Well I got screwd!!

Well I am at work sick as hell, must have that stomach virus that is going around. I hate sitting at home so I came to work late. Well I got the 426 out yesterday before I got sick with whatever I got and decided I would turn a few laps and fiddle with the air screw on the bottom of the carb to see if I could eliminate the backfiring under decel. I am back at stock jetting by the way. Well I turned it out 1/2 turn before I went riding. Well I did not go half a lap on the track and noticed that the bike was backfiring under decel more than ever. So I pulled over and needless to say (You all know what happened) the screw had fallen out. Dammit. So I rode her back down to the house to put her up. She was now backfiring more so than ever without the screw. So I think I should have turned the screw in instead of out. Help guys, how does that thing work again, does turning it out give it more fuel or more air. I feel terrible I can't seem to think straight. Where can I get one of these screws for a reasonable price shipped to me before the weekend. Any help as usual would be great. I did notice also that the boot coming from the head to the carb was loose so it could have been sucking air from there too. I just want to get this bike back to prime condition. Any ideas where might have that screw in stock and I could order it and get it here before the weekend? Thanks, as usual, Frank

Frank, trial & error. That is how we all learn. Try your local Yamaha shops for the screw.

Originally posted by FASTFRANK:

[QB Help guys, how does that thing work again, does turning it out give it more fuel or more air.

Any ideas where might have that screw in stock and I could order it and get it here before the weekend? Thanks, as usual, Frank[/QB]

out = more fuel

You can probably pay $20 to have the screw overnighted by your local dealer.




That screw is an idle mixture screw. Out is more fuel.

Keep these things in mind: Backfiring out of the exhaust (sometimes you get to see a pretty flame) means the carb is jetted too fat (too rich). So lean it out with the appropriate nob (pilot jet, needle, or main jet depending on rpm range). If it backfires out of the carb/air cleaner, it's too lean. I haven't done any fiddling to my 2002 YZ426F and it runs fine....all stock. Hope this helps.

yeah I wish I would have left mine alone instead of fiddlin with it. It is backfiring out exhaust not airbox or carb. When I do get a new screw I will try about a turn out and see what that does. I can probably order the screw from my local dealer but they are so slow it would take a week to get it and they charge over retail for pricing. I paid like seven dollars for a freaking spark plug through them. I hate giving them business unless I have no other choice. Just thought someone might know where I could get one. How can I be sure that the screw won't fall out again once I get a new one. anyone put anything on the threads?? Just curious and frustrated, Frank


The fuel screw shouldn't fall out as long as its not turned out to far. There should be some slight decel popping for better throttle response. My screw is about 1 1/8 turns out running with a #40 pilot jet. I also made an on-the-fly adjust screw like motoman 393 has on his site. It works great for fast adjustments. Very easy to make.


If the screw does not have internal thread interference when you drop the new one in, put some blue loc-tite on it. This stuff doesn't dry too hard and will keep it from moving. I use blue loc-tite on about every fastener on my chopper...and luckily no parts have fallen off lately. Haha. Good luck.

The interferance on the screw is the spring and washers.

The screw is a fuel meter. Out is more fuel. Rule of thumb: if the screw is on the backside of the carb its an air screw. ON the front/engine side of the carb its a fuel screw. If the mix cant be corrected between 1/2 and 2 full turns from bottomed, you need a different jet. If you bottom the screw and the bike still runs, its too rich on the jet, get a smaller one. If its out 2 full turns,and seems like it wants more, go up on the jet. This is the pilot jet. Not the main. Call a company named Sudco Southern Ca. They have what you need at a decent price. Good luck


In a recent issue of MXA (can't recall the month just now), they had a artical about fuel screws on the YZFs. Basicly, you put the bike on a stand and turn the idle screw up high while the engine is running. Then turn the fuel screw one way, and then the other till you have the highest cleanest idle. Once that's done, turn the idle back to where you like it and ride. I did this on my 400 and it rarely backfires and runs good as ever.


Everyone is right, out is more fuel.

The reason that you think out is less is it sounded or acted more lean without it. It is more lean without it because now, unlike when it is turned almost all the way out, air comes in the open hole. The air is making it lean, despite the fact out is more fuel.

I lost mine, plugged the hole, and rode until the new one came in. The plug wasn't a perfect fit (grew up at small sawmill so I whittled one) but it not being perfect worked out perfectly.

Sudco has them. You'll have to order the screw, the washer, and the rubber grommet. There are three thread types for this screw. I didn't know which one, just said ship the most common, and it worked fine.

Another, perhaps better, alternative is to get the Kouba fuel screw. They have two fuel screwish products. One is a tee handle that press fits over existing screw, the other is a screw with a builtin tee handle that replaces the existing screw.

Good luck,


appreciate the replies guys. Kouba screw huh? That sounds interesting. What is that website? I figured there was more parts that I would need to replace other than just the screw. Will the Kouba screw come with all the parts needed? Let me know if so I will order one today. Thanks, Frank

Hey guys,

I was just doing some searching and read about the Kouba extended fuel screw with the T-handle for like 20 bucks. My bud has a CRF 450 on order so as soon as it comes in we can put the extra one I have to order in his bike. Now with this screw do I get the oring, screw and spring and t-handle???? Everything I need???? Just want to make sure. I called Norman Kouba and left a message for him to call me back so I can get one on order. I saw that local dealers wanted 30 bucks alone for the screw. So I am sure that my dealer will want 50 bucks for one!! Just let me know guys about this Kouba screw. Thanks, Frank

A few observations:

1. The Kouba tool is not a fuel screw substitute! All it does is attach to your existing fuel screw to make adjusting it easier.

2. Unless they changed the carb design between the 400s and 426s, I don't know how your fuel screw could just fall out. Mine will not come out without first removing the float bowl and the float bowl will not come off without first unscrewing the fuel screw (I've got a Kouba tool on mine so the float bowl will not just slide off.)

3. Putting loctite on your fuel screw is a bad idea. Mine has a lot of friction already and I wouldn't want any more. That carb is made of pot metal and who knows what the threads will do under too much stress. If yours fell out I have a feeling it wasn't installed properly to begin with. I've heard stories of Kouba tools vibrating off, but those are just press-fitted on.

4. Just order a fuel screw from your dealer (or from www.powersportspro.com or www.hlsm.com -it's way too complicated to have to order the screw, spring, washer and o-ring all separately from Sudco.

That was my two cents.

I tried to order one from Sudco, and they don't sell the fuel screw for our carbs... Apparently they sell a slightly different FCR carb, and they sell parts for that. Some of those parts are interchangable, some aren't.

I had to get the set (screw, o-ring, washer) from my dealer $24...


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