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Backfiring

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I have an 09 yamaha yz250f with a vance and hines titanium pro exhaust. I cannot get it to stop backfiring with I let off the throttle. I have put a 48 size pilot and a bigger main and have a gytr fuel screw that I have adjusted but it still backfires.

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I have an 09 yamaha yz250f with a vance and hines titanium pro exhaust. I cannot get it to stop backfiring with I let off the throttle. I have put a 48 size pilot and a bigger main and have a gytr fuel screw that I have adjusted but it still backfires.

Is it backfiring (which means an explosion that fires back through the carb) or is it popping, upon deceleration.

Follow the fuel screw adjustment procedures in the sticky at the top of the forum. It takes a few minutes.

You may never get the popping out, as any race motor with a race pipe is not designed to run clean at closed throttle.

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The backfiring is probably unburnt gas firing in the hot exhaust if there are no leaks in the exhaust.

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I have tried to adjust my fuel screw but it seems to not respond to any adjustment at all.

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Lower your idle speed. However, as Krannie said, you have an open pipe on a race bike. Race bikes do not coast, They are either WOT or hard on the brakes.

No, a jetting kit will not make a difference. The popping is nomal when the engine is reving and the throttle is closed. A stock exhaust would not pop as much. Dpuble check you have no exhaust leaks (hold a towel over the end of the muffler, listen for hissing at the cariuos connections).

Here is a fuel screw adjustinging in greater detail.

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

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Keep going up on the pilot jet size until it stops. You're too lean on the pilot if it's popping on decel. Adjust fuel screw as required

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Keep going up on the pilot jet size until it stops. You're too lean on the pilot if it's popping on decel. Adjust fuel screw as required
Sorry, but that is bad advice.

He has a race bike. You must set up the pilot and fuel screw for a perfect hot idle. If you richen it (not the right thing to do) to deal with decel popping, you'll never start it hot.

If you must own a race bike and ride it like a play/trail bike and have to have a loud exhaust, the correct (if there is such a thing) to do is get a woods bike carb that has a ACV (Automatic Coast Valve) on it. They are designed to add fuel during decel to deal with or reduce popping.

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yeah,do not put a bigger pilot in it,you need a 45 pilot,and a 178 or 180 main.i had the same problem with our 09.although mine had a ported head,i did get it to run better with the red needle from a jd jet kit.also make sure that the rubber gasket on the slide is not messed up,that will make it backfire too.and follow williams instructions for ap timing and fuel screw adjustment.

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