I have a 1999 YZ400F with alot of backfiring on deceleration. I have owned the bike about only 3 months, so Im not sure of the entire history of the bike, but it runs strong. I have checked the valve clearances and they are within spec. I removed the pilot jet thinking it would be the stock 45 and I would replace it with a bigger jet, but it was a 50, so I dont think that is the issue. I cleaned the pilot and turned the air/fuel screw out to about 1 3/4 turns, the recommended stock setting is 1 3/8 turns. It is still backfiring. If i pull out the choke slightly on deceleration, it doesnt backfire. This makes me believe it is a carburetor issue, not an air leak. What about taking the needle down a notch? I did notice recently that the header mount to the engine is missing a stud on one side. Could that be allowing air to enter and cause a lean condition?

Thanks

the fuel screw should be closer to 2 turns out. YOu need a smaller pilot, try a 48 and get the fuel screw to about 2 turns out. IF not, change the pilot.

Also check the header and mid pipe for exhaust leaks

Wouldnt a smaller jet make my problem worse, because I have a lean condition? What about muffler leaks? I noticed after I made the post that I am missing a few rivets in the muffler that are probably pulling air.

You don't necessarily have a lean condition. When you go too rich on the low speed jetting combustion which is already lazy at closed throttle gets even slower. This delayed combustion dumps even more raw fuel into the pipe than normal and it gets re-fired by the fresh air leaking in at the head where the exhaust stud is missing. Someone likely bumped the pilot jet up to cure the backfire, and only managed to make it worse.

Fix the exhaust leak or learn to live with the popping. You aren't going to jet around an air leak at the head/pipe junction.

Thanks Rich

Would you go back to the original 45 pilot after the leaks are fixed?

Depends on the conditions and your riding style. In dry or slippery conditions it can be worthwhile to run a pilot that is a bit too rich to soften the power on the bottom and make the bike hook up better. It can make the initial throttle opening as you are standing the bike up in a turn easier to manage.

Pilot jets are cheap and easy to change. Try a 45, 48 & 50 and see what feels right to you.

I can change the pilot jet in my old RM125 in about 20 minutes or less and the main in about 10 minutes (due to the bowl plug) and the jets are available for $6 or less at my local motorcycle shop. The Yamaha is a whole different animal. My local shop doesnt carry those particular jets, so I would need to get them from Yamaha at $18 each...ridiculous. Removing the bowl on the Keihin carburetor is also complex because there are so many obstacles in the way and it is a big carburetor. I didnt remove the carb, I only slid it to the side and it took me over 2 hours, (it was my first time doing it, but it wasnt as easy as I thought) In other words, I would like to leave the 50 jet in it if I can. I put a header bolt in it and pop rivited the muffler together where it was missing 5 or 6 rivits, then put some high temp RTV at the muffler joint..maybe that will help. I will know tomorrow after the RTV sets up.

So you dont want to fix the problem cause it is to hard. hrm..... that will never get solved. you can change a pilot in 15 minutes on a 4 stroke if you know how to access the carb properly.

Also for the cost of the jets, it is a standard kehin pilot. Any shop should have aftermarket brass from Tucker Rocky, WPS, PArts or SUdco. If not, find a new shop.

Matt

Once I fix my air leaks, there may not be a problem. I worked on the leaks yesterday, but I havent ran it yet. Why change the jet if my problem is fixed? If it isnt fixed, I will change the jet. I would like to see someone change the pilot jet on my Yamaha in 15 minutes, that carburetor is packed in tight. Im sure it will take me less time than before, now that I have done it.

I use to have a 426. So I know it has been done. Remove the coil, seat/tank and the hotstart and loosen the clamps and take out the 17mm bowl cap there and you can turn the carb no problem.

you already said you have a problem with the pilot by the turns on your fuel screw. So you are going to have to address the problem.