Question about 2003 Yamaha YZ 250

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04 yz250...goes straight to full throttle when started...trying to ops check it after i changed the main & pilot jet and adjusted the needle

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Did you put the jet in correctly ?? are you sure it aint upside down ?

Posted on Apr 02, 2011

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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.


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

Apr 19, 2014 | 2003 Yamaha YZ 250

1 Answer

I hve a yamaha xj750 1981, cleaned carbs, bigger jets(122) because of exhaust, starts very weel, pullaway good when cold, as soon as heatd up it stutters under 3000 revs, above 3000 it goes well but t


Check to make sure that you have no intake or exhaust leaks anywhere in your system. Once you have made sure the system is sealed you need to perform a plug chop and read your spark plugs to find out if you are running rich or lean in that particular throttle position and adjust the carburetor accordingly. Judging from the ramp range you stated, it sounds like your jet needle may be running rich. If the needle has different clip positions you may want to move the clip up (so the the needle sinks lower into the carb) to lean it out a bit on that circuit. You may also have to adjust the pilot screw as depending on the throttle position it may also be affecting this rev range. Another possibility is that you may have gone with a main jet that is too large. You stated that the new main jet is a 122 to compensate for an aftermarket exhaust. What was the original main jet size? Usually exhaust changes do not need much if any jetting changes. One to two sizes up is all you would need at the most.

Dec 10, 2012 | 1982 Yamaha Xj750 Xj 750 Y140 Maxim Air...

1 Answer

What should my carb settings be on a 98 yz 250,all pro-circuit exhaust,living in timmins ontario,im running 91 octane


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.
Some help with my yz250 carb settings please Yamaha 2 Stroke
Two stroke Carburetor jetting How to performance tune your two stroke carb...
YAMAHA YZ250 LC Owner Service Manual
OEM parts for Yamaha
http://mybikemanuals.com/yamaha/yamaha-yz-owners-manuals

Feb 21, 2012 | Yamaha YZ 250 Motorcycles

1 Answer

I have a 94 yz250 has plenty of power but is spitting oil and after about 20 min of riding goes dead but will crank back up right after. doesnt do it if I ajust the carb to run rich.


Spitting oil indicates that it is running rich. It could be down on compression which will cause it to run rich. The rings don't last forever in these bikes and need to be changed every once in a while to restore proper operation.

I am betting you are turning the air screw to "adjust" it to run rich as you stated. This screw only effects the lower RPM - it is actually a fine tuning screw to adjust the low end - specifically the pilot and needle jet. It has very limited effect on the jetting above maybe 1/2 throttle. The proper way to alter the way the carb acts is to physically change the jets in the carb. The pilot is for lower end performance, the needle is for the lower-mid range and the main is for mid to mostly upper RPM range. The float level also plays into all this.

To attempt to answer your problem - it could be that your spark plug is incapable of firing once it gets overheated. An overly rich mix will prevent it from doing it's job also and can burn a plug out. Try replacing it with a fresh plug and you might want to clean your air filter and check your compression. You should have at least 150PSI. I've seen bikes run with as little as 95PSI but they run poorly and are hard to start.

Good luck!

Oct 22, 2011 | 2003 Yamaha YZ 250

1 Answer

I have a 98 yz125, i race it but seem to have a lack in power in certain areas, ive changed my jetting a few times but seem to find the power else where and NOT where i need it, as of rite now im running a...


I would first make sure the power valve is decarboned and moves smoothly. Jetting is trial and error as so many things can affect the mix. Temp. humidity, gas quality, oil ratio etc. Tht being said you can make small changes and keeping a log with results. First write down baseline settings. Include main jet and pilot jet numbers found stamped on the jets and needle number. You don't say where it is you are trying to get power. 125s have poor bottom mid power, it is all on top for racing. Don't change and race or you may keep going when you should stop and melt something. Try to test at the altitude where you will race and read the plug often, cleaning or changing the plug between tests. Be alert for the surging feel of lean mix with racing idle do not let this continue. Remember 0 to 1/4 throttle, pilot and needle base width. 1/4 to 1/2 throttle controlled by needle mid taper. 1/2 to 3/4 throttle needle tip and main. 3/4 to full is the main jet. Find out the stock settings and make sure to start near there. I don't know about a spes pipe but pro circuit has charts for jetting and maybe a support line. I really feel you will get more from a clean powervalve, repack the silencer, clean airfilter and maybe v-force reed. Gearing can offer some options. I just read about a James Dean jet kit for cr250 that puts more power in the low mid. I might try that for my bike. Good luck, just remeber " too lean, too long- too bad"

Apr 06, 2011 | Yamaha YZ 125 Motorcycles

1 Answer

I have a primary jet and secondary jet in the carburator of the 500 gl silverwing . and the jets I have is 75 and 125 which goes where . and what should be the setting on the air jet ?


The secondary main jet (125) goes into the jet needle holder in the center of the carb (needle). The primary main jet (75) is between the secondary main jet and the pilot screw (this one is slightly off center) . The start setting for your air screw should be 1 3/4 turns. Warm up your engine and drive for ten minutes. Adjust your idle speed to 1100 + or - 100 rpm. Now turn pilot screw in or out to get highest idle speed. Readjust idle to 1100 rpm. Now turn pilot screw in till idle speed drops 100 rpm. If pilot screw seats go to next step. Adjust the pilot screw out 1 full turn from 100 rpm drop point. Now reset Idle to 1100 rpm. That is how Honda says to set your air screw. Standard setting is probably sea level and going through the procedure will set you up for the altitude where you ride. Do the adjustment during good weather at the altitude you plan to do the most riding.

Aug 27, 2010 | Honda CX 500 B Motorcycles

1 Answer

How do I clean the carborator on my 1994 kawasaki ninja zx600-c


Carburator Theory and Tuning
carb_jet_usage1a.jpg

For some reason everyone seems to think tuning a carb is just real easy. Change a jet or two and boom, your there. Yeah, right ! There are quite literally millions and millions of jet combinations. A rough check on Bing carbs shows there are at least 13,860,000 different combinations of jets. If you are going to change carbs you'd better be prepared to spend some time and money on the job.
venturi1a.jpgmainjet_1a.jpgIf you look at a carburetor, you will notice a rather large hole going from one side to the other. This is called a Venturi. Air passes into the engine through this hole (Venturi). As the velocity of the air entering the carb (and then the engine) increases, it's pressure decreases, creating a low pressure or vacuum in the venturi. This vacuum moves around in the venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the venturi. The way the jets are made causes the fuel to vaporize as it goes into the venturi. Where the jets are placed in the carb and where the jet's outlet is located in the venturi, determines what part of the throttle opening that jet controls. The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening. The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%. This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet. As you can see, the different jets over lap the operating range of each other. That is, the jet needle starts to effect things before the effect of the idle jet ends. This is something to remember when working on carbs... everything is interconnected. Change one thing and it will effect other things.
OK, let's go over the different systems in the carb and see what they do.
  1. Fuel level. The fuel level is controlled by the fuel floats and the fuel float valve. The floats are hollow or made of something that will float on gasoline, such as cork. Part of the float presses against the float valve, sometimes called a needle and seat. Most times the part of the float that touches the float valve needle is bendable so you can adjust the level of the fuel in the floatbowel. All plastic floats are not adjustable. If this level is way too high, gas can leak out the carb overflow tube or into the engine. If fuel gets into the engine it will thin out the engine oil, ruining it's ability to lubricate. This will, sooner or later, blow up your engine ! If a full tank of gas in the evening turns into a half tank by morning, check your oil. If it's thin and smells like gas, change it and replace your float valve and/or check your fuel level. If the oil is OK, check under the overflow tube. If it's OK, then check where you are parking your bike 'cuse someone is walking away with your gas ! If your fuel level is just a bit high, the mixture will tend to be a bit rich. If it's low, the mixture will tend to be a bit lean. This is because a high level takes less vacuum to **** fuel into the engine and a low level takes more vacuum to do the same.


  2. Pilot or idle jet system. The idle jet controls the idle and on up to quarter throttle, give or take a bit. On some carbs, like Mikuni there is an air jet too. In conjunction with the idle jet there is an idle jet air screw. This screw leans or richens the fuel mixture for a smooth idle and on up to one quarter throttle. From the idle jet, there are little passages cast into the carb that lead to holes just in front of the throttle valve or plate. There can be just one hole or there can be several, depending on the carb design. They effect the mixture as long as the vacuum, in the venturi, is over them. As the throttle opens further, the vacuum moves to the needle jet and jet needle.
  3. The Throttle Valve. The big slide that opens and closes your throttle has a bevel angle cut in one side of the big round (can be flat, too) slide, toward the air cleaner. This angle comes in several sizes and helps control the fuel mixture from idle to about 35% open throttle.
  4. Needle Jet. This jet doesn't really even look like a jet, but it is ! It controls the fuel mixture from 15% to 60% open throttle. It sets in the center of the carb, right over the main jet.
  5. Jet Needle. This is the needle that rides in the throttle slide and goes into the needle jet. This needle controls the fuel mixture from 20% to 80% open throttle. It can come in many different sized tapers. Sometimes, one needle can have several tapers on it. The top end of the needle has grooves cut in it, usually five, and you can move the little clip on the end up or down to lean (down) or richen (up) the mixture. Most late model bikes have needles with only one groove cut in them. This is so you can't richen the mixture, thereby keeping the EPA happy.
  6. Main Jet. This jet controls the fuel mixture from 60% to 100% open throttle.

Apr 16, 2010 | 1994 kawasaki ZZR 600

1 Answer

What should the stock carb jets be for a 94 yz250 with the stock pipe and aftermarket silencer


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

Dec 12, 2009 | 2003 Yamaha YZ 250

1 Answer

Kx 85 04,bike had a slight bog when throttle was gassed then i cleaned the carb,now it really bogs and running very rich.i also messed with the air & idle screws hoping i could fix the problem &...


Start with setting the air and throttle screws. Turn each one inward until it GENTLY seats. Back each one out one and one half turns. Clean the air filter and starter plunger. The plunger may need to be rebuilt or replaced. Check the pilot and main jet sizes. There will be a number stamped on the side of the jets. The pilot jet should be #45, the main jet should be #140.

Aug 01, 2009 | 2004 kawasaki KX 125

1 Answer

Re-jet the bike


than it was the day before you can actually go back to your sea level jetting a 260 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference! Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny sometimes pinging and running HOT. What happened?!?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually leaned out our mixture! A 1 to 2 sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and grey hair in the long run! PILOTS NEEDLES, MAINS: So far we have only talked about main jet changes to compensate for altitude, temperature and fuel density. As most of you know there is a pile of jets in a carb. While main jets are the most critical for ensuring full power operation and engine longevity, the other jets are equally as important for a good running engine. Let's run through them quickly. Pilot jets control the low-speed and idle mixtures. Many times an adjustable jet is used in conjunction with the pilot jet. The adjustable jet allows a precise setting of the idle mixture. If the adjustable jet is located to the rear of the carb and usually on one side it is a AIR adjustment. It controls the amount of air that mixes with the fuel coming from the pilot jet. If the adjustable jet is to the front of the carb, on the side or bottom, it controls the amount of air/fuel mixture going into the engine. In either case if adjusting the mixture screw won't improve the low-end running speed it's time for a different pilot jet. Throttle valves (the slide) control the off idle, to 1-quarter open ,mixture. Some aftermarket carbs have replacement slides available with different cutaways. Changing the cutaway changes the mixture. More cutaway is lean, less cutaway is rich. Some carbs do not have different slides available, so you have to compensate by changing the mixture on the idle circuit or needle circuit. Partial throttle hesitation or rough running can be caused by the slide cutaway. Needle jets control the amount of fuel going by the needle and into the engine at low to mid throttle. There are 2 types of needle jets used in a carb. One is a primary type that has a very precise hole hole drilled through the middle of it, along it's length. The size of the hole relative to the size of the needle determines how much fuel goes into the engine. The other type of needle jet is constructed essentially the same except for a bunch of holes drilled into the side of the jet. These holes allow air to mix with the fuel before it's metered into the engine. Either type of needle jet works well in most cases but there is power to be gained on HIGH PERFORMANCE 4-STROKES by going to the needle with the holes in the side. These are called bleed type needle jets and produce more midrange power in a 4stroke. In any engine going to a leaner (smaller) needle jet is the easiest way to rejet the midrange running when going to higher elevations. Changing the needle jet leans out the mixture evenly at all the midrange throttle settings moving the needle clip doesn't. Jet needles more commonly know as the needle control the fuel mixture throughout the midrange. The shape or taper of the needle dictates how much fuel goes into the engine at a given throttle opening. The needle must work in conjunction with the fuelling requirements of the engine relative to slide position. If you have an engine with a strong hit in the midrange the needle will probable have a noticeable reduction in size the the slide is half open. Remember it takes fuel to make power and when the engine makes power it needs fuel NOW! If it doesn't get the right amount of fuel it pings or misses. You many have cleared up a little midrange pinging by moving the needle up a notch but at the same time you may have overrichened some other areas. If the problem isn't too bad you won't even notice the rich condition. If the machine stutters before it comes on the power that part of the needle's taper is too small and the only way to cure it is to get a needle with a different taper. Finding the right needle can be difficult so hopefully moving the clip will do the job. Finally the good old mainjet comes into play at 3-quarters open to full throttle conditions. Most of you already know a bigger mainjet has a bigger hole so it lets more gas into the engine! Pretty simple!! As simple as it is the mainjet is absolutely CRITICAL to high-speed engine operation. Not only does it meter the gas into the engine, it can aid in cooling the engine as well. A properly sized mainjet will let the engine make good power for a long time. A one size smaller mainjet may make greater power for a while. A slightly rich mixture burns cooler than a lean one so be sure the mainjet is big enough! One final note on jets. All of them and the carburetion functions then perform tend to overlap into some other jet's territory. If you mess with one jet, you may have to mess with a few of the others. My best advice is to not change more than one jet at a time. Slowly work out the correct jetting and keep notes on what you are doing. If you get totally fouled up at least you can go back to where you started. SIGN, SYMPTOMS AND CAUSES: How would you know if there was something wrong with your jetting? If you listen, your engine will tell you! All you need is an interpreter. Since I speak and understand several different engine dialects, I will give you a hand. Let's start with lean conditions because they can cause the most damage. In a lean condition the engine will surge and sometimes ping under acceleration. The engine will also be cold-blooded (hard to start and keep running) but will run better when hot. The spark plug will look bone white or burned in extreme cases. The engine may spit back or sneeze thought the carb once in awhile too. If the engine is running rich the throttle response will be fuzzy and not too quick. The engine will burble, miss and blow black smoke. It will start easy but will run funny when fully warmed up. The plug will be dark, wet or fouled (possible all 3!). Ok so what do you do first to cure the problem? The very first thing is to check and adjust the float level. If it's off one way or another it can throw the jetting off too. Set the float to the specs and retest the running. The next item is to determine a rich or lean condition. Let's say the engine gets hot and doesn't pull well. This is a lean condition so the engine wants more fuel. Stick in at least a two size bigger main jet and try it again. If it's better but still not right go even bigger on the jet. and try it again. Bear in mind that drastic or sudden changes in jetting usually mean an air leak has developed somewhere in the engine. Find it and FIX IT!! When the engine burbles on the top end come down 1 jet size at a time until it winds all the way down. Don't drop and more sizes! If the engine seems sluggish and lumpy or want to load up on the bottom end the mixture is TOO RICH. Adjusting the low speed mixture screw helps a little but doesn't cure the problem completely. What you need now is a new pilot jet. Go 1 size smaller and try the adjustment again. When the engine runs smooth with the adjustment screw about 1 and a half turns out from the seat you have it!! IS THAT ALL THERE IS TO JETTING??: There's a lot more to jetting than just stuffing jets in holes and hoping the problem goes away. If you can understand what your engine is trying to tell you when it runs funny you will have a better chance or correcting the problem than someone who doesn't have a clue. When you rejet, go slowly and carefully until the problem is solved. As a final thought let me remind you that jetting is a lot like life, if you have a choice it's always better to be a little rich!!,,,

Nov 10, 2008 | 2003 CCM 604 DS Dual Sport

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