Jetting questions, HELP

[MikeT]

Bike 1996 YZ125

Saturday I went trail riding and there were a bunch of long downhill sections of trail where I had the throttle closed or barely open. This caused my bike to LOAD up REALLY bad. I even ended up fouling a plug. I know my bike is jetted well up top and runs cleanly there but I have never been able to get the bottom clean. To date I have gone one size leaner on the slow jet (also called the pilot I think) and turned my air screw out 1 1/2 turns. If I'm on an open MX track 1/2 to full, the bike will NEVER foul a plug and always runs clean (Maxima SuperM 32:1). I am happy with the oil and ratio and want to stay with that. Does anybody have any other ideas to help clean up the bottom? What different jets should I change??? :think:

 

[skhasky]

Is this a YZ-125?
OK - I had very similar issues. First - sounds like the pilot could be rich. Warm it up and then run it along in 2nd gear with ... like 1/16 throttle.When running at 1/16 was it smooth or did it crackle? Whack it full open! Did it wheelie? Did it take off? If so, the pilot is ok. If it bogged and went (buwaa) then you are too lean. If it crackled and sputtered then you are too rich. Either case, try adjusting your air screw first. On the 36TMX carb it should be around 2 turns out. If it is too rich (sound to me like it is) then turn the air screw out 1/4 turn at a time and try intil you get great response. If you hit 3 turns on the airscrew and it is stil rich, go to a smaller pilot - reset at 2 turns- and try again! Settle on the pilot that runs well at 2 to 2 1/4 turns out. My bike came with a factory 50 pilot. I went throught this until I finally settled on a 35 pilot (4 steps leaner then stock) at two turns - runs great / no load. Now, I live at 5,000 so don't try this uless you do too! 35 is aweful lean.
A couple more things... which spark plug are you using? Spec is a B9ES, however you can run a hotter plug. I run a BR8EVX. The lower the number, the hotter the plug. The "hot' means it retains more heat in the insulator and will run hotter in the engine. This will help with plug fouling, as there is more heat to burn off the stuuf that's fouling! Watchout though... hotter plugs make things leaner... you may find yourself lean in areas with this - so prepare for possible jet changes. I wise man (Eric Gorr - I think) once said to run a plug that leaves the first 3 threads of your spark plug dirty, and the rest clean. If they are all dirty, get a cooler plug. If they area ll clean, the plug is too cold so you need a hotter one.
Good Luck!

 

[MikeT]

Sorry, yes its a YZ125. I guess it sounds like I need to go another step leaner. I think the stock was a 50 and I have a 45 in there. I guess I'll have to go to a 40 and still work with the air screw a little more. The plug came out wet and black when it fouled. I am sure the bike is running rich. I also have the clip up one position (leaner) from stock.

 

[skhasky]

45 at 1 1/2 turns out... you still have a bit to go with that air screw before changing that pilot. 45 seems right for sea level - how high do you live? Also, it is possible that your float height is too high - making it run richer. If you live at sea level and you still can't lean it out with the 45 - I would look closely at the float height. However, if you live at - say 2,500' then it is reasonable for the 45 to be too rich of a pilot. 40 would sound better at that elevation. Let me know if you need any help with this.

Skip

 

[MikeT]

I'm at about 700-1200 feet depending on where I ride. I have never adjusted the float height.... It is stock as far as I know. I'll have to check the actual amount of turns out on the air screw. If the float height needs to be adjusted, how much should it be moved??

 

[skhasky]

....maybe too much info, but.... I thought of something else. Another jet that comes into play at small throttle openings is the needle. You said you raised your clip (leaner). At the smaller throttle openings, the diameter of the needle has more to do with the mixture than the clip position. Let's say you had a #69 needle in clip position 3. You noticed it was running rich at 1/4 to 1/2 throttle, so you raise the clip - perfect! Howerver, let's say that the problem was more in the 1/8 to 1/4 throttle opening. You could theoretically be perfect at 1/2 throttle and rich at 1/4 throttle. In this case, raising the clip would fix the bottom but lean out the top too much. When dealing with the needle, adjust the taper of the needle to fix problems at the lowest throttle openings, and the clip to fix problems at around 1/2 throttle. So, in your case, if it runs fine at 1/2, but rich at 1/4 or less, you may need to change the taper to a number #70 needle - for example - and adjust the clip from there.

Skip

 

[skhasky]

700 - 1200'.... your float height is set at the factory. The dealer has a tool for measuring the feul height. To adjust, simply bend the tab between the floats and measure again. Commonly... the floats leak and take on fuel. If you take the float out and shake it you can hear the feul in the little floats if they are leaking. If that is the case, replace them.

As for your altitude, i checked and your bike came with a 55 pilot and a 350 main. I'm thinking 340 is probably the right main. What main are you running? Pilot - somewhere between 45 and 50 should be right. You should be able to tune that air screw and get the 45 to work for you. As for Needle/Nozzle - this is tough. I don't think you are high enough to warrant the leaner taper, but... if it was rich from the factory - well. It's worth a shot! I think you probably did right by raising the clip. Finally...the nozzle. I think yours came with an S-9 nozzle. I'd have to say the S-8 would probably be the ticket. Try it and see. You may have to lower that clip again, but would get rid of the loading at low throttle. So much to do! Oh -well, most people don't have their bikes jetted properly, and you would be amazed at how much better things are with perfect jetting. It sounds like you know this already, though...

Skip

 

[MikeT]

I said I have a 45 in there, but what I really have in there is one step leaner than stock. If it came with a 55 then I have a 50 in there, if it came with a 50, then I have a 45. I just can't remember off the top of my head. I have to reread your post because it sounds like you definately know how to jet a bike and I don't want to get anything wrong. I should probably get a call into the Yamaha dealer and put some of those jets/nozzles on order and check the float height.

 

[skhasky]

Here's what I think should be in your 'Kit' - for what it's worth.
(1) #350 Main
(1) #340 Main
(1) #330 Main
(1) #50 Pilot
(1) #45 Pilot
(1) S-9 Nozzle - should be in your bike
(1) S-8 Nozzle
(1) Needle 1 step leaner taper.. if your bike came with a #69, then get #70
(1) spare float
(1) Spark plug 1 number hotter than the factory. I think yours came with a B9ES, so get a B8ES.

This is about $75 in parts, however, you will have everything you need to achieve perfect jetting anywhere from the coast to 2,500 feet and from 60 degree autumn to 105 summer heat and humidity. This is what I would have in my 96 jet kit.

Skip

 

[MikeT]

Where were you last year?? Thanks man!

 

[MikeT]

I'll keep you updated

 

[skhasky]

Where was I last year? Not in Tarheel country unfortunately! I love N.C.! Definitely keep me posted. I am wierd enough to 'enjoy' sorting jetting out, and love to see how people react when it is 'spot on'. I run a 98 YZ-125 that is what I consider 'perfectly' jetted. The bike is TOTALLY STOCK - stock pipe/silencer/piston/jug/etc... and I roost all over the local guys who haven't figured out that there is much more to it than setting the main and checking the plug. It's a wierd science, but once it 'clicks' you will never sweat jetting again... and I guarantee practically no hop-up mod has as much of an effect on performance as proper jetting. Also, you'll find when the bottom is sorted out, YOU WILL see improvements at the track (not just the trails). If I am not around the board, just email me at skhasky@hotmail.com and I will help you get this totally sorted. Good luck!

 

[MikeT]

Nozzle?

?? What will the nozzle do as opposed to the needle?? Do they complement each other and should they be changed at the same time?? What part of the throttle position does the nozzle effect?

 

[skhasky]

Here's your Yamaha parts list. Don't let your dealer dick you around. This is suggested retail, so don't pay more. If your float is ok, you don't need one, but it is nice to have as a spare. I always keep an extra float lying around. Also, get Genuine Yamaha parts. I few dollars more, but the Mikuni Jets are much better quality, and there are pilot variations that can screw you up. For example, the pilot for your bike should have a larger emulsion tube and a ring at the thread/body junction. If your dealer just grabs a No45 - you could have problems if it is the old style.

4PE-14185-00-00 FLOAT $28.39
5A0-14116-E0-00 NEEDLE(6BEG9-70) $8.83
4KM-14142-45-00 JET,PILOT(#45) $5.90
4KM-14142-50-00 JET,PILOT(#50) $5.90
137-14143-66-A1 JET,MAIN #330 $7.35
137-14143-68-00 JET,MAIN #340 $7.56
137-14143-70-00 JET,MAIN #350 $7.56
3TC-14141-S8-00 NOZZLE,MAIN (S-8) $19.98

Take inventory of what you have so you don't buy double on some items.
Your bike shipped with the following:

350 Main
69 Needle - clip 3
55 Pilot
S-9 Nozzle
5.0 Slide

If your dealer put in a 1 step lower pilot, and that's it, I'd guess your current jetting is:
350M/69-2/50P@1.5 turns/S-9/5.0

Just a hunch, but my money wagers you will end up somewhere around
340M/69-2/45P@2.0 turns/S-8/5.0 for 80 degrees at ~1,000'

Skip

P.S. When you get your carb apart to inventory your jets, clean the carb up with spray carb cleaner. Pay attention to the tiny orifices and the pilot. These clog and wreak havok with jetting. You don't want to be chasing two problems. Also, make sure your hoses are attached and clean.

 

[skhasky]

Nozzles and needles and jets - oh my! The main jet screws into the bottom of the nozzle. The nozzle screws into the bottom of the carb. The needle rides in the hole in the center of the nozzle. At 0 to 1/16 throttle. a little air seeps past the slide, and the vacuum draws fuel from the pilot circuit. As you twist the throttle, the slide/needle raise allowing more air, and allowing fuel to flow from the nozzle (which has the main at the bottom). If you have a leaner nozzle, then it has a smaller hole in the middle. Think of it this way. the needle has a diameter - let's say tapered 1mm to 2.5 (not really, but for example). The nozzle has a 2mm hole. As the needle withdraws from the hole in the nozzle, there is space between the needle and the nozzle walls. This is where fuel gets by. If the needle were leaner (thicker) then there would be a smaller gap ther and the bike would run leaner at part throttle. If the hole in the nozzle were leaner (smaller) same story.... smaller space between the needle and the wall of the nozzle. So... you can get basically the same results by using a fatter needle or a smaller nozzle. See the relationship? It works the other way around too. You can change one or both the other way to get a richer part-throttle mixture. Now... your twisting the throttle to 3/4 and the needle/slide are coming up farther (more out of the hole in the nozzle) this allows more air, since the slide is lifting - and more fuel since there is less obstruction by the needle to the nozzle. Finally, you get to 4/4 throttle. The needle no longer effictively obstruts the flow of the nozzle which is being fed by the main jet attached at the bottom. Now here's the $40 million question. If needle taper and nozzle diameter do the same, which is better? If you run lean at 3/4 to full then the nozzle may be obstructing the flow from the main... use a larger nozzle. But that may end up in a rich condition at 1/4 throttle since the space between the needle and nozzle tube is too wide (too much flow). So, use a leaner needle. If it is rich on top and lean on the bottom, switch them! Use a richer needle with a smaller (leaner) nozzle. Don't get wound up to much as to what jet controls what. It is a system. To some extent they all effect each other!

 

[MikeT]

You are right
I have 350M/69-2/50P@1.5 turns/S-9/5.0

 

[skhasky]

You may try backing that airscrew out before putting in the 45 pilot....

 

[MikeT]

Originally posted by skhasky
If you run lean at 3/4 to full then the nozzle may be obstructing the flow from the main... use a larger nozzle. But that may end up in a rich condition at 1/4 throttle since the space between the needle and nozzle tube is too wide (too much flow). So, use a leaner needle. If it is rich on top and lean on the bottom, switch them! Use a richer needle with a smaller (leaner) nozzle.

I forgot to check how many turns the air screw is out last night, I'll do that tonight. I have it pretty dialed in (a tiny bit rich) on top and REALLY rich on the bottom, so from what you wrote, I think I'll try a 340M/70-3/45P@2turns/S-9/5.0 Does that sound like the perscription for making the bottom a good deal leaner and the top a little leaner?? I think if I did the 70 nozzle I might make the top too lean.

 

[skhasky]

>I think I'll try a 340M/70-3/45P@2turns/S-9/5.0 Does that sound like the >perscription for making the bottom a good deal leaner and the top a little >leaner??

The 340 Main will make the top a little leaner, and switching to the 70 taper needle will make the mid leaner. The 45 pilot kicks in at the low, and with 2 turns on the air it sounds reasonable to me!

>I think if I did the 70 nozzle I might make the top too lean.

Do you mean the S-8 nozzle?

Skip

 

[MikeT]

Ya, S-8 Nozzle :confused:

 

[skhasky]

Give it a go! Be careful, though... these are speculated jetting calculations... You want to test enough to know that you aren't overly lean somewhere. Sounds like you know what a properly jetted main feels like, and I described a properly jetted pilot. For the mid (needle/nozzle) for me, I test by rolling on the throttle at various points - being ever-watchfulr for the dreaded buuwaaahhh that indicates too lean of a mixture.

Skip

 

[MikeT]

I think if anything, I can switch out the main pretty easy back to the 350 if necessary. Currently this bike is so rich down low I don't think it even knows what lean means!

 

[MikeT]

skhasky, I checked the air screw and it is currently turned out 1 3/4 turns, So, I'm going to order the jets/needle.

 

[Jaybird]

Mike, you may need to vary a bit from the recommended turning of the air-screw +/-.
It is just a guage point. I like to set mine at high idle.

 

[whyzee]

Mike, another thing to remember, our bike does not like getting close to one turn out or less on the air screw. A 45... I'm at a 60 @ 300'ASL and just a tad lean still at 1 1/2 turns out. Skip sounds like he knows his stuff re: nozzles and needles. This is the needle information I was trying to confirm about a month ago. Thanks Skip, and good luck Mike. Skip, give me your feedback about running a leaner needle vs raising the clip.
Thanks, Dave