Question about Yamaha YZ 450 F Motorcycles

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I have an 04 yz450f and have a pretty bad backfire. it is mainly when I let of the throttle. I have rebuilt the top end and cleaned my carb. I have owned the bike since it was new and always lived at the same elevation. I have masked the problem with jetting and by adjusting the fuel screw but can't get rid of it. what can it be?

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Give the exhaust system a thorough inspection...leaks especially close to engine where the exhaust is hottest leak out under load...but can draw air in under deceleration....any unburnt fuel will ignite with the fresh air and high temperature.

Posted on Mar 16, 2011


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2003 yz450f running too lean

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.

Nov 22, 2013 | Motorcycles

1 Answer

2003 yz450f too lean backfiring on deacceleration

clockwise is rich & cc is lean which way are you adjusting?
clean the small air port in carb with adjustment screw removed.
Screw should be set 1 1/2 turns fron light seated position.

Nov 21, 2013 | Yamaha 2003 Yz450f Yz450 Yz Wr 450 Stator...

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Spits and sputters

it\'s your TPS! well Greg first clean intake with toothbrush and carb cleaner! when it gets dirty the throttle position sensor doesn\'t know where it it so it will act like that, so it\'s a good idea to clean it first.

Mar 25, 2013 | 2004 Dodge Intrepid

1 Answer

Had carbs rebuilt and suppose synced but bike starts hard and when running backfires whin you let off the throttle

1. choke circuits still plugged up or carbs not cleaned correctly; most people that attempt to clean their own carbs do not have the proper equipment to do it with. Most carbs have really small orifice(jets) and require really small jet drills/orifice cleaners to clear them out. Cleaning them in a caustic soak tank will clean them out but the carbs must be taken completely apart or it will damage any gaskets or rubbers parts left in it. Carb cleaning should be left to professionals unless one has experience doing it.

2. Open/broken vacuum lines can make it hard to start and backfire when letting off the throttle; jets that are not cleaned out correctly can also make it backfire.

3. If the backside of the inlet grommets are not sealed at the flange and have a vacuum leak, that will cause hard starting and backfire also; this applies to carbs with a flange style grommet. If the intake grommets are cracked or leaking it too will cause hard starting and backfire. Remove the carbs, wipe the head and mating flange surfaces with laquer thinner, let it dry and then reseal the grommets back to the head with rtv silicone rubber. Make sure to wipe any excess rtv(before it drys) from down inside the port before putting the carbs back on.

Feb 20, 2013 | Honda F6 C Valkyrie Motorcycles

1 Answer

I have a 2008 yz450f , I was practicing at Glenhelen sat . After 2 practices without any problems. 3 laps into the 3rd. Moto it backfired and died. The motor.kicks over but wount fire. Carb. Is getting...

Hi, Ssfichter1 and the usual suspects are:
1. Severely discharged or a damaged battery, should have 12.5 volts or better and be able to pass a load test if necessary.
2. Failed alternator/generator and or voltage regulator.
3. Loose or corroded battery terminals and or cables especially the "NEGATIVE" cable, look for loose, corroded, or broken connectors inside the cable harness at "BOTH" ends.
4. Failed main circuit breaker or ignition switch, check for loose connections and continuity.
5. Failed system and or ignition relay, check for continuity.
6. Failed ignition coil, ignition/electronic module.
7. Failed CKP, CPS, CMP, MAP, TPS, or BAS sensor, corroded, loose or broken wire connector pins/sockets.
8. Throttle cables and or idle speed improperly adjusted hot idle speed should be 950 RPM to 1000 RPM.
9. Air/fuel mixture screw improperly adjusted.
10. Accelerator pump damaged or not working.
11. Water or dirt in the fuel system, carburetor or filter.
12. Restricted, blocked or kinked fuel line.
13. Fuel tank empty.
14. The gas cap is not venting properly or fuel tank venting system blocked.
15. A failed fuel pump, pressure regulator and or fuel injectors.
16. Vacuum line from intake manifold to petcock broken, cracked, or not attached, carburetor vent line plugged.
17. Needle and seat stuck closed in the float bowl.
For more information about your issue and valuable "FREE" downloads that you need please click on the links below. Good luck and have a nice day.
2006 YZ450f backfires then dies YZ 400 426 450
2010 YZ450f starts but soon stops bike stalls FOUND ANSWER
YAMAHA YZ450F Owner Service Manual
OEM parts for Yamaha

Feb 26, 2012 | 2007 Yamaha yZ 450 F

1 Answer

My 1982 Honda 200e starting missing about a month ago, sometimes it will backfire, it runs but poorly, plug was black. Had the fuel changed, carb cleaned and still the same!!

With any naturally aspirated motor, aside from timing, your fuel/air mixture is crucial at part-throttle as well as at wide-open throttle and off-idle. Sounds like your running too lean on the top end and too rich at part-throttle and off-idle. You should focus on either preliminary adjustments to your carbs; rebuilding or replacing them. You may also consider re-metering and re-jetting your carbs.

Jun 07, 2011 | 1974 Honda CB 200

1 Answer

2007 CRF 450 R- from 1/2 TO full throttle it backfires and cut out,as soon as you release the throttle it comes back to normal.Below 1/2 throttle all is fine.Cleaned the carb ,checked all jets and fuel...

This can only be lack of fuel for some reason backfire is nearly always lean fuel condition and the cut out pretty much confirms lean or no fuel?
I would be checking the size of main jet (they numbered) double check it is factory size? I would check the needle clip position god knows why but people change the position so double check it is factory setting (usually 3 or 4 notches from top is factory setting I forget which one it is for Honda's) I would double check your air filter is clean and/or not oil drenched
refer to your manual or ring your Honda dealer for all the specs
Some other possibilities would be spark plug fail at high load (but unlikely?) stator failing at high RPM ( unlikely but worth the two minute multi meter test to make sure resistances is still in spec)
good luck regards Jamie

Jun 01, 2011 | 2005 Honda CRF 450 R

1 Answer

Briggs and Stratton outboard. I have a 5 hp briggs outboard that we have cleaned out pretty good. The carb has been cleaned also. The engine will start and idle perfectly but when you start to throttle it...

If you have started the engine out of water, you may have locked up the lower end, the prop gear.  If you think it is the carb or a fuel problem, try and get your hand over the carb when you rev it up. if you put your hand over the carb right as you rev it, then pull it off, she will pull more fuel in and should go on to rev up. if this works you still have a port cloged in the carb. 

Aug 11, 2009 | Boating

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

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