This has probably been covered several different places. but I will be tackling the task of jetting a zilla carb for my 250. I asked Skeans how a good way to do this would be and this is what he pulled from a honda 250r site, I feel as though it is a good way to go about doing this job and I thought I would share with the group, Might also be a decent sticky.
For whatever reason it becomes necessary to re-jet a carburetor, it is without a doubt a nightmare if you do not have a procedure to follow. The following is nothing more than a technique, procedure, steps or whatever you want to call it to help identify and isolate the carburetor circuit involved.
You can only begin re-jetting your carburetor if the following conditions are met:
1. Top end is in good condition.
2. Bottom end is in good condition. Crank seals.
3. Spark plugs, air filters, reeds and so on.
If your bike's motor is not mechanically sound, then all the jetting in the world will not help. With all of the above conditions met, you should be able to jet your carburetor following these steps:
Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.
Step 2: DETERMINE THE CORRECT PILOT (slow)JET.
Step 3: DETERMINE THE CORRECT MAIN JET.
Step 4: DETERMINE THE CORRECT NEEDLE TAPER AND CUT AWAY.
Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.
Whether or not your carburetor is a MIKUNI or a KEIHIN, it does not matter. This is the most important step in jetting your carburetor--period!
1. Remove the main jet.
2. Place needle clip in mid-position.
3. Start motor and run it on the stand.
Condition: Motor running and main jet out. Needle or needle jet is correct: Carburetor should run clean to approximately 3/4 throttle. From 3/4 throttle to full throttle, the motor should start to break up as a result of too rich condition.
Correction: None needed.
Condition: Needle or needle jet is too rich. Carburetor runs clean to approximately 1/2 throttle but breaks up before 3/4 throttle as a result of too rich condition.
Correction: Mikuni replace needle jet with next leaner and test again. Keihin replace needle with next leaner diameter and test again.
Condition: Needle or needle jet is too lean: Carburetor runs clean beyond 3/4 throttle and has an erratic throttle response.
Correction: Mikuni replace needle jet with next richer and test again. Keihin replace needle with next richer diameter and test again.
The emphasis here is to find the correct needle or needle jet diameter, which will allow more fuel to pass than is needed but not so much that the needle itself has no control below 3/4 throttle.
Step 2: DETERMINE THE CORRECT PILOT JET.
1. Make sure the bike is warmed up if at all possible.
2. Main jet out.
3. Needle clip in mid position.
4. Turn air screw all the way in then 1/4 turn out.
5. Start motor and run it on the stand.
6. Adjust idle so the bike will just barely idle.
Condition: Motor running and main jet out.
PILOT JET CORRECT:
With one hand on the throttle maintaining RPM at approximately 1/8 throttle, turn air screw 1/4 turn at a time clock wise until you bottom it out. Motor should become slightly erratic and you should have to play with throttle to maintain RPM. Start turning air screw counter clockwise, 1/4 turn at a time until you have reached 2 3/4 turns out. Between 1 1/4 and 2 1/4 turns, your motor should have reached its highest RPM maintaining a steady throttle. Adjust air screw again between 1 1/4 and 2 1/4 until you have determined highest RPM. Quick throttle response should be clean without bog.
PILOT JET TOO RICH:
RPM does not reach a peak between 1 1/4 and 2 1/4 turns, stays the same or keeps rising out to 2 3/4 turns.
Correction: Mikuni replace pilot jet with next leaner and test again. Keihin replace pilot jet with next leaner and test again.
PILOT JET TOO LEAN:
RPM does not become erratic and motor maintains throttle when air screw is turned all the way clockwise.
Correction: Mikuni replace pilot jet with next richer and test again. Keihin replace pilot jet with next richer and test again. Remember, with a steady throttle approximately 1/8, there should be a distinct difference in RPM from 1 1/4 turns to 2 1/4 turns if the pilot jet is correct. The emphasis here is to find a pilot jet that will run crisp without bog and without the main jet.
Step 3: DETERMINE THE CORRECT MAIN JET.
The main jet selection process is easy once you have the correct needle diameter or needle jet. You now only have to correct a rich condition from 3/4 throttle on up and you know what a rich condition sounds like. Your pilot circuit is correct and without bog.
1. Replace main jet with one that is at least two sizes smaller.
2. Needle clip in mid position.
3. Start motor and run it on the stand.
By replacing the main jet with one that is too small, you are looking for a condition that is too lean. You adjust your main jet from a too small to lean condition.
Condition: Motor running and main jet in.
MAIN JET CORRECT:
Carburetor should run clean and crisp to full throttle.
Correction: None needed.
MAIN JET TOO RICH:
RPM reaches a peak slowly with a deep sound. Excess fuel and oil mixture at end of silencer. Spark plug fowls easily and is dark in color.
Correction: Mikuni replace main jet with next leaner and test again. Keihin replace main jet with next leaner and test again.
MAIN JET TOO LEAN:
RPM reaches a peak quickly but erratic. A quick full snap open of throttle causes the motor to hesitate BEWAH sound or a complete bog. Motor sounds like it has a ring to it. End of silencer white. Spark plug is white in color.
Correction: Mikuni replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. Keihin replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. The emphasis here is find a main jet that is just rich enough to allow you snap the throttle wide open without the motor bogging as a result of the main being too lean. Should be a quick crisp throttle with no hesitation.
Step 4: DETERMINE THE CORRECT NEEDLE TAPER AND CUT AWAY.
This step in the jetting process can be made very simple if you remain close to stock. However, your needle taper is adjusted for 1/2 throttle to 3/4 throttle. Start off with a rich taper (shallow taper angle) and keep going leaner (steeper taper angle) until it will not maintain constant RPM at 1/2 throttle (runs erratic). Go back to the leanest taper angle that ran the smoothest at 1/2 to 3/4 throttle and that should be the correct taper. The needle taper final test should be under track conditions with the greatest effect entering and exiting corners. Do not change the needle diameter or needle jet size during this process because that has already been determined. Adjust taper and throttle cut away only.
Throttle cut away affects from idle to 1/4 throttle. The correct cut away will maintain steady 1/8 throttle with quick throttle response. Generally the stock cut away is very close. Experiment with different cut away until it maintains the best response to 1/4 throttle.
QUICK TIPS:
Keep it simple, buy the optional OEM needles or needle jets that are available, as this may speed up the taper selection process. Don't skip any steps or you're just guessing.
FINAL THOUGHTS:
That's it, if you spend the time jetting correctly, the benefits you will gain definitely out-weigh way the time spent. Take the guesswork out of jetting by following a procedure that has been given or one you have laid out yourself. Keep the black magic process out of your tuning tricks and you will be better off for it.
ALSO, I was wondering where a good starting point would be for my jetting? or what works for who? I have a stock port, an LRD pipe, and a newly rebuilt motor that is fouling plugs like crazy
The above jetting procedure is filled with WRONG INFORMATION AND THEORY. I would look for another jetting procedure.
I would recommend that this procedure be removed. Following this procedure is going to cause inexperienced readers to be replacing a lot of burned pistons. I wish I could go through the above procedure line by line and show all of the errors, but I do not have the time with my busy schedule.
Ok, so with that being said. What are some methods that others have used or do use that work well?
I could just throw the carb on with its current set up, and adjust all over the place one piece/section of throttle at a time, but I feel that is just a shot in the dark and it will never get 100% right.
Thanks for the input Jerry. I'll be more then happy to remove the procedure but I feel we should replace it with a better solution.
Keep looking and posting them and I will look them over and give my opinion on their accuracy.
I do not know of any veteran tuner that learned his craft by reading and not making many expensive mistakes. Jetting is one of those skills only acquired by much trial and error. The information published by the manufacturer of the carburetor you are using is one of the best places to start as far as which circuit is responsible for fuel control at a particular throttle position. The above procedure did not even get these basics correct.
One of the most important things to remember is: the circuits overlap and if one circuit is too rich it can or will overlap into neighboring circuits and make them also rich. The same goes for one circuit being too lean and overlapping and causing neighboring circuits to also be lean.
Do not speed too much time trying to get one circuit perfect because you will more than likely have to retune that circuit many times before you get all of the circuits working together.
Most tuning procedures that I have seen published do not make their readers aware that changing the needle jet size or changing the constant diameter portion of the needle often affects the idle mixture. Changing the needle jet size or the constant diameter portion of the needle has a huge influence on the mixture on closed throttle high RPM deceleration. Moving the clip one position can affect the main jet size. Changing the taper of the needle can affect main jet size. These last four tips are some of the most miss-understood and overlooked areas that are responsible for addressing the wrong circuit when tuning a carburetor.
Ok I just did the TM38-85 swap on my 92 small Reed motor lt. My motor has been built by lrd some years back and was a backup ice race motor for my friend. It is fresh and I now have it in my bike. I have an 89 zilla needle middle clip, Q2 needle jet (I think the only numbers were Q2, could be missing some but whatever), air screw out 1 1/8 turn, slide at just under 3mm, 30 pilot and was told by Carl to start out at a 460 main and work down from there.
I started out with a 35 pilot, to rich the 30 is perfect (for my bike) and my 460 main is too lean, I am now at a 480-490 that's at 80 degrees temp outside and mild humidity. So maybe 65 I'm running the 490 and 75-80 I'm at the 480. (For my bike)
I am not going to throw out "tuning advice" I got my baseline starting points from Gilles and Carl, and I just got MY bike set to where it needs to be. It was a lot of trial and error but I am happy with it as of now. Runs very well.
I am also running a full lrd exhaust, no airbox, 185 compression with 110fuel.
I found your procedure very good.
If that can help you there my setup with tmx 38mm carburetor
I use tmx 38mm on my lt250r 1988, fmf pipe/exhaust
I have porting, boyesen ported intake /reed. well oiled uni air filter using maxima air filter oil.
25 pj /410 mj with removed airbox lid
27.5 pj / 380mj with closed airbox lid
the jet needle is s-7 and don't remember the need jet.
Thanks guys.
I just got the 38mm in (that was one hell of a fight) using the stock boots and a hair dryer. I did not mess with the needle or the pilot just yet, so I could get an idea of what needs to be adjusted before I start changing everything. I have a 240 main in it, needle in the mid position, I think shes running a little rich on the bottom side. I posted up a video:
https://www.youtube.com/watch?v=HBhV7t66PEs&feature=youtu.beThis was a cold start and run, I have not taken it for a run yet because I gotta change the tires out.
What do you guys think?
Well, it's idling. When you turn the air/fuel screw does the idle change? I assume that's how you set the idle.
So that was another question that I had, my 34mm had a big screw that I could turn by hand for that. This 38mm does not, it does have a common screw head on the side that is brass is this now my air/fuel screw?
why are you running 38mm carb with the stock boot ?
I suggest you put bigger main jet until you found witch one will net let the engine rev. act like a rev limiter
if I am you I will start with 350 mj
also, you must ride the bike to test the main jet and the needle position.
your engine must be hot
a engine that is lean is smoking a lot when cold ....
you should have 2 adjustment on the carburetor one is the idle air and the other is the slide height ( idle speed )
as Nekro said you should have a brass screw that is on the left side of the carb, this is air/fuel screw. than on the right side of the carb there should be a knob with a spring under the knob. this is your slide height adjustment. you will want to set the slide height to maybe 2.5-3mm open using the adjustment knob only, you adjust the idle with the air fuel screw. you may have to adjust the slide height just a tad to dial in YOUR motor. its just a starting point. you do not want your slide more open than 4mm. shouldnt have to be that high. your bike seems to be idling pretty fast.
also you are going to want to go buy youself some jets man. your gunna want to start with a 350 main and i would imagine a 30 pilot. buy youself a bunch of jets 350,340,330,320,300 for mains and maybe like 3 pilots 32.5,30,27.5 maybe. your going to need the jets unless you only ride in say 80 degree weather. but here in wisconsin i have 100 degree summer days and in winter i have 20 below zero. so i have jets from 540 all the way to a 440. and numerous pilots.
dont blow your motor and richen that thing up.
brand new plugs are a must when tuning a new carb. keep in mind if your running just a little bit to lean your bike will run better than it ever has right before something bad happens.
Thanks guys, So I see the brass screw on the left side of the carb the air/fuel screw. I am not seeing the other adjustment though, here are some pictures of the carb that I am using.
http://www.suzukiquadracerhq.com/lt250/what-carb-is-this/msg47336/#msg47336 I very well could just be blind. I agree with you guys it is idling VERY high. and I would much rather run it a little rich then potentially screwing up my motor. I did try changing the air/fuel screw but it did not really change anything. Another thing that I noticed is that when I pull the choke the idle slows down, instead of speeding up. The idle is also not very consistent, it raises and lowers without even touching the throttle.
I was going to grab some jets today however, the bike shop closed early. and of course they are closed Saturday-Monday so it'll have to wait until next week. Last time I was there I did make sure to grab plenty of plugs, as well as a new clean air filter so were good on that side.
I'll probably grab several sets of jets next Friday, I'm hoping to have this thing running decent by the end of the month for dunefest.
on the left side of the carburetor ( intake facing you ) there is a crew ?? at the place you should have a thumb adjusting screw , if you remove the slide you should see a screw that is adjusting the slide height .. ( idle speed )
when that screw is screw in, that raise the slide ...
the lt250r thumb idle adjusting screw is the same part for a lt500r carburetor like your.
It doesn't have or even have the boss for the idle screw on the carb body.
It doesn't have or even have the boss for the idle screw on the carb body.
you are right.
i guest the only way to adjust the slide height is using the throttle cable adjuster ...
using a stock booth you should use yout stock carb man ...
Yep that is exactly what I decided to do. At least until I can get a better carb. I do not like that 38mm just like skeans said it does not have all of the adjustments like it should. So right now I have the 34mm in and I'm just going to be working on jetting that.
Quick question: What should my float height be on this 34mm I saw online 22-24mm can anyone confirm this?
Thanks for the help guy
I'll post back when I make some headway.