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Constant Backfiring

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I have an 04 yz250f and it is constantly backfiring. When I am going pretty fast it seems to stop, does any idea what it is. any help or sollutions is appreciated

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constantly?

Does it do it when you are accelerating or when you hold it at a steady throttle position? or when you let off of the throttle?

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I just bought a 04 this summer as well. Mine backfires when i am off the gas and there is heavy engine braking going on and others riding with me say they can see fire out the pipe.

Does yours also spit fire?

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If you're blowing fire out the back and are constantly backfiring, you are definitely running lean. You need to up your jets a step or two and it should cure your problem. If you don't, you may end up burning up your piston or worse.

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assuming there is no air leaks in the boot.

if it's doing it at a low steady throttle and not doing it when slowing down sharply, that says to raise the needle a notch. (raise it a notch at a time)

if it's doing it at a low steady throttle and also doing it when slowing down sharply, or just doing it when slowing down sharply, that says to turn the pilot screw out (CCW) turn it a 1/8 turn out at a time. more than 3 turns and you need to go up a size in the pilot jet.

but as B86 suggests you can indeed burn up a piston if the main jet is too lean. so make sure you check it by checking the plug via a throttle chop test.

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Thanks for all of the help. It only backfires when I am slowing down and when I am riding at low throttle. I hope that helps. Another thing, I guess you could say that I am not very Mechanically inclined when it comes to dirtbike engines, plus I am scared I will mess it up so if you could give me a more detailed directions on what to do as far as needles and jets. I don't even know where they are located.

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A little bit off "popping" through the exhaust on deceleration for these bikes is normal. This is usually 2-3 pops, maybe a few more on a hard decel. This is NOT backfiring. Backfiring is flame through the back of the carb into the airbox.

With that said, popping is typically a sign of a lean condition. If popping only on decel and excessively this is lean on the pilot circuit. Adjust fuel screw out or go up one step on pilot jet and readjust fuel screw.

If the popping occurs from 1/4 - 1/2 steady throttle you need to richen your needle by moving the clip one (or two) notches down the needle. This is called "raising" the needle.

Accessing the Jets:

The MJ is a small brass threaded jet with a 6mm hex end and threads on the other. You can get to it with the carburetor in place on the bike. Turn the fuel petcock OFF. Open the float bowl drain tube and allow the gas to drain out. Take off the 17mm hex nut at the very bottom of the carburetor. A small amount of gas will pour out that is in the nut. If you feel in the middle of the hole where you pulled the nut from, you will feel the small hex nut of the MJ. A 6mm 1/4" socket can take it off. It should only be about 1/2" long. If it is over 2" long, the needle jet came out also. No problem, just put both back in together. Loosening the boots and rotating the carburetor helps.

The needle is under the very top of the carburetor. On a WRF you will need to remove the tank and seat. There are two 3mm hex bolts on the front and back of the top cover. Clean off the top of the carburetor and the bottom of the frame above the carburetor with mineral spirits or WD40 and a rag. Take off the two hex bolts. Carefully remove the top of the carburetor. It has a rubber gasket attached so try to do this without rubbing it around too much. Looking down into the carburetor you will see the throttle slide. Twist the throttle and it will rise. In the middle of the throttle slide is a 4mm hex screw. Remove it and it will come out along with a spring and a collar. Don't loose the spring and collar. Under it is the round top of the needle. Using tweezers or needle nose pliers, gently lift it. Have someone twist the throttle to raise it up if you have trouble getting to it. Pull it out.

The PJ is in the same hole under the 17mm nut in the bottom of the carburetor that the MJ was in. It is in front of the main and has a flat screwdriver head. Simply use a small short screwdriver to remove it.

The leak jet is located in the float bowl. You will need to remove the float bowl and then use a small screwdriver to remove the jet.

The PAJ and MAJ are in the intake port of the carburetor. You will need to remove the air boot and the intake bell to access them. You may need to take the carburetor off for the air jets. Use a small screwdriver to remove the jets.

The fuel screw is at the bottom of the carburetor in the front (engine side) recessed in a small hole cast into the float bowl. You can't see it and you probably don't have a short enough screwdriver narrow enough to get in that hole. You will probably have to make one by cutting down an existing screwdriver or use a "carb tool" like the ones from Motion Pro.

Jetting the Pilot Circuit:

When the pilot circuit is jetted properly, starting the bike should not be a problem. You will need the choke to start cold and it won't run well off of choke for 30 seconds or so. When hot you will probably need the hot start. The bike should idles smoothly and have no hesitation of idle and minimal backfiring on deceleration. For more info about the pilot screw, see this article that recently appeared in Motocross Action Magazine.

Setting the Fuel Screw / Pilot Jet "by Ear"

1. Adjust the idle with the black knob until it is too fast. Then adjust it back down until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.

2. Before you start adjusting, count the turns required to tighten it up lightly.

3. Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw.

4. Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle.

5. If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone.

6. If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again.

7. If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again.

Setting Fuel Screw / Pilot Jet with Tachometer

1. Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct it into the radiators (A YZF will start to boil out if you take too long to do this, WRFs have a nice catch tank).

2. Turn the fuel screw 1 1/2 turns out. Read the RPM for about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write down average RPM.

3. Turn the fuel screw 1/2 turn out. Write down the average RPM.

4. Turn the screw 3 turns out. Write down the average RPM.

If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM. Go on to step #7.

If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.

If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.

5. Adjust the fuel screw in 1/4 turn increments around 1 1/2 turns out and find the maximum RPM fuel screw position. If the idle is now above 1900 RPM, turn it down to be in spec.

6. If you get a little deceleration backfiring on closed throttle, try adding another 1/4 turn out. Remember you will need to redo this if the altitude or temperature changes significantly.

Don't worry about screwing anything up. As long as you keep good notes and only change one thing at a time you can always go back to where you started or reverse a change that seems to have an adverse affect.

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First, let me advise you to pay the most attention to Mr. Kenle's post.

Nevertheless, a couple of comments to add to it. Exhaust backfire on decel is common and, unless it starts a fire or seems likely to blow off part of your exhaust system, it's harmless. One of the most common causes besides a lean idle circuit is an air leak in the exhaust system. Since some fuel and a lot of heat are already present in the pipe, a little air is all that's needed to go bang. Check the joint at the header/mid pipe and the pipe to head seal.

My YZ450 pops out of the exhaust fairly steadily at times when I have the revs up at a "steady" throttle over really nasty whoop sections. I found that it does not do this on a smooth surface under the same throttle and speed conditions unless I also work the twist grip slightly, mimicking the rhythm of a set of whoops. My conclusion is that my "steady" throttle condition isn't so steady after all, and since I didn't care for how the bike ran when I raised the needle (in my case, not yours, necessarily), I decided it wasn't anything to worry about.

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My '04 YZ250F also backfired terribly (at sea level) when I first got it. A friend suggested it was lean, and we just backed out the fuel screw a total of 3/4 turn (didn't do the whole turn it in, then turn it out as we were in the middle of the desert and ready to ride), and its WAY quieter now, and starts easier too!

Seems to be a factory adjustment issue for the '04s or something...? :thumbsup:

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Speaking of factory adjustments. My fuel screw was screwed all the way in from the factory and it ran, not good, but it ran.

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If you're "front-firing" through your exhaust with fire and explosions, you might consider that as a "rich" condition on the needle and MJ - not ALL popping is from lean conditions, especially if there is too much unburnt fuel left over (i.e. a rich condition) in the exhaust system causing fire and explosions, keep that in mind...

You need to note/write down what your stock/current setup is - fuel screw setting, PJ size, needle setting and MJ size, from your current setup and your riding climate, you can figure out if your setup is rich, lean or about right. I've read that the 03 and newer(?) WRs comes with a 185 MJ stock, that is pretty darn rich, going richer (depending on your climate) will only make your bike run more sluggish, possibly foul the plug and cause MORE front-fire through the exhaust.

An engine tuner told me to jet just slightly rich to get the best engine response and performance. Take it for what it's worth, but this is what I do now and I've since read a few articles indicating the same thing, i.e. run as close to lean as possible and even slightly lean for the best throttle response but I would NOT run our 250Fs slightly lean (those racing teams have lots of $$$ for replacing blown engines :devil:), go with slightly rich jetting :thumbsup: I run a 178 MJ on my 02 WR250F and my buddy's 02 YZ250F is at 172 IIRC, we ride close to sea level and we also ride on some of the East Coast mountains, probably 200 to 1500 feet high.

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An engine tuner told me to jet just slightly rich to get the best engine response and performance

It makes the most power under heavy loads. Acceleration baby. :thumbsup:

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Setting Fuel Screw / Pilot Jet with Tachometer

1. Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct it into the radiators (A YZF will start to boil out if you take too long to do this, WRFs have a nice catch tank).

2. Turn the fuel screw 1 1/2 turns out. Read the RPM for about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write down average RPM.

3. Turn the fuel screw 1/2 turn out. Write down the average RPM.

4. Turn the screw 3 turns out. Write down the average RPM.

If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM. Go on to step #7.

If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.

If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.

Perhaps I'm mis-reading this or just confused. In numbers 3 and 4 shouldn't it technically read "Turn the fuel screw in until it lightly seats--then turn it out X turns" ? That would take the fuel screw to its extremes to test lean and rich. I figured this to be the case as 1 1/2 + 3 would be 4 1/2 turns out (a seemingly useless setting)

If so then I'm confused with "If #3 is greatest, you need less fuel. Install the next smallest number pilot jet...." If the idle were right at 1 1/2 turns out and then the fuel screw were turned in, thus leaning the mixture because of less fuel flow, wouldn't the result naturally be that the idle would increase? If so then why would it be that the fix would be a leaner pilot jet? What am I missing here? Does the statement "you need less/more fuel" only pertain to the fuel ratio with the fuel screw at 1 1/2 turns out?

I think I understand #4 in that the carb could be close to starving and supplying more fuel would bring it to life but but am missing something with #3. :thumbsup:

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Perhaps I'm mis-reading this or just confused. In numbers 3 and 4 shouldn't it technically read "Turn the fuel screw in until it lightly seats--then turn it out X turns" ?

Yes.

What am I missing here? Does the statement "you need less/more fuel" only pertain to the fuel ratio with the fuel screw at 1 1/2 turns out?

Yes. Sorry, I guess the wording could be more clear. You are trying to find the pilot jet that gives you the best mix close to 1 1/2 turns out on the fuel screw.

So if you have the highest idle at #2 (1 1/2 turns out) you have the correct PJ.

If you have the highest idel at #3 (1/2 turns out) your pilot jet is too big. You need to install the next smaller PJ. Your engine is getting too much fuel with the screw 1 1/2 turns out.

If you have the highest idel at #4 (3 turns out) you current PJ is too small and you need to install the nest larger PJ.

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New Kid on the block here. I would like to refine the pilot circuit on my new "05" yzf450. I also had the exhaust popping issue, did a leak test on the exhaust system & have no leaks, had to back out the fuel screw out three turns before it got better. Put a #48 pj, one up from stock. Now I am ready to dial it in. I have a tach & need to know were to connect it. By the way, Thanks for the good info in your post!!

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