Answer

Need 3000 ft elevation jetting for 2002 yz250 with fmf SST pipe and fmf spark arrestor


Hey Anon,
Proper and correct jetting is something that can only be achieved in person by a REAL MECHANIC that KNOWS how to "read" a spark plug,,,With the mods you mentioned.. you need to know the stock jet sizes..a close guess would be reduce the main jet by two or three steps and lower the needle one clip position... oil mix should be at least 32:1....or better yet 28:1.. KEEP the oil mixture and jet the carb for performance....( I see TOO MANY IDIOTS that instead of jetting a carb correctly just reduce the oil ..some as low as 50:1(INSANE) just to keep from fouling plugs.).. then WONDER why their engine doesn't last long... hahaha????
YOU > BE SMART!!

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1 Answer

Problems with stalling at low speeds and not idling well


Hi, Jonathan and the usual suspects are:
1. Idle circuit and transfer ports clogged.
2. Air fuel mixture adjustment too lean.
3. Pilot jet clogged.
4. Faulty accelerator pump.
5. Float level too low.
6. Float bowl contaminated old fuel, water, dirt, rust, etc.
7. Float bowl vent or overflow tube clogged.
8. Fuel flow to carburetor restricted.
9. Intake manifold leak.
10. The gas cap is not venting properly or fuel tank venting system blocked.
11. Vacuum line from intake manifold to petcock broken, cracked, or not attached, carburetor vent line plugged.
12. Needle and seat stuck closed in the float bowl.
For more information about your issue and valuable "FREE" downloads that you will need please click on the links below. Good luck and have nice a day.
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals
OEM parts for Yamaha
2001 2013 Yamaha YZ 250 Service Manual YZ250 Moto Data Project
YZ250F terrible low end bog stutter Intake Jetting Fuel Injection
Dirt Rider Magazine

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1 Answer

What is the size of main jet of YZ125?


  • Main Jet: 410
  • Main Air Jet: 2.5
  • Jet Needle: 6BFY43-74
  • Needle Jet: S-1
  • Pilot Air Jet 1: #40
  • Pilot Outlet: 0.7
  • Pilot Jet: 40

Jun 05, 2011 | 2003 Yamaha YZ 125

2 Answers

How do you set the timing on a 01 yamaha yz 125 or why else would it be melting the spark plug prob off with a 32:1 fuel mix and the plug that goes in the bike from the factory


melting spark plug is too lean. exhaust gas high temps caused by too much air. your oil to fuel ratio is correct, but your air to fuel ratio is not. adjust your needle clip position to add more fuel to the carb by lowering the C clip. this will raise the needle, giving you more fuel for the same amount of air. also check air filter element for tears.

Nov 22, 2010 | 2003 Yamaha YZ 125

1 Answer

2001 yz 250 starts right up but as soon as the choke gets shut down the bike won't idle and die's out . However will idle nicely with choke on.i'm lost with this.


Sounds like the pilot jet is blocked, or stale fuel. drain the caby bowl and fill with fresh fule.if it still does it you have blocked jets,More so if the fuel has been sitting in the carby for more than 3 weeks...Remove the carby and remove the main and pilot jet, the emulsion tube under the main jet clean them out make shur you can see light through them. reassemble and refit, try again

Jul 01, 2010 | Motorcycles

1 Answer

Need spects for carburetor main jet on mikuni ucal and spects for pilot air jet 1 and pilot jet 2


http://www.cmsnl.com/yamaha-yz125r-2003_model11245/partslist/A-12.html

Site above has all the parts and specs for your bike. Jet ranges are large for a YZ as many mod's are available and riding conditions contribute largely to a peaky 2 banger. Start with #40 pilot, and #420 main. Trial and error from there. Cheers.

Apr 26, 2010 | 2003 Yamaha YZ 125

1 Answer

My spark plug is going black all the time


Your too rich with fuel. Two stroke? 32:1 is what you should mix to, then jet to make the plug run chocolate brown. Run it at an idle and read the plug, and adjust the idle or pilot jet to get a good color. Then run it 1/2 to 3/4 throttle for a good while, kill the engine and re-check the plug for the main jet. The needle clip adjustment is for between the pilot and main.

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Stock Jetting for a 1994YZ250 is:

350 main
45 pilot
J8-6EJ33-61 needle

Adjust your jetting down one size for each 1000' of elevation above sea level. Needle clip position on the needle fine tunes the pilot circuit.

Scott

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NGK brand spark plug BR8ECMVX is the stock plug for the bike. The proper gas / oil mix is 32 to 1. That means 4 ounces of 2 stroke oil per gallon of gas. The posting was not clear as to just what the problem is. If you are burning up plugs there may be a air leak in the intake system making the bike run lean. A lean fuel mix runs hot.

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