Pocketbike Forum banner
Not open for further replies.
1 - 3 of 3 Posts

1,043 Posts
Discussion Starter · #1 ·
39 and 47cc Pocket Bike FAQ

This FAQ is a compilation of information from many forums, websites and people. The purpose of this FAQ is to help someone who is just breaking into the sport, get a fair understanding of how these bikes run and what's needed to keep them running.

I hope this helps everyone out there, and I'm sorry if I haven't mentioned your name, if I used your information please let me know and I will gladly add your info! If you cannot find the information you are looking for in this thread, use the SEARCH feature right here:


Section 1
The Basics!

What is a pocket bike?

A pocket bike is what it says it is, a miniature version of a real GP racing motorcycle, it just doesn't go as fast as one. There are many different makes and styles available, many of which look just like real racing bikes. They stand about 15 to 18 inches high depending on the model and brand and they weigh anywhere from 35 to 55 pounds. THEY ARE NOT TOYS!

What is a pocket bike for?

A pocket bike is not just your average ride around everyday scooter. A pocket bike is a high performance tuned racing machine… believe it or not! It must be well cared for and must not be misused or abused in any way.

How fast are these things?

Depending on which type you get will depend on the speed of the bike. The speed also varies according to the rider’s weight and road conditions. Most pocket bikes are capable of doing anywhere from 28-50 MPH.

What kind of protective gear do I need?

You should invest in a DOT approved helmet, pads and a body suit. Going at 25mph + does tear jeans and leave some pretty bad cuts and scrapes. NEVER ride without a helmet...Not only do you look stupid but you WILL hurt your self.

Do I Need A License?

These bikes are not for road use, so driver's licenses are not necessary. See your local DMV website or office for more information.

Can I Ride My Pocket Bike On Public Roads?

NO. They are not street legal. They do not possess headlights, turn signals, and brake lights. And do not have Department of Transportation approved tires (since they come with racing slicks). There are smaller 50cc bikes that are street legal, but again they tend to be larger and slower.

How Much Power Do The Bikes Have?

Stock is around 2.6hp. Simple modifications as mentioned on the parts page, including the Stage 1 2 and 3 kits, can increase power to 3.1hp at the tire. It is likely that the motor is actually generating 20% more than that at the crank. These engines are capable of more power than that if you are interested in spending the money for engine work.

What kind of motor does a Pocket Bike use?

Pocket bikes have 2-stroke engines.

Section 2

Well I have my pocket bike… what now?

Following the following steps will ensure that your first ride wont be your last!

1. Clean out your tank and flush with fuel to get any debris out.
2. Clean out the carb to assure it is free of debris.
3. Move the needle clip on the carb to the 5th position, which is the bottom notch if the needlepoint is pointing down.
4. Get an Air filter
5. Check your tires to see if they have enough air. 28lbs seems to be right.
6. Make sure all fasteners are tight and secure, possibly add loctite.
7. Make sure the gas cap is in some way vented so the tank does not vacuum. (Use a pin to punch 2 holes in the cap)
8. Make sure chain is well-lubed
9. Adjust your brakes so that they are not dragging too much, especially the rear. A dragging rear brake can cause your clutch to fry prematurely or break a clutch spring, among other things.
10. Mix your oil/fuel mixture accurately with good oil meant for 2 stroke bikes
11. Check over the engine bolts and steering head bolts for tightness, make sure wheel and steering head bearings are functional and not sloppy. Do not over tighten your foot peg bolts because they frequently break off.
12. NEW riders, read the starting procedures
13. Starting the bike- To properly start the bike, make sure the fuel **** is open, the choke is on, if the engine is cold. Make full complete pulls to the engine, while adding a little gas. Caution, do not add too much gas, your bike will fly out of control if it is not started on the stand! Even if you're starting it on the stand, it is not necessary to add too much gas. Once the engine has started, you can not turn the choke back to A if it was on the choke. Be sure to warm up the bike for at least a minute before riding off into the sunset.
14. Have fun!

Will it operate on pump gas?

Yes, premium pump gas should be used. Use fresh gasoline only, do not use any gas that has been stored for more than 2 weeks. Only mix enough fuel/oil that will be used for the day or weekend.

What oil is best to use?

Any good quality (brand name) pre-mix 2 cycle oil, preferably synthetic

Good oils:

Any good fully synthetic oil is fine.

1 pint = 16 oz

1 liter = 33 oz

1 quart = 32 oz


3 pints = 48 oz

3.2 oz = 15 fills [40:1]

2.6 oz = 18 fills [50:1]

General guide for the number of Oz to be mixed in one US gallon and what ratio it comes out to be.

50:1 = 2.6 oz oil to 1 US gallon of mid grade gasoline
40:1 = 3.2 oz oil to 1 US gallon of mid grade gasoline
32:1 = 4.0 oz oil to 1 US gallon of mid grade gasoline

By fills I mean the number of gallons you can mix.

What do I use to flush out my tank?

Whatever mixture you plan on using (ex. 32:1, 40:1 or 50:1) Use that same mixture to flush out the tank.

How do I break in my new Pocket Bike?

Start with a 40:1. Don't full throttle it and don't keep the RPMs at one speed. You want the RPMs varying up and down. Let it cool down after every 10-15min. of riding. Run the second tank on 40:1. After that switch to 50:1 using full synthetic of your flavor.

What position should the choke be in?

The choke should be in the upward position when your doing a COLD START, and it should be in the down position when your riding or doing a WARM START.

What is the difference between air-cooled and water-cooled?

Water Cooled tends to last longer between rebuilds, because the engine runs cooler - but 40cc class European racers used to favor the air-cooled bikes because they could be a tick faster...having less parasitic loss (no water pump drag), and less weight (no pump, radiator, brackets, hoses, overflow bottle, and water).

Is my spark plug fouled?




1,043 Posts
Discussion Starter · #2 · (Edited)

Section 3
Performance Tips

Performance! How do I get more?

Carb- The bigger the carb (meaning bore and jets sizes) the more fuel you can introduce to the cylinder. A venturi bore speeds up the airflow across the main jet and pulls more fuel into a somewhat compressed mixture. (There is a limit on how large to go though)

Reeds- Stiffer reeds close tighter and faster sealing your precious air/fuel mixture into your case. This aids in the pre compression stage of your mixture. Stiffer reeds also don't flutter at higher rpm, which creates turbulence before the charge reaches the combustion chamber.

Ported head- (this also includes the porting of the case at the transfer port bottom) this allows more fuel/air to enter the combustion chamber via lesser resistance and larger cross sectional area. (Up to a limit, this gets WAY more complicated) This includes matching the ports between the case and the head and smoothing any obstacles to the mixtures flow.

Indexed/matched piston- Indexing the piston (the way I do it) effectively increases the transfer port size at the top. It also advances the intake timing, allowing more fresh charge into the combustion chamber. An angled "index" will guide the charge up and to the rear of the chamber which will swirl and create a vacuum at the front of the chamber; this helps scavenging and creates more space for the return pulse charge. This makes your power band peak stronger and allows your expansion chamber to be more effective.
Matching the piston is cleaning up the underside to maximize flow and to enlarge the transfer port at the bottom, this is mostly effective on the Cag piston port style motors.

Tuned pipe- A properly tuned pipe will create a return pulse at precisely the right time to pack the left over fresh charge back into the combustion chamber. At your power band peak this essentially acts like a turbo.

Performance clutch- I'm mainly talking about the clutch engagement time here. If a clutch is set to engage at later rpm, you can build more horsepower before it engages. This will give you substantially more acceleration. The motor will also rev faster at the lower rpm due to the lack of a load on the motor. This also acts as if you have a lighter piston/crank, until engagement that is.

Sprockets- Bigger rear sprocket or smaller front pinion = more acceleration, Smaller rear sprocket or bigger front pinion = less acceleration, more top-end
Pinion to Sprocket ration ~= 1:10, that means 1 pinion tooth ~= to 10 Sprocket teeth. If you're looking for a good all around gear ratio to ride with everyday, and you want some good clutch pad life, stick with your first thought...7/72 or even 7/74. You'll have much better acceleration, but not quite the top speed as a 7/66 or a 7/68 would have. Also, try a 6-tooth pinion. It’s easier for your engine/clutch (as far as these centrifugal clutches go) to push a 6t pinion than a 7. Try a 6/68 if you want.

What should I mod first?

1. Air Filter induction kit / exhaust can upgrade

2. Re-gap spark plug spacing to .025 Inches. You may also want to pick up the NGK BM7A (Use the spark plug disc you get at any auto parts counter for like 99 cents)

3. Fuel filter and Higher Quality fuel line

4. Carb porting and polishing

5. Primarily an engine teardown, modification and reassembly. It includes bumping the compression, indexing the piston and other porting. Depending upon the builder and methodology, it can be done to yield more bottom end, midrange or top end power.

How do I maintain and keep my bike running smooth?

The spark plug must be clean, and must be replaced promptly when signs of wear begin to show. The condition of the spark plug can tell you a lot about how your motor is running. The spark plug should be a light brown or golden color.

The air filter is used to clean the air going into the gas motor. Periodic cleaning of this filter is recommended to protect your motor against the damaging effects of dirt and abrasive particulates. Your air filter can be located on the back and it has 2 screws.

The chain on your gas-powered scooters should be lubricated before every ride. The chain should also be checked for tightness on a regular basis. If the chain is too loose a popping sound can be heard and the gas-powered scooter will jerk under acceleration, or the chain will just jump off altogether. If the chain is too tight it will be noisy and bind, this can be felt by pushing the scooter with the motor off.

The carburetor has many adjustments on it and it is recommended that experienced persons make these adjustments only, however we will point out the major points. The carburetor is responsible for pumping the fuel and mixing it with the air at a precise measurement. The carburetor does this with very small diaphragms, orifices and ports, which are easily clogged by contaminants. This is why is extremely important that you use only clean new fuel mixed with 2-cycle oil. KEEP YOUR CARB FREE OF DEBRIS!!!

Section 4

Adding a tachometer

Any tach that is digital will work fine.

Adding a speedometer

You can purchase a speedometer from your local wal-mart or the following websites. (Note you do NOT need one that costs 100$ a 15$ one is just fine!) Links will take you directly to the product.

1,043 Posts
Discussion Starter · #3 · (Edited)
Painting your bike:

Tools Needed:

1. Whatever tools that is required to remove the part to paint
2. Airbrush

1. Sandpaper, 400-600 grit
2. Tack cloth
3. Plastic adhesion promoter (primer)
4. Color paint
5. Clear coat
6. Reducer
7. Flex agent (If paint requires it)
8. Whatever tools that is required to remove the part to paint
9. Airbrush

When using an airbrush, many times you are required to thin the paint. Check out a local hobby store for an inexpensive airbrush kit (or a compressor and paint gun for larger projects).

If you're unsure of painting, try a practice piece or the back of the plastic piece you are painting.

Now for the good stuff!

Step 1- Remove part to be painted
Step 2- Clean thoroughly, Remove all grease and wax using a degreaser and a detergent.
Step 3- Wet sand, Wet sand lightly, so that the promoter will adhere better.
Step 4- Clean again, clean again with detergent and dry, or use a tack cloth.
Step 5- Plastic adhesion coat, Follow instructions on the can. Apply 2 coats and allow for drying time (time on can). DO NOT OVERSPRAY!

If you notice imperfections, you can wet sand again and apply another promoter coat.

Step 6- Paint. Follow instructions on the paint can. Wipe down lightly with a tack cloth to remove any dust or debris. You'll likely have to use a reducer at this point to thin out your paint mix (refer to the can or your airbrush instructions for details). Apply about 3-4 thin coats, allow for drying time per instructions on can between coats, in many cases over an hour. DO NOT OVERSPRAY! More coats may be needed, you want a smooth surface. Allow drying for 6-24 hours, the more coats, the longer the drying time. Don't touch while drying, yes it is wet, yes you will leave marks.
Step 7- Clear Coat. Follow instructions on the clear coat can. Wipe down lightly with a tack cloth to remove any dust or debris.

Apply about 2-4 coats, allow for drying time per instructions on can between coats, in many cases over three hours. DO NOT OVERSPRAY!

More coats may be needed; you want a smooth, bubble free surface. Allow to dry for a full 24 hours. Don't touch!

Step 8- READ THE LABELS. Check with the paint and clear coat maker about waxing information before using any wax. Most recommend not to in the beginning. The labels will also tell you any additional instructions the manufacturer recommends you follow.
Step 9- Reinstall part

PPG, DuPont, and ColorRite are good paints to use, here is some PPG info:
Plastic Adhesion Promoter - SXA1050
Reducer - DT870
Urethane Clearcoat - DCU2021
Hardener - DCX61

Where can I buy parts to mod my Pocket Bike?

Pocket bike Planet has partnered with the best pocket bike dealers and specialty equipment manufacturers in the industry. When you need a new bike or a high-end part, buy it from someone you can trust and who knows the product. Just click on our partners advertisements and you will see the quality products that they offer.




Carburetor- A device used in internal-combustion engines to produce an explosive mixture of vaporized fuel and air.
Clutch- Any of various devices for engaging and disengaging two working parts of a shaft or of a shaft and a driving mechanism.
Engine- A machine that converts energy into mechanical force or motion.
Exhaust- A duct or pipe through which waste material is emitted.
Horse Power- A unit of power, used in stating the power required to drive machinery, and in estimating the capabilities of animals or steam engines and other prime movers for doing work. It is the power required for the performance of work at the rate of 33,000 English units of work per minute; hence, it is the power that must be exerted in lifting 33,000 pounds at the rate of one foot per minute, or 550 pounds at the rate of one foot per second, or 55 pounds at the rate of ten feet per second, etc.
Intake- The place where water or air is taken into a pipe or conduit.
Piston- A solid cylinder or disk that fits snugly into a larger cylinder and moves under fluid pressure, as in a reciprocating engine, or displaces or compresses fluids, as in pumps and compressors.
Reeds- The reed(s) on a reed valve induction engine function essentially as a one-way doggie-door, admitting air when the motor is drawing air/fuel mixture, and closing when crankcase compression (known as "pre-charge") begins. (Thank you pocketbike.com)
RPM- revolutions per minute, rate of revolution of a motor; "the engine was doing 6000 revs"
Spark Plug- A device inserted in the head of an internal-combustion engine cylinder that ignites the fuel mixture by means of an electric spark.
Sprocket- Any of various tooth like projections arranged on a wheel rim to engage the links of a chain.

ZR Performance
1 - 3 of 3 Posts
Not open for further replies.