A little help with my s-s-s-sputter

I am getting a little bit of a sputter or hesitation out of my 01 426. I'll try to describe it so it makes sense. On my local track there are four jumps close together and i take them 2 at a time. I come out of the corner leading up to them and my bike pulls like my 69 Mustang Mach 1, take flight and let of the throttle ready to crank it back on upon landing. I land on the downside of the 2nd jump and give the throttle a good twist the second the rear tire makes contact, this is where the sputter comes (I should say ONE of the places) I get a little hesitation causing me to case the second double. My jetting is stock and I adjust the fuel screw until decel poping just goes away. Is this the problem the BK mod will solve? My squirt seems to be about 2 sec and it clears the slide. Is it something else? Help me out.


The BK mod will help crispen the throttle. It might also be a float level problem. The bk is too easy not to try.

Please post your elevation, a lean spot on the needle can also create a "bogging" sensation.

My elevation is about 4500. Temp is usually between 80-100. My jetting and needle position are totaly stock. I know I should rejet, not sure where to start.



Good answers from above and right in line with what was my first impression. I might add that once you start to zero in on the problem and it gets better, try to keep in mind that a slight pop on decel is probably a good thing. This is an indication of a slightly lean pilot circuit which can yield good throttle response right off idle. This is an area where a good sensitive feel of how the fuel screw(pilot screw) can add to the overall fine tuning. Whatever you do, make ONE change at a time before testing. This is a good time to bring along a small notebook to make a record of what you are doing. If you go out for a day of testing, you can make a record of where you started and then how the engine responded to each change. Making detail notes is valuable beyond description for future reference. Good luck and post back as you make progress.

Did this start suddenly when riding at this particular track or has it always done this? At sea level the stock jetting is fine, but I've only ridden at sea level so I have no idea how high you have to go before you have to rejet. That said, if you're high enough then rejetting would probably help. When my bike has done this in the past then either cleaning the carb or tweaking the fuel screw has fixed it every time (at sea level).

*Waves to Cy from the other side of the valley.*


I took the carb off my bike this morning (dont you just hate us self employed people, not working on friday!) I timed the acc pump squirt to over 3 secs. Now on my way to get a tap and screw and bring that puppy down a little. By the way, thanks for the helpful pics and descriptions on your sight motoman. Will report back soon.



Stock jetting didn’t work well for me at all, and I am in a similar climate and elevation, 4,500 ft. in the desert.

The stock needle isn’t too far off, but you should try a clip leaner. I think the stocker is too rich off the pilot too, I have an EKQ which is ½ clip leaner and one size leaner off the pilot. Yamaha PN is 5JG-14916-E1, and I run it on the 3rd clip (so I’m 1.5 clips leaner than stock).

On the pilot I’m one size leaner, 40 vs. 42, ditto on the main, 160 vs. 162. The pilot and needle clip make the biggest difference, but the needle change and BK mod really rounded out the jetting giving me perfect (as far as I can tell) carburetion. The only glitch is that the smaller pilot will make a bit more decel popping than the 42, particularly with an aftermarket silencer. To fix this lean spot at higher revs you need to go smaller on the Pilot AIR Jet. Sudco sells an adjustable one, or you can swap one from a WR (Yamaha PN is 7A9-14943-17-00). Stock YZ size is 100, stock WR is 75, I ended up at about 85.

Sudco 800-998-3529, PNs 021-230 & 021-235 for spring and screw. The way this works is one turn out is 100, 3/4s is 85 etc. For fifteen bucks or so it is a great investment.

Now realistically I don’t expect you to run home and change all this Air Jet this and New Needle that, but I would encourage you to drop the needle 1 clip and put a 40 pilot in there. That should wake you up to the possibilities. Life is too short to ride a poorly jetted YZF.

Here is my jetting again, 4,500 ft. +, summertime temp.s 85 to 105, humidity 15% to 40% (when it’s rainin’).

160 main

40 pilot (don’t have a PN but the Flat CR carb uses same pilot jets as PWK two stroke carb).

EKQ needle # 3 clip

~ 85 PAJ

200 MAJ (stock)

1.5 turns out on idle mix

Pump stroke limited to 1 mm (measured at the actuating rod) w/ BK screw.

The throttle hanging is an indication of being lean in the idle circuit. Maybe go back to the 42. Otherwise ignore it and it might make you coast thru faster LOL. Good Luck. If you change fuels you might also get a little different reaction.

Stock needle is OBEJP, I guess you couldn’t read that last tiny little letter :)

The last three letters indicate the jetting spec of the needle. First letter is taper, ignore that. Second letter is clip position. So stock is “J” and each letter difference is a half clip position. So an ELP on clip # 5 is identical to an EJP on clip # 4. Last letter is the diameter of the straight portion, before the taper. This governs jetting just off the pilot before the slide lifts the needle to where the tapered portion starts coming out of the main. My EKQ then is one size leaner on the straight portion than stock (and a half clip leaner also).

So now you know more than the parts guy at your dealer about FCR needles.

Hanging idle. Hmmm. The air jet may affect this, but that ain’t real likely. You don’t want to really be more than two turns out on the idle mix. At the same time I don’t think there is any way that a 40 pilot is too lean for you. Could you get back to us after you’ve ridden it some more? Maybe it wasn’t fully warmed up?? Try a radical change, like all the way down to one turn out with the bike warmed up and see what it does.

I forget exactly, but keep in mind the idle speed spec. is like 1,400 rpm or something. Should be a nice healthy lope. Like the pulse of a Jack Rabbit. How’s that for descriptive?

The popping on decel was exactly as predicted, and in my experience most aftermarket pipes make this a little worse. A good way to get the most out of this carb circuit is to get on a road in fourth or fifth gear and try some trailing and nuetral throttle, as if you were cruising on the freeway. When the PAJ is too lean (large) the bike will want to surge a bit. So keep going smaller until you have nice smooth operation in this higher rpm / lower throttle range. At this point the decel popping may not have totally disappeared, esp. w/ an aftermarket can, but it should be tolerable.

Hope this helps.


Everthing is back together thus-ly:

160 main

40 pj

stock needle 3rd clip (i think obej?)

BKed to less than .5 sec (didnt have a way to be exact)

1.5 turns out on fuel screw

Took a spin up the road and back. The lower end response is noticably better. I am going to the track in the morning to get a better idea of how it is working. Will report back.

However, Hick, I have a slight hang on the idle. When i blip the throttle, the idle hangs about 100rmp high for about .5 - 1sec then returns. I assume i am a little lean on the fuel screw or needle. I turned the screw out to 2 and it didnt fix it. Thanks for your help.


BTW I now have slight decel poping at higher rpms, I am going to order than air jet today.

[ August 16, 2002: Message edited by: CyPrice ]

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now