Modification Options

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Deleted member 12676

To Mod Or Not To Mod, that is the question

Some people have the fear that modifying their engine will make it too fast and too unreliable. But here's what I want you to know;
Opening the engine ports can make it faster but you can lower the speed back to normal with a larger rear sprocket and then you will have a bike that accelerates really good and climbs hills better. But maybe a larger sprocket isn't necessary if you put on an intake extension because that increases low rpm power while taking away from high rpm power.
And the only reliability issues are with two inexpensive parts: 1) the upper connecting rod bearing which you can easily replace at a cost of $6, 2) the primary gear woodruff key on the crankshaft The screw holding the gear on needs to be loctited and checked regularly. If you port for racing then you may want to make a steel woodruff key from a bicycle sprocket (make it 8x3.5x2.3mm) or buy a set of 25 alloy steel ones (40% stronger) from McMasters for $5.85. The part # is 98525A060.
When you lighten the piston/wrist-pin you will actually increase reliability because that eliminates engine vibration. And replacing the stock CDI with the Jaguar CDI greatly improves reliability because the stock CDI fires too early which causes excessive compression (stressing the crank and con rod bearings) and vibration (which loosens all engine nuts/bolts).
Concerning expansion chambers, the top speed can be set by the header length. You can lengthen the header for less top speed and more low rpm power. You see, you are in control. With these small engines it takes all the modifications I recommend to really get some decent power out of them. Making one little change will not turn it into a fire-breathing monster overnight. I wish it were that easy but it isn't. Doubling the power of these will still leave you with just a few horsepower.

With increased power you may need to increase the tension on your clutch pads. That is done by tightening the flower nut, pulling in the clutch lever, and then tightening it one or two notches more. Then readjust the clutch cable free play.

Reliability and Power Suggestions

for Chinese 2 stroke RELIABILITY:

Use sandpaper on glass to plane flat the cylinder head and top of cylinder for a good seal.
Replace the cheap nuts and bolts that come with the chinese kit and replace them with good ones from Sick Bike Parts for $10.
Buy a good carburetor. Choices: 16mm or 18mm Mikuni, 16mm Dellorto PHBG
Replace the stock CDI with a Performance CDI and coil ($70) that will reduce vibrations at high RPM and increase power with a stronger spark. The stock engine has way too much vibration which causes nuts to loosen which is the #1 reliabilty risk.
Replace the stock upper connecting rod bearing (where the rod connects to the piston) with a better one.

for More Power:
Lathe off 1mm-1.5mm from the stock head (till the cranking pressure is 135psi) for more compression and power.
Use tubing to extend the intake tract length which gives more engine power at low RPM.

For even More Power:

Make a Jaguar Torque Pipe.
Use JBWeld to redirect transfer port flow more rearward in the cylinder.

For More Speed:

Use a rotary tool to change the cylinder ports for more power and higher RPM (more speed). If vibration is more annoying at higher RPM then buy a 7.1mm carbide drill bit and drill out your wrist pin (best to take it to a machine shop) or even more troublesome but better: remove the flywheel and drill holes in it for better balance.
Buy a Rock Solid reed valve and ask them to size the part where the carb connects to the reed valve for the needed diameter for your new carb (or just have your machine shop make one if necessary.).

Two Main Options for Grubee Engine Modification

#1: Stock piston port intake engine:
It is OK for most applications with an extended intake and possibly an extended exhaust. This gives even better power than a reed valved engine when taking off from a stop.
#2: Reed Valved intake engine:
A reed valved engine with an expansion chamber with baffle extension gives better mid range and top end power. A regular non-modified expansion chamber is not the way to go because it robs mid range power to boost top rpm power. Right when that power hits you want to grab a higher gear but unless you have a shifter kit then there is no higher gear to grab. Pictured below: 55cc engine, Rock Solid reed valve, nylon adaptor, 18mm Mikuni, torque pipe

Basic Necessities for both types of engine:
COMPRESSION: Lath the stock heads mating surface down for more compression and buy an upper connecting rod bearing because the stock one wears out too fast with increased compression. Extra compression, especially from the very low stock compression, gives very noticeable power increases all thru the rev range. Unfortunately increasing the compression is not recommended if you still have the stock CDI. That is because the overly advanced ignition in conjunction with increased compression causes too much ignition compression before top center which causes excess engine heat which can melt the piston and damage the upper wrist pin bearing.

CARBURETOR: Replace the good-for-nothing stock carburetor. For a carb with the same 19mm connection you need to buy a Dellorto SHA or a CNS carburetor, neither of which are very good. Better to get a 16mm or 18mm Mikuni or Dellorto PHBG which are super excellent. JNMotors sells the Walbro carb and also adaptors for use with both types of engine. For the Mikuni you should buy the long (to provide 7" intake) zig-zag offset intake (so the carb is besides the vertical frame post) and right connector (O.D. 21mm for 16mm Mikuni, 20/23mm for 18mm). If you are getting a reed valve also then request Rock Solid to make the intake size the extra 23mm (for VM18) or 21mm (for VM16) instead of the standard 19mm.

CDI: Replace the crappy stock CDI with the Jaguar CDI. It will run smoother (less vibration) with less engine heat. And it makes the engine more reliable. And you can then safely increase compression.

CRANK SEALS: The stock seals don't last very long, especially if you stuff the crank for more crankcase compression (for more top end power). Both the left side and right side seals are available from Bicycle Engines. You can use a jewelers screwdriver to pry out the old seals instead of splitting the cases to get to them. On the right side of the crankshaft there is a sharp lip that makes it hard to install the seal since it catches there. Use a file or your Dremel to round off that lip.

PISTON WRIST PIN: The stock one is too heavy and causes engine vibration. You can lighten your own by drilling it out or buy lighter ones from Treatland.

NUTS & BOLTS: Replace all the nuts and bolts with a kit from Sick Bike Parts. The stock ones are too soft and loosen and wear out too fast. This is a reliability issue. Use medium grade Loctite when assembling.
CLUTCH: If the extra power causes the clutch to slip then these are the necessary steps to increase clutch spring preload:

The main clutch spring that squeezes the clutch together is inside the transmission case, between the clutch and final drive sprocket. It is adjustable from the outside of the cases.
1. Remove clutch cable from engine.
2. Remove sprocket cover with clutch release lever.
3. Remove clutch cable stop from top of transmission. It just unscrews after loosening the lock nut that binds against the top of the transmission housing.
4. Find a strong flat bladed screwdriver that fits snugly down into the hole that the clutch cable stop was threaded into.
5. The spring adjusting nut is basically a threaded collar with 4 notches, equally spaced, cut around it's circumference. The trick here is to rotate the final drive sprocket with channel lock pliers or similar tool that will not damage the sprocket teeth.
Rotate the sprocket while pressing down on the screwdriver until you feel the screwdriver slip down into one of the 4 notches of the spring adjusting collar.
Once the screwdriver fits into one of the notches, the sprocket and sprocket shaft should be difficult to turn.
6. Hold the adjusting nut inside the cases stationary with the screwdriver and turn the sprocket clockwise to tighten the main clutch spring.
Do not adjust is too tight as doing so will only make squeezing the clutch lever harder, strain the clutch cable, and accelerate wear on the clutch release components inside the cases.
1/2 to 1 full turn should be plenty, but you may have to experiment a little (read do all of this over a few times).
7. re-assemble everything back together, adjust your clutch cable and flower nut back to "normal" (2-3 notches tight), and enjoy a clutch that actually puts proper pressure onto the clutch pads.
I've read a little about correcting transfert port flow and some sugest to use Durabond 950. But some said it can cracked. SHEETS/950N NP.pdf

Want to know if you fill JBweld in all the transfert to the jug or just the opening cylinder port.

Or some said it's better to shim that by a machinist.
I put JBWeld at the front of the transfer ports. Having it there at both corners also helps to keep it in place.
What you are doing is aiming the transfer port so it points toward the intake port. Thanks for the tip on Durabond 950. As far as cracking, JB Weld will also crack, but who cares? As long as it stays in place all it's doing is redirecting port flow, and is not under any great stress. The temperature and thermal expansion properties look to be ideal. So the main thing would be the bonding properties, you don't want any to come loose and get sucked into the intake. I'm thinking about trying it, if the cost isn't prohibitive.
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