Yamaha TY-350 Modifications

Any or all of the modifications listed below may cause permanent and irreversible damage to you or your motorcycle, and/or may result in a motorcycle that is unsafe and dangerous to ride. Neither my self, my heirs, nor my associates are responsible for any damages or injuries that occur as a result of the following modifications and suggestions.


Bob Burkart recommended the carburetor modification. He says that it helps with the transition from idle to higher throttle settings and helps with the Pinging problem that 350s seem to have when you shut the throttle down. It involves increasing the throttle slide cutaway, and changing to a larger pilot jet, or drilling the present pilot jet. The larger pilot jet enriches the mixture across the entire range of throttle operation. But it makes the mixture far to rich at idle. Increasing the throttle slider cutaway leans the mixture at idle only and so compensates for the richer pilot jet. This is for the stock carburetor only.

Be sure you check to see if this modification hasn't already been done before you change anything or spend any money. To check remove the slider and look at the bottom of the slider and the bottom edge of the cutaway. If the bottom of the slider has a 25 stamped on it and the bottom edge of the cutaway still has chrome on it then the mod hasn't been done yet. If the slider is a 35 or the bottom edge of the cutaway is brass colored then it's likely that this modification has already been done.

You will need a couple of sheets each of WetOrDry sandpaper in the following weights; 180, 400, 600, a FLAT surface for grinding. The best thing that I've found for a Flat surface is a piece of glass. A piece of thick Mirror, about 12" x 12" would be ideal. It seems that mirror glass has fewer defects than window glass.

Remove the carburetor slider. Looking at the slide with the cutaway facing you make a mark that 1mm up from the bottom front edge of the cutaway on the center line of the slider. After you make that mark then measure down from the top of the slider to the mark and record that distance. If you lose the mark during the grinding process you will use that distance to figure out when to stop grinding.

Make some soapy water using dish detergent. Take a piece of 180 grit WetOrDry wet it down real good with the soapy water and place it, cutting face up, on the glass, which should also be on a solid flat surface. If you wet the glass it will tend to stick to whatever flat clean surface you put it on and make the grinding easier. Being extremely careful not to alter the point where the cutaway arc intersects the side of the slider, sand the slider down to your 1mm mark. Keeping the sandpaper soaked with soapy water will ease the cutting and speed the process. As you get closer to your mark go to progressively finer grits of sand paper finishing with the 600. When finished you should have a cutaway that matches the factory in finish and sharpness at the edges. Clean, dry and reinstall the slider. Test the motorcycle. It should run but not Idle.

If you can get it to Idle with Air Jet Screw between 1 and 2 1/2 turns then stop. You're finished. If the airscrew adjusts greater than 2 1/2 turns then you need the next smaller pilot jet. If the adjustment of the airscrew is less than 1 turn you need the next larger pilot jet or you need to drill the one you have.

If you want to drill out the Pilot jet I would recommend going to a Good Welding supply store and get a Tip Drill set. This should include a set of small drills including a .015(#?), .018"(#77) and .020"(#77) and a drill vice. This size of drill bits are very fragile and may be too small to fit in a regular drill, so the drill vice included in the tip drill set makes the whole process easier and you get a lot of other really small drills. The drills will be used to drill out the pilot jet.

DO NOT use these drills to drill Main jets!! The sizing of the holes in the main jets is far more precise that can be achieved with these drills.

Remove the pilot jet. Starting with the smallest drill in the set try to fit it into the jet. If it fits then move up a size until you find the size that won't fit. Using that one to drill out the jet. Reinstall the Pilot Jet and do the Idle adjustment again. Again if you can get it to Idle with Air Jet Screw between 1 and 2 1/2 turns then stop. You're finished. If the adjustment of the airscrew is less than 1 turn remove the pilot jet and drill it out with the next larger drill and repeat the test. If the air screw adjusts greater than 2 1/2 turns then you have over drilled this jet and now to buy a new pilot jet.

I had a 36 pilot jet as an original and then went to a 32, which was too small. So I drilled it out with a .015 tip drill and it works perfect

Cylinder Head

The TY-350 often pings when it's warm. Part of the reason is that the Squish Band is poorly designed. A cheap remedy, is to remove the head gasket. Remove the head, and the studs that hold the head to the cylinder. Put some valve-grinding compound on the gasket mating surfaces, then lap the head to the cylinder. Remove all of the valve-grinding compound, and clean the surfaces with brake cleaner, acetone, etc. You need a clean dry surface where the cylinder and head meet. Reinstall the studs, and put a thin layer of high temperature silicone gasket sealer on the mating surfaces and reinstall the head. This will reduce the squish band clearance from about 0.080 to 0.040 inch. Which works a lot better, and will reduce pinging. To find out why, you will have to study two-stroke theory.


If you remove the flywheel weight from the flywheel the motorcycle will hit harder, quicker and will wind down from high rpm quicker. It will be more responsive and have less run on (same thing different words). It makes the 350 feel frisker. The downsides are that it's an all or nothing procedure, it removes all of the extra weight and is irreversible, and also the motor is less forgiving of improper clutch technique at low rpm.

A considerable amount of the flywheel effect comes from a metal band that is apparently heat shrunk on the flywheel. There are two ways to remove the weight. They both require you to cut across the wide part of the weight down to the flywheel whereupon the weight will separate from the flywheel. The difference between the methods described, is that one I've tried and the other I haven't HO HO!

In accordance with the maintenance manual, remove the skidpan, and the flywheel cover. Find the timing mark stamped on the flywheel weight and transfer that mark to the flywheel. Make the new mark permanent but DO NOT HIT THE FLYWHEEL WITH A HAMMER TRYING TO MAKE THE NEW MARK. Impact will tend to dislodge the flywheel on the crankshaft, bend the crankshaft, dislodge the crankshaft bearings, break the crankcase seal and demagnetize the Ignition and lighting magnets in the flywheel. So, it might not be a good idea to use a hammer and a chisel to make a new mark. Try scratching a mark with a sharp tool; just don't make the scratch to deep for reasons that will be explained below.

Then, if you can, fit a SHARP hacksaw blade between the outer edge of the flywheel weight and the case, then start sawing. REMEMBER, take your time and try to stay square with the wide part of the band. DO NOT CUT INTO THE FLYWHEEL!!! If you nick the flywheel it will provide a place for a stress crack to start (same thing for a deep scratch). I put about a .003 in nick in mine and I don't anticipate any problems but you do want to be careful not to let the hacksaw cut into the flywheel.

Also try to control the metal chips that come from the sawing. Temporally wrap the flywheel in Saran wrap to prevent chips from entering the flywheel. Since the Yamaha has no moving parts in the flywheel section, a few chips may not cause any problems, but good practice dictates that ANY metal shavings are a no no. The magnets, that are a part of the flywheel, will attract the chips, so try placing the hacksaw blade so the cut takes place on the pull stroke, that way the chips will tend to fall out of the flywheel area.

The above method will require more time to cut the band than the one that I used but it doesn't require any special tools, just lots of care and patience

The method that I used was pretty much the same except I removed the flywheel and used a metal cutting band saw to cut the band. If you remove the flywheel and put it in a vise, it will take less time to cut the weight off, it will be easier to remove the metal filings, and easier to transfer the timing mark from the weight to the flywheel. You'll need a flywheel puller, a flywheel holding tool, a 19mm (3/4" will work) thin wall socket, and a torque wrench.

The flywheel puller is a special tool that you should be able to get at almost any Motorcycle Shop, or the Trials Shop.

The flywheel holder keeps the flywheel from turning when you remove and install the flywheel nut. It will be harder to find. If you can't find one, try putting the bike in gear and holding the rear brake. Try a higher gear first. If that doesn't work try wrapping a tie down strap around the flywheel and hook it to the foot peg to hold the flywheel. Wrap it so that as the flywheel turns it tightens the strap.

A thin wall 19mm socket may also be hard to find but cheap 3/4" sockets are generally thin wall. I used a Sears Craftsman 3/4", 3/8" drive, #44337 with a 3/8" to 1/2" adapter and it worked real well.

Shock extension
The Stock Yamaha suspension is not very progressive, and the bike has an awful lot of rake for a trials bike. One cheap and easy way to fix this is to extend the length of the stock shock. If you extend the stock length 0.31" then it moves position of the shock actuation on the arc of the transfer link. This increases the progressivity of the suspension. It also raises the rear of the bike, which increases ground clearance, and decreases the rake. The disadvantage is that it also raises the rear fender and seat height.

Remove the bottom shock attachment from the shock and Carefully grind or cut off the U-bracket at the weld. Fabricate another U-bracket with the holes extended 0.31". Weld it on to the attachment, reassemble and reinstall. You will notice the difference.

Shock Preload

I have heard that preload should be set so that the bike sags 1/2" under its own weight. If you extend the shock you might need to increase the preload past what the maximum stock setting is. This can be done by cutting a groove be cut in the body of the shock. Remove the shock and the spring and adjusting hardware from the shock body. Then take the shock to a machine shop and cut a groove identical to the one already cut into the shock body, 12mm closer to the shock rod. This will place the lowest possible setting, one setting above the highest stock setting.

Shock mod

Foot Peg Mounts

The foot pegs were lowered 1.75". I took measurements from the center of the seat mount bracket hole, the center of the swing arm pivot bolt and the center of the foot peg spring hole.

The drawing included shows the new measurements. Before you cut the pegs off double check these measurements to see if they will result in the location that you want. There could be differences between frames, which may affect the accuracy of the above distances. . Remember to check the angle of the mount before removing it and be prepared to be able to duplicate that angle when you jig it up. The foot peg mount angle is relatively critical.

Remove the foot pegs and hardware. Remove rear brake lever and hardware. Cut the triangular foot peg brackets off of the frame. Make the cut just on the peg side of the welds. I used a die grinder with a cutoff wheel. WARNING Ensure that you DO NOT CUT INTO THE FRAME when you remove the Bracket. After you have removed the brackets grind off the old welds down to the profile of the frame tube.

You will need to make a spacer to go between the frame and the old bracket. Jig the old bracket in it's new location and then using cardboard slightly stiffer than a 3X5 card make a template that fills the gap between the old bracket and the frame. You will need 4 templates, inside and outside each peg both sides.

I made my templates so that they were slightly longer than the distance that the template contacts the frame and bracket. That way when I made the metal spacer plate I was able to bend the tops and bottoms of the plate over so that when welding them to the frame the inners and outers joined at the tops and bottoms.

Fabricating the brackets will require some patience. Take your time here because a good fit will make the new bracket much stronger. Weld the spacers on the frame and weld the footpeg brackets to the spacers. Clean up the welds and paint.

With the Pegs lowered 1.75" the Center of gravity of the bike is lowered which results in much more controllability, and gives the rider a more stand up stance. The down side is that your feet are closer to the bottom of the skidpan and I've caught my toes on some obstacles. Also the brake cable will rub along the swing arm. This may affect braking slightly, but I haven't noticed it.

NOTE: Some folks claim that the front end of the TY-350 is too light. If you lower the foot pegs, you might want to consider moving them forward as well. Moving the pegs forward 1" will transfer more of your body weight on to the front wheel. The modification may also be easier because you might not have to make a spacer, or at least the spacer will be smaller. I rode a TY250 in Oklahoma where the guy had just cut the peg brackets off the frame and then slid them down the frame tube to where he wanted them and rewelded them back on the frame. I don't know how far forward this places the pegs. When I rode that bike, all I noticed was how much easier the bike was to balance and how stable it felt, so I was convinced that lowering the pegs was the hot ticket. If I was to do it again I might consider moving the pegs forward.

Throttle Spacer

The Throttle Spacer is a piece of plastic that fits in the cable groove of the twist grip. Its purpose is to increase progressively the throttle action. The stock twist grip is an approximately a 3/8ths grip. This means that that it is about 3/8ths of a turn of the twist grip to go from closed throttle to wide open. When you place this piece in the grip it changes it to a progressive 1/4 grip. The first part of the turn is the same rate as the 3/8ths, but from about 1/4 throttle and up the throttle opens faster for each degree of twist. The result is a fine throttle control at low throttle settings combined with the ability to get to full throttle without having to move your hand on the grip. Basically it makes the groove increase in diameter as you turn the grip which pulls more cable quicker as you turn the grip.

It's a piece of plumbing plastic. I think I made mine out of 5/8ths-inch schedule 40, the white stuff. Take a fine saw and cut a small slice off the pipe so that it will fit in the cable groove. Then split it in one place and file at the split "B" to make a ramp for the throttle cable. The thickness "C" could be adjusted to increase or decrease the rate of change. Trim away the excess length to fit, "A" install and test. I used 5-minute epoxy to glue it to the twist grip. It made the install a bit more permanent. Another way to do the same thing would be to tape a waxpaper dam around the part of the twist grip where the throttle cable goes and fill that area with J B weld (another epoxy material) and then shape the semi hardened J B weld with a file to get the right shape. Test some J B weld first to see how it works and files.


I've shortened the bars to 31.5" and they work well for me. When you shop for new ones look for bars that have the bends close in to center. Then you will have more room to adjust the location of the clutch and brake perches.

Clutch Arm

The stock clutch feel was too stiff for me to use the clutch with one finger so I extended the length of the stock arm. The length of the external clutch arm is 2.5". If you extend the clutch arm don't just tack a piece on the end of the clutch arm on the centerline of the existing piece. That will place the hole for the cable too far away from the end of the cable itself. Instead, with the clutch and cable adjusted properly, remove the cable end from the clutch arm and swing the cable end toward the cylinder until it is 2.5" from the center of the clutch arm pivot. This will show you where to place the hole on the extension. As you will see it has to be at an angle to the centerline of the clutch arm.

Disk Brakes

I installed a disk brake on my TY-350 and one on my son's. They both work better than the stock drum brake. I got most of my setup from Bob Ginder of B&J Racing by trading a 1969 Greeves for a Fantic front wheel, a Grimica master cylinder, a stainless steel braided brake line, and an mounting bracket. Jesse's was purchased from Mike McLaughlin. It included a 21" rim laced to a ?????? hub, a brake hose, a Honda master, and a Honda single puck slave.

I used the Honda master and hose with the B&J bracket, the Fantic front wheel and a Honda Rebel slave on my 350. On Jesse's I used the Grimica master, the stainless steel braided line, the custom wheel and a Honda MB50 slave.

Here is the data that I gathered while looking for a Disc brake setup. The information is incomplete because I stopped looking when I traded the Greeves for the Yamaha Kit. The best combination that I came up with before I stopped was as follows;


1. Kawasaki KX-80, 28 spokes 2. Suzuki RM-80, 32 spokes


1. Honda type unknown 11mm 2. Yamaha RX-50 10mm

1. Honda Rebel 250. 2. Honda MB-50


1. B&J Racing 2. Custom Made 3. Weld Slider

The Honda Master cylinder is the only one that I found that has an 11mm bore. I don't know what it came from because it came with the kit from Mike Mc.. The RX-50 master was listed because it is the only one that I found with a 10mm bore other than a Grimeca.

The Honda Rebel Caliper was selected because of the way that it mounts on the stock motorcycle. It and the MB50 caliper are located on the rear side of the right slider. The Trials installation mounts the caliper on the forward side of the left slider. So you must look for a caliper that mounts on that side, or on the rear of the right slider. Also the Rebel caliper has 2 pucks and a pad with a larger surface area. Although it is heavier then the single puck calipers, I thought that the extra weight would be offset by an anticipated increase in braking effectiveness. The MB-50 caliper was my 2nd choice because it is a single puck, and the pads have less surface area. Most motocrossers have the brake on the rear of the left slider and so are not desirable for a trials installation where dropping the front wheel into a crevice might hit a caliper installed on the back of the slider.

The choice of brackets was made based on ease of installation and ability to return to stock brakes if necessary. The B&J Racing Bracket had some fore and aft play, so I put some mold release on my slider and filled the gap between the brake tab on the slider and the bracket with epoxy filler. It seems to work great, no slack or slop. I also had to make an adapter plate between the bracket and the caliper because the B&J bracket was made for a Grimeca or Brembo caliper. A Custom made bracket could be more precise and require no adapter, but it would be time consuming and probably cost more. Welding to the slider was the last choice because of the potential problems with distortion of the fork tube and metal compatibility in the welding process. I didn't like the looks of the welded brackets that I've seen on other TY-350s with disk brakes, which led me to believe that there may be a welding problem with the aluminum of the sliders. On the plus side, a welded bracket would give a positive and lightweight attachment point.

After installing Disc brakes on Jesse's and my 350 I've decided that it was well worth it. Both motorcycles' brakes are easier to operate over a wider range of engagement. Neither release as well as a factory disc brakes. There is a slight drag that I attribute to the difference in precision between the installations.

The Jesse's has a Grimeca 10mm master and a MB-50 30.15mm slave.

10mm dia. = 78.5 sqmm
30.15 dia = 713.6 sqmm
ratio = 9.09/1

The mine has a Honda 11mm master and a Honda 2 puck 25.4mm ea. slave.

11mm dia = 94.9 sqmm
25.4 dia = 506.5 sqmm
x 2 = 1013 sqmm
ratio = 10.7/1

The mine seems to be more progressive, powerful and precise than the Jesse's brake. I think that's because mine has a larger pad area and the fact that it is riding on a solid disc. Jesse's has a smaller pad area and is riding on a heavily vented but larger diameter disc.

Before I got the above parts, I did some looking around in an attempt to put together a disk brake setup for the TY-350. The list below was made by calling parts people and cross checking numbers. I did not verify the accuracy of the information because I was just in the initial stage of the process. It also shows the KX-80 with 2 sizes of bearings. I believe that the parts man gave me the bearings for 2 different variations of the KX-80, the one with the 6300 bearings was the one that I selected because the bearings from the TY-350 would (should) slip right in and ease the installation. I never did get my hands on a Kawasaki rim to verify the sizes. The RM-80 was my 2nd choice because of the number of spokes would allow me to use the rim from the TY-350, although it might require some welding and machining to fit the proper size bearing.


Yamaha TY-350 32 6202 15x35x10 ?

Yamaha YZ80 6001 12x28x8

Honda CR80 28 6201 12x32x11

Yamaha RX50 6301 12x37x12

Kawasaki KX80 28 6200 19x30x9
6300 10x35x11

Suzuki RM80 32 6200 10x30x9


6001=12x28x8 6002=15x32x9 6200=10x30x9 6201=12x32x11

6300=12x35x11 6301=12x37x12

Front Forks

I haven't done much to the front forks, but here it is. I cut the tightly coiled end off of the spring. The remaining length was 13 inches. With that spring, I use a 6-inch preload spacer, made out of conduit. I run 300cc of Dexron III transmission fluid in each leg. Shortening the spring stiffens it, and the Dexron seems to provide adequate dampening. For additional modifications contact Chris Johnson <papazit@juno.com>, and/or Jon Stoodley <stoodley@oknet1.net>, and thanks to both of them for their help and advice.

All of this stuff is taken from letters I've written to friends, or they have written me, and so some of it may sound odd. I have edited parts of it to make it clearer but that's all if you have any questions, feel free to contact me.

If you give me money, I'll try to spend it.