For any "TTR Old Timers": During the format change (05/16/04) I copied this posting by Donkey and saved it on my PC. During the format change, this ENTIRE posting disappeared. Hence a very slight change in this post: my name appears as the poster vice Donkey who gets ABSOLUTE 100% credit for this!!!! 


1. Stock carb jetting 
Mikuni VM20SS 
17.5 pilot 
110-120 main, (depending on mods) 
The needle works best in the stock position, or middle clip. 
Jets can be found here: http://www.sudco.com/ The mains are the N100.604 Large Round Type, and pilots are the VM28/486 Type. 

You can order the correct TTR125 jets right from the Thumpertalk marketplace. 

Large round type Mikuni main jets (N100.604) 
107.5 TT part #MK-183 
110.0 TT part #MK-184 
112.5 TT part #MK-185 
115.0 TT part #MK-186 
117.5 TT part #MK-187 
120.0 TT part #MK-188 

VM28/486 Pilot Jets 
17.5 TT part #MK-031 
20.0 TT part #MK-032 

They're $1.93 each 

Here's a link to a detail drawing of the pilot and main jets in the carb of a 2002 TT-R125L. Bear in mind this bike is a California model: 

TT-R125L Pilot & Main Jets 


2. Airbox Mod 
Consists of removing the snorkle/plug from the top of the airbox and removing the screen from behind the filter. Every TTR should have this performed immediately upon delivery. Even more air flow can be achieved by cutting out the entire top section of the box and adding a high flow aftermarket air filter. This will require a richer pilot and main jet and tuning of the fuel screw to get the best response and power. However, it should be taken into consideration that the more extensive airbox mods may not be a good idea for someone who rides in extreme conditions often. With the top completely removed water or sand could easily get into the box and cause the bike to run poorly. 

3. Flywheel mod 
Basically, the flywheel creates rotating mass in your engine, keeping it spinning when you let off the gas. The "flywheel mod" involves machining material (up to 18oz.) from the flywheel, making it lighter, and in effect, allowing your engine to rev faster, or "spin up" quicker. The drawback is that the engine is easier to stall. Some internet companies charge over $100 for this service, but most local machine shops will usually do it for as little as $25. Overall opinion is that this is a very worthwhile mod for the money, but is better left to the more experienced riders. 

4. BBR frame cradle compliancy 
The BBR frame cradle is an essential item for the TTR. It adds much needed rigidity to the frame and also protects the engine, while at the same time, being very light weight. 

These pipes have been proven to fit well with the BBR frame cradle: 

BBR (of course) 
M4 
Pro Circuit 
GYT-R 
FMF 
DMC 
Engines Only 
Powroll 
Big Gun 

Pipes that have been proven not to fit: 

Stock pipe (oddly enough) 
Yoshimura 
White Brothers 

5. Handlebars and Handguards 
Handlebars: 
The stockers are weak, but there are many aftermarket bars to choose from in all shapes and sizes. However, most every rider prefers a slightly different setup, so you need to put some thought into how you like to feel and position yourself on the bike beforehand. The best thing to do is measure the stockers up, and compare those #'s to what's listed on the Renthal/TAG/ProTaper web sites. That way you can get a good idea of what you want, and what to expect. Oversized, 1-1/8" bars are great, but will require adapters or a new triple clamp. They damp out most of the vibration and resist bending like you wouldn't believe. Not to mention the fact that they look cool. But the extra thick 7/8" bars aren't bad either, and will bolt right up with no extra parts to purchase. You can buy your handlebars from a million different places, but look around for the best deals. TAG, Pro Taper, and Renthal all make oversized billet adapters for around $75, but remember, they raise the bar height 3/4 of an inch themselves, so keep that in mind when figuring out which bend you want. Triple clamps with oversized mounts can be found here: http://www.rswracing.com/ here: http://www.precision-racing-components.com/ and here: http://www.bbrmotorsports.com/Home.htm At prices from $100-200. I suggest the RSW triple clamp. Ron also makes a very nice and highly effective billet fork brace, which at $70, is probably the best deal on a performance accessory that you'll find anywhere. 

Handguards: 
The Acerbis Rally Pro's are the best thing goin' at $75. But reguardless of brand, the key is the sturdy reinforcing aluminum backbone. The plastic brushguards do little to protect your hands or the levers during a get off. If you are using oversized bars, you'll also need adapters for the handguards. Of course, all the major players can supply you with their version, and all seem to work well. With good bars and handguards your bike becomes almost indestructable. No more bent or broken levers. And no more fingers smashed between the bar and the ground. 

6. Other Basic Mods 
Sprocket changes are sometimes necessary. For more top speed go with a larger front sprocket or smaller rear. For more low end power do the opposite, smaller front or larger rear. The 14 and 15 tooth front sprockets work well and are readily available from Sprocket Specialists here: http://www.motorcyclesprockets.com/html/main.shtml They also have a wide selection of much lighter aluminum rear sprockets. 

An actual chain guide is another well advised security mearsure for the TTR. There are many on the market, more popular versions being sold by BBR and PRC. 

A stainless steel brake line is one of the best $$$ value mods available, and adds great feel and extra power to the TTR's front disc. I prefer Goodridge lines and have used them on all my bikes for a long time with great success, but there are several other brands available. Galfer, Speigler, Russell, ect... Most any manufacturer can make a custom line in any length you need with all the hardware in any configuration you want. Prices range from $50-90. Keep in mind, the stock rubber lines are supposed to be replaced every two to three years. Stainless lines are good for at least 10. 

Brake pads can also be upgraded. The stock pads offer decent feel and power, but they overheat and fade quickly with the small rotor. Again, there are several brands on the market to choose from, and many times it comes down to personal preference. 

7. YZ carb mod 
The last generation of round slide carbs from the YZ80 make great improvements to the TTR for little $$$ and fit right up with no modification. (2.5mm can be milled off the engine side for a perfect fit, but is not necessary.) They are particular about jetting, but make very good power. Look on Ebay and in junk yards for a clean model from 1982 to 2001. They are all basically the same and can be had for as little as $20. Strait from a YZ, they will be very rich on top and bottom, and lean in the middle on the TTR. A good place to start with rejetting is a 20-22.5 pilot and 140-150 main. The best needle is still undecided, but any of the richest Mikuni 4 or 5 series needles could work. (The stock YZ needle seems to fall just on the rich side of the middle of the charts in the Mikuni catalouges.) Slide sizes could also be changed. The YZ's slide is a #3. A #2 or even #1.5 could work well? Many folks have opt'ed to replace the stock O-0 needle jet with a P-4 size and keep the YZ needle. This seems to work well too. But remember, jetting will be different on nearly everyone's bike, so what works great for one, may not work as well for another. These are all just generalized places to start. Getting your bike finely tuned is half the fun. But overall, the YZ80 carb mod is very much worthwhile for the added power and response. It also enables the use of a pod filter, which is impossible with the stock carb. All necessary jet's and needles can be found here: http://www.sudco.com/ and here: http://www.mikunioz.com/ 

8. Suspension mods 
The best money you'll ever spend on any motorcycle is in getting the suspension dialed in. Not only will it enable you to go much faster, but you'll be doing it with less effort and more direct control of your bike. There are several routes that can be taken to upgrade the TTR's stock suspension. The easiest, and most common thing to do is add heavier aftermarket springs front and rear, thicker oil to the stock forks (10-15wt.), and purchase stronger billet triple clamps and a fork brace. 

Those mods alone make terrific improvements over stock, but can still fall short for many larger and/or more aggressive riders. The ultimate suspension upgrade for the TTR125 is to replace the entire front end with an inverted forked, and much more rigid, '93-present YZ80/85 or KX80/85 front end. You'll need to change the springs to match the heavier bike and there's some simple adaptation required, but a kit for the YZ-TTR swap can be found here: http://precision-racing-components.com/yamahaparts.html  The YZ forks are much longer than the stock forks and can be shortened to maintain the stock ride height or a BBR swingarm can be purchased to raise the rear 1.5". They will also need to be matched up with a shock in the rear that can equal their performance level. The Works Performance shock seems to be the popular choice, but the newer generation TTR shocks with the remote reservoir can be rebuilt with a good deal of success. 

9. Motard Conversion 
The most common modification is to lace wider aftermarket 17 inch rims, such as Excel or Sun, to the stock hubs with heavy duty spokes. They are available here: http://www.buchananspokes.com / and here: http://eastcoastwheels.com / 

All year model TTR's with a stock or YZ/KX front end can run either a 2.5/17" or 2.75/17" front rim. The 2.5" is the preferred and standard 125GP size, but restricts your tire choices to full blown race tires if you want the proper profile to be maintained. The 2.75" rim is wider than necessary, but allows the more common 110/60 size sportbike front tires to be used without distorting their intended profile. Whether you are able to use the entire tire surface with that size has yet to be proven, though. 

All '02 and older TTR's, and newer standard and "L" models without the aluminum swingarm, are restricted to a 2.75/17" rear rim with a 110/70 tire. The '03 and newer "LE" models with the wider aluminum swingarm have the choice between the 2.75/17", or the preferred 125GP sized 3.5/17", rear rims. The 3.5" wide rear rim allows you to run tires up to a 120/70 without any issues. 120/70 is the most common size of sportbike front tire, and they work great as rears if turned backwards. High performance street tires, DOT race tires, and full blown slicks are all available. 

The 125GP setup (2.5"f, 3.5"r) is more restrictive as to front tire choice, but offers the best performing tires and sizes, and easily available rears in the most common size of all. Only three manufacturers make tires for the 125GP bikes, Michelin, Bridgestone and Dunlop, but they are all very good. A set at retail is expensive though. $200-300 front and rear. You do have the option of slicks or rains, and Dunlop and Michelin offer two compound choices. Much cheaper take-offs are out there to be had too. The Dunlops feel great and last a very long time, but are a harder compound rated for higher temps, so they take a while to "come on".. They also have a very stiff carcass so they tend to chatter more than slide sometimes. The Bridgestones and rated for a much lower optimum operating temp, so they heat up faster and work better at the lower temps that the TTR is likely to see. However, they are softer and don't last nearly as long as the Dunlops. They are cheaper though. The Michelin front tire doesn't suit my riding style, but I do like their rear tire. 

The 2.75" setup has it's advantages in the fact that many decent tires are readily available, and can be had for cheap. However, it doesn't offer the all-out performance potential of the GP setup with slicks and a fatter rear. The steering is a little slower, and the response isn't as good with the wider tires. But there's plenty of grip to be had with good street tires. It also takes a lot more suspension to deal with the super grip of the slicks. Street tires may be just as fast, if not faster on some bikes. 

10. Extreme Tuning 
Of course, there are those that will never be satisfied with the amount of power their bike makes. Thankfully, I'm one of you. 

The sickest power mods for the TTR begin with the camshaft. BBR, Powroll, PRC, and HotCam all make excellent cams for the TTR. I love my HotCam and Wiseco makes a drop in 11:1 high compression piston that works great with it. 

Next up would be porting, polishing, and head and valve work. In this area it's really all about who you know and how good they are. I doubt you'll pry any secrets from the real competitors about the specifics here. Just have to find out for yourself. But the TTR's do have a good bit of power in there. 

Big bore. What else needs to be said? There are several 150 kits on the market, and Powroll even offers the mac-daddy 170 stroker. And if you can't find enough juice for your TTR in 170ville then you've got the wrong idea entirely. 

Links: 
BBR - www.bbrmotorsports.com/Home.htm  
Powroll - 
PRC - www.precision-racing-components.com/  
RSW Racing - www.rswracing.com  
Sprocket Specialists 
Sudco - www.sudco.com  
www.ttr125l.com     
 

I think this is from Kevin of ThumperTalk.com 
Jetting 101:

Quote:

I thought it would idle without the choke. But with when the choke was off it would not idle. But the opposite happened with the choke. 

Is there any particular reason for this? 

Your carb meters the fuel to your engine. The carb has three different sub systems: 

Pilot Jetting - for up to 1/4 throttle position 
Needle Jetting - 1/4 > 3/4 throttle position 
Main Jetting - 3/4 > full throttle 

These jets allow a fixed amount of fuel, the needle allowing an "operating band" of fuel metering. 

To alter the amount of fuel, you can either increase or decrease the orifice (the hole in the jet). This applies to your pilot and main jet. 

The needle jet is actually a needle. The fuel first passes through the main jet, then the needle limits the fuel flow (kinda like putting your pinky finger into a garden hose. Water still comes out, just not as much). 

The reason your bike won't idle: 

Fuel Screw 
The Fuel Screw is mis-labeled in the Yamaha manual. It is labeled as an "Air Screw". This is 100% backwards. Air screws are on two stroke carbs. The last I checked, this carb has been on this 4 stroke engine. 
The fuel screw allows more fuel through the pilot circuit. This very definitely affects low speed operation. To allow more fuel flow or richen up the bottom end, the fuel screw is turned counter-clockwise (or turned OUT). This IS richening up the bottom end. 
To lean it out, the fuel screw must be turned clockwise (or turned in). The problem with this low speed/idle jetting is it is too lean. The only time I think you would turn it in is if you decide to ride your TTR into the Rockie Mountains, where bikes are starving for air, which ain't there! 

Pilot Jet 
First, your pilot jet is absolutely too small. You need to go up in size to a 17.5. The fuel screw on your carb allows a VARIABLE amount of fuel to pass through the pilot jet. This fuel screw WILL ABSOLUTELY affect your idle. The fuel screw has a spring behind it that allows you to turn it without it falling out of your carb. Turning it in REDUCES the amount of fuel (leaning the fuel/air mixture) and conversely turning it out INCREASES the amount of fuel (richening the fuel/air mixture). The bike IS starving for fuel at idle because the pilot jet is too small. Regardless of the fuel screw position, your bike NEEDS MORE FUEL. If you end up turning your fuel screw out 3 turns or more, your pilot jet IS TOO SMALL. 

Needle Jet 
The needle jet can be changed as well, or raised or lowered. So far, I have not seen any recommendations to swap out needles. Needles vary in diameter, AND taper in size, i.e. the tip of the needle is skinnier than the diameter half way up the needle. So as the needle is being pulled up, the needle gets skinnier. This allows MORE fuel as the needle is raised up via the throttle cable. 

There is a clip on the needle that allows you to raise or lower the needle, effectively changing the amount of fuel flowing in this 1/4 > 3/4 throttle position. So, if you move the clip down, this effectively raises the needle. Since the needle is now higher in the carb, and we know the needle is skinnier at the bottom, MORE fuel will pass through in the midrange than before you moved the clip. 
As for swapping out the needle, the skinnier the needle, the more fuel allowed into your engine. 

Main Jet 
At 3/4 throttle and higher, the ONLY thing metering fuel is the main jet. If your bike has problems at wide open throttle, the main jet is your culprit (unless it's your ignition...???). A larger main allows more fuel. 

When you modify your bike, i.e. cut your airbox lid, install a high air flow airfilter, you are changing the amount of air into the engine. 
When you install an aftermarket exhaust pipe, more air (exhaust) is coming out of your engine. 
In BOTH of these conditions, you MUST increase the fuel going to your engine to offset the air going into (or out of) your engine. 

Moving more air means you have to move more fuel. 

Elevation & Weather Changes 
When there is a change in the weather or riding elevation, the amount of Oxygen or moisture will be the culprit. At higher elevations, there is less O2 available. You will need to jet lean to decrease the amount of fuel to compenstae for the lower O2. 
If it is humid, there is MORE water in the air, you MAY have to jet lean. 
In the winter, bikes run lean. Because of the lower temps and humidity, there is more O2 due to density changes. Your bike will run lean, and you will benefit w/ RICHER jetting. 

I hope this helps you understand carbs better. 


Kevin