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2003 yz450f running too lean

Have a 2003 yz450f running too lean, backfiring. have a 50 pilot jet and air/fuel screw almost all the way out. shouldn't be anymore than 3 turns out with a 45 pilot jet. took the carb apart and cleaned it. sprayed carb cleaner through the jet holes and replaced the intake boot/joint.

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First, try to avoid ethanol. It melts carb lines, and can goo up small passageways. Get some good gas and try again. Hi test if necessary. Check your specs, maybe the bike requires hi test.
Taking apart and cleaning involves soaking in fresh carb solvent to clean the small passageways. Is there an air cleaner? It may be clogged, test with it off.

Posted on Nov 22, 2013

Testimonial: "yes it shouldn't be anymore than two turns out. it only stops backfiring when the screw is about 4 turns out. I took the carb apart and sprayed carb cleaner through all the ports. I should only be running a 46 at the biggest with about two turns out but its not working out"

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2003 yz450f too lean backfiring on deacceleration


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clean the small air port in carb with adjustment screw removed.
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2003 yz450f popping on deacceleration too lean


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1 Answer

Yz450f running too lean


Hi, Anonymous 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.
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How To 4 Stroke MX Fuel Screw Adjustment YZF CRF KXF RMZ FCR Part 1 of 2
YAMAHA YZ450F Owner Service Manual
OEM parts for Yamaha
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Will not idle without choke


Hi, Anonymous and the usual suspects are:
1. Fuel cap or fuel tank is not venting properly.
2. Fuel filter clogged.
3. Fuel line pinched or kinked.
4. Float needle and seat sticking.
5. Float level too low.
6. Carburetor bowl vent line clogged/blocked/pinched.
7. Idle adjusting screw set too low.
8. Air/fuel mixture screw set too lean.
9. Idle port, transfer ports, slow air jet clogged.
10. Slow fuel jet clogged.
11. Faulty fuel pump.
For more information about your issue and valuable "Free" downloads that you will need please Click on the links below. Good luck and have a nice day.
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1 Answer

05 YZ450F wont idle just not sure how to adjust the idle?


Hi, Bryce and the usual suspects are:
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2. Air fuel mixture adjustment too lean.
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4. Faulty accelerator pump.
5. Float level too low.
6. Float bowl contaminated old fuel, water, dirt, rust, etc.
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Jun 25, 2012 | 2004 Yamaha YZ 450 F

1 Answer

Drained old gas put in new new now its poping from exhust


Crackling and backfire is nearly always lean on fuel condition...
open your air fuel mix screw tiny amount at a time and test to see if helping this should clear up the popping..
Please note if you need to open the air fuel mix screw more than 3 full turns from fully closed then the easiest solution is leave the air fuel screw at 2 turns out from closed and increase the size of your pilot jet by one or two sizes.. they very cheap to buy few dollars each.
I would suggest if your only popping and crackling a little then small adjustment on the air fuel mix screw will cure it for sure
Being YZ I find an occasional pop or backfire when de acelorating is exactly how they should be if you don't get a pop or crackle every now and again then chances are it running to rich.
but if it is regular occurance then definitely needs fine tuning or larger pilot jet installed
good luck regards Jamie

Sep 09, 2011 | 2003 Yamaha YZ 450 F

1 Answer

I was riding my 03 yz450f and it back fired and just died. then i tried to start it had a hard kick and had to use hotstart to start it once started ran 2 seconds and backfried and died. then tried...


hi mate backfire usually points to lean fuel condition but in your case being quite severe maybe blocked pilot jet. I would remove carbi and compress air clean it with pilot jet out and clean or replace pilot jet they only cost a few dollars...
I would also highly recommend a leak down test to double check your valves are sealing well and compression is all within range.
any Yamaha shop can do this test for you if you don't have the equipment, may be hundred or so but could save you heaps with accurate diagnosis in case something major is wrong??
regards Jamie

Jul 24, 2011 | 2003 Yamaha YZ 450 F

1 Answer

HughesWhat main jet for a 2009 yz450f with a factory 4.1 slip on and mid pipe at 85degrees and 500 to 1000 elevation


Hi, Bdunlap216 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.
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How To 4 Stroke MX Fuel Screw Adjustment YZF CRF KXF RMZ FCR Part 1 of 2
YAMAHA YZ450F Owner Service Manual
OEM parts for Yamaha
http://www.yamaha-motor.com/assets/service/manuals/2004/lit-11626-17-11_784.pdf

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1 Answer

Just bought a 2001 WR 426. Has an FMF pipe, no air box lid. Main is a #165 and pilot is a #42.these are stock jets according to shop manual. My elevation is 2500-3000 Ft. Any ideas on jetting?


Hard to give accurate info with out seeing the bike with jetting but I would suggest take a reading from the fuel air screw if the bike running better with the fuel air screw in carbi at 2.5 turns out or more then definitely increase pilot jet up 1 or 2 sizes if you get the bike to run well between one - two turns out on the fuel air screw that is where your jetting should be
I wouldn't be concerned with main jet at all - it should be fine for wide open throttle unless you feeling power loss when you open it right up
also if you are getting backfire or popping when de accelleration that also hints toward lean pilot so a size or two change in pilot jet would be worth trying

Jan 20, 2011 | 2001 Yamaha WR 426 F

1 Answer

2001 WR250F WILL NOT IDLE AND BACKFIRES ON DECEL. AND SPITS/SPUTTERS WHEN THROTTLE IS APPLIED WHEN UNDER LOAD


these symptoms usually point toward lean on fuel for some reason
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depending what altitude you are a bigger size pilot jet may assist this issue also but clean and replace original pilot jet and circuit first to confirm

Jan 02, 2011 | 2003 Yamaha WR 250 F

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