Porting: stock exhaust duration is 169 degrees, and the transfers/boosts are 124°. In order to get more high RPM power I had to raise the exhaust port so its duration is 184 degrees.

Engine Compression: stock is only 120psi which is too low. I removed 1mm from the head mating surface to bring the pressure up to 155psi. That made a noticeable difference in power but eventually I brought it back down to 135psi with an extra gasket because the engine was getting too hot in the summertime.

Exhaust Pipe: The stock pipe is the perfect design for a 4 speed bike but its internal stinger will gradually get smaller in diameter if you don't use a synthetic engine oil (so there won't be hardly any carbon deposits). If you want to race this bike then you need a normal expansion chamber but I warn you that it's a pain-in-the-ass on the street with one. That is because one designed for this bike has an extra long baffle return wave that interferes too much with intake when below the pipe powerband. The extra long belly/baffle is needed to produce the extra long return wave because there is a 3,000 RPM step between gears since the gearbox only has 4 gears. Stretching out the return baffle wave stretches out the RPM range by which the pipe has its supercharging effect. Below is my design for this bike. But I don't recommend it unless you also have a boost bottle to lessen the pre-powerband dip in power. In the drawing below the blue pipe is the internal belly stinger which greatly reduces noise. (Click here to read all about expansion chambers.) This design is the 4th I tried. For even better acceleration you can switch to a 46 tooth rear sprocket and a 13 tooth front sprocket.



This drawing shows the difference between the original pipe and a better version of it which I've made. My latest piston isn't ceramic coated and the pre-powerband dip just became too annoying with the previous pipe. I don't remember exactly what powerband the original pipe had but this latest pipe is really to my liking. It has good low RPM torque and it's powerband covers 3500 RPM. It would get beat in a race with my best expansion chamber listed above, but not by much. Click here for the page about it's powerband.


This design of silencer is very quiet and totally non-restrictive. All engines with limited engine intake flow need zero restriction from the pipe/silencer. When I switched to this design I got a big power boost.



Power dip before getting "on the pipe": All expansion chambers cause a dip in power for about 1000 RPM right before the engine comes into the pipe powerband. The Yamaha boost bottle was designed to counter that dip. I designed and installed one and it did exactly that without any drawbacks. But I had to richen the idle jet with a larger .64mm hole and glue shut the two holes I had in the carbs slide. You can read up on the subject and get the details by clicking here. I don't recommend doing it unless you are skillful at such things because there was hardly enough room in the intake manifold for the hose connector and it is prone to cracking the JBweld I used there to cause an air leak. It is essential that the boost bottle be secured to the frame to lessen movement at the hose and connector.



I JBwelded the hose tube in place and then secured it all with a strong wire.


Ignition Timing: stock timing is correct for the stock engine but too advanced for a modified engine. So I installed a racing ignition coil and modified a KDX CDI that I made to suit the AX. It gives a timing curve better suited for the modifications listed here for better acceleration. (Let me know if you want one.) The stock CDI is not optimal for this engine. In all humble truth this CDI and ignition coil has made a really big difference in engine performance. I am very happy with them. I don't advise increasing engine compression without this CDI. Too advanced ignition timing with moderately high compression will cause too much engine heat causing seizures or piston deterioration from detonation.  I can sell you the performance CDI or sell you its construction plans. My email is health@dragonfly75.com




Carburetor: It is only a 20mm which is small but since the reed valve is so restrictive there is no sense in putting a bigger carb on it. Make sure the float bowl height is 21mm from the gasket. After installing my expansion chamber I increased the main jet size by drilling it out with a .9mm micro drill bit). And I increased the idle jet size by drilling it out with a .45mm micro drill bit (equal to a #42.5 idle jet). With the expansion chamber the mid throttle position gave too rich a mixture, even with the needle lowered completely. So I used my dremel to gouge out the part of the slide assembly that the needle clip rests on top of. So in effect I lowered the needle about 1.5 clip positions and drilled two 2.6mm holes at the bottom of the intake side of the carb slide and the engine ran clean without sputtering after that. (By "clean" I really mean it ran great all thru the RPM range.) Instead of modifying the slide to let the needle sit lower you can just buy the 4DH11 needle to lean it out more. Before switching out the ignition system I thought the off idle mixture was too rich because of inconsistent combustion but it turned out that the stock timing at low RPM was just too advanced because my CDI cured most of that problem. After putting in a 4DH11 needle my final jetting was #25 pilot jet and #95 (1mm hole dia.) main jet and the bike ran even better than with my modded stock needle.

Piston: I drilled two holes in the stock pistons intake skirt which helps with more flow and more power at high RPM. The photo at the right is a GP100 piston showing the piston holes and where I cut off metal impeding the intake flow from the reed valve.

Intake Tract
I used my rotary tool with cutting disc to make grooves along the walls of the intake tract and transfers to prevent gasoline from forming streams on the walls which become big droplets which burn much slower than gasoline mist. This gave a definite boost to power.

Dealing with engine heat:  Ideally any engine that revs past 9000 or has 150psi or above should have a cylinder head with squish band to lessen the heat that the piston absorbs. But this engine is without a squish band so i decided to have a thermal barrier (ceramic coating) put on the piston top to help when the engine is run at top speed with outside air temps at least 90F. I sent off a spare piston I have to Swain Tech for application of thermal barrier coating. (If you send a piston to them first send it to a friend in the States for him to remail it to them. They will reject any shipment to them from outside the USA.) With the ceramic coating the engine maintains more power on hot summer days since excess engine heat robs engine power. Make sure you are not running as thin an oil to gas ratio as possible or you could get a "cool down seizure". That is because the ceramic coating, in addition to preventing absorbing as much combustion heat, prevents the piston from cooling off as quickly as the cylinder does when transitioning from high RPM to low RPM. When I ran too lean an oil ratio (by mistake) it seized on the 2nd run after coming off the highway onto a bumpy road that I went real slow on. Since then I've had no problem with it and I run either regular engine oil at 20:1 or semi-synthetic oil at 30:1. That was all with their "TBC" top coating ($26) which sometimes still lets the engine lose power at high temperature although not as much as before. Next I tried their "Gold Coating" which is 50% thicker (cost: $42.50) but it wasn't any better. I want to make a ram air scoop over the head like the Suzuki GT's have to force more air flow over the head to keep it cooler.

I have glued jeans cloth onto the top of the gas tank since the the sun gets the tank so hot I can hear gas vapor escaping out past the gas cap. That should provide some insulation to keep the gasoline from heating up as much. Some professional mechanics actually take off the gas tanks from motocross bikes that are being raced on summer days and keep them on ice till its time to get ready for the next race. And they have plastic gas tanks which conduct heat much less than our steel tanks! Here's a pic of my piston with the gold coating:

Notes:

My home-made reed, made out of .5mm thick carbon fiber sheet, lasted till around 8000 km when I made and installed another one. I've tried it with and without a reed limiter with no observed difference which probably means the reed is too stiff to need one. 

At almost 9000 km the red wire exiting the battery broke off which caused a voltage rise which blew out my headlamp, tail-light, and speedo bulb. So I advise everyone to tape that wire to the body of the battery to not let it move any which will prevent it from breaking. I still haven't found a correct headlamp replacement bulb (6v 25/25W) so if you know of one then please inform me.

It finally started to rain occasionally here and I found out that my Dunlop Mystic rear tire was terrible on wet roads. I replaced it with a tire with more grooves in the tread and that is flatter in the middle so there is more contact area with the road.

This engine starts to lose power at a head temperature of around 290 degrees F which happens often here when the weather is hot. So I removed the engines side fins rubber noise absorbers for more air flow on the fins. I didn't notice any increase in noise. And I used my knife to scrape off dirt that had hardened onto the fins. That reduced the temperature at the head about 30 degrees to my surprise. But it still got hot too often before I had the piston coated. 

Cylinder Removal: It's easy once you know the final trick. Look at my pictorial on removing it: (click here) 

I hate the rubber that the rear carb boot is made of since now it is rock hard and has lost all flexibility. I have to soak it in hot water to be able to get it to stretch onto the carburetor. This is critical because the smallest amount of muddy water sucked in at that junction will wear down the engine really fast. I put silicone sealant there.

The swingarm hole at the right side has gotten bigger and allowed some forward/backward movement in the swingarm so I advise everyone to take out the bolt and lube it and the hole with a good quality grease. Suzuki had little or no grease there.

The piston/rings I buy from EBay is for the A100 (predecesor to the AX100) which is the same except that the rings are thicker and the piston ring grooves are thicker to match. So rings aren't compatible between them. Here's a picture of the A100: