Pocket Bike Engines Tutorial



Howdy Fellas.
I've read a few posts recently from guys interested in using pocket bike engines on a bicycle.
To date , I've built and sold over twenty of these and have actually just started using kit motors recently.
I'm going to start building another one tomorrow .

I will be using a 49c.c. 2STROKE cag (cagliary) POCKETBIKE ENGINE, simply called a CAG from here on, with a CHAIN PRIMARY drive to a HOMEBUILT JACKAXLE, followed by a VBELT SECONDARY DRIVE to rear tire in a REAR RACK MOUNT on an old ccm Cruiser.

A good engine with the right gearing will cruise close to 40 mph but 30 mph will give you alonger lasting engine , a safer ride , and a little throttle left over to pull yourself outof harms way if need be
On a slight downgrade , I was clocked and pulled over on my schwinn by the local RCMP at 77 kmh.(48mph) . lol was a 49cc pocket bike engine and believe me for those who don't know , thats as fast , excuse me, thats faster than a person wants to go!!

I will post pics and detailed instructions of the build as I go for those who might want to give it a try but arent sure where to start.
If you have an old pocket bike or some pockt bike parts hanging around, the price of this build will be vurtually nil.

This is what were going to build .

Will post the first set off pics and instructions tommorow along with alist of some materials/ tools etc.



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BELT OR CHAIN??I've used plenty of vbelts as secondary drives from ajackaxle to rear tire.
Yes Quiet , smooth . no chain oil everywhere!!! God I hate that !! betweeen the 2 stroke oil and chain oil, , well You guys all know what it does to your paint job and clothes. alot more forgiving than a chain also in terms of alignment.

If your pulleys are out of alighment up to an inch even , they'll still track if tension is right. BUT try to get as true as possible as being off centered will cause undue friction as the belt is pulled up against the side of the pulley causing heat and you do not want a hot belt .

I love the maintenance free aspects. It took me a few bikes to get it down to a science but was excellent once I did.

On my schwinn ,I went over three months of daily riding (my only transportation as I dont even bother to insure the truck anymore) without the belt giving me any problems what soever or even needing adjusting!!

If the belt isnt extremely tight she'll slip. If it slips even a little , the belt will get hot , stretch, loss its integrity and expandability and you'll be just as good off using an old chunck of rope as using a belt once you've cooked it . If you 're not tentioned properly, one day you'll be out riding in the rain and she'll slip and even just a few spins is going to cost you. Dont chance it and figure you will just take it easy, stop, rght away. , let the belt cool off and dry off., get a rag and thoroughly clean and dry out your belt and pulleys.

Once they've slipped and cooked, tightening won't help as the belt will just roll over onto itside going through the pulley ( twist if you will) which makes the belt longer (Different inner circumference if its riding on the side instead of the inner face as it should be) SO tighten properly . Even if a belt is one piece , doesnt mean it isnt garbage.

start with a new belt, if you accidently cook it, it'll give you continous problems so spend the 6 bucks and buy anew one.
Wish I could tell you in foot pounds or something but have no measurements to the actual tention.

If you feel its causing alot of drag as you roll the bike , its too tight,
Iif it isnt tight enough to cause a minute amount of drag its too loose .

To avoid slippage in wet conditions, extremely tight is the answer but this causes another problem!! AHUGE amount of pressure on the spokes that your pulley is attached to. I was mounting pulleys to the wheel in basicaly the same fashion as we mount the sprockets but was going through a wheel a week due to spoke breakage .

To overcome this, I took some street signs , like stop signs but stop signs aren't quite big enough. and cut disks that fit tightly around the rim of the wheel. You have to cut a circle a little larger that the area your covering as once you cut a small wedge out of it from the center to the outer edge.approx 1' to 2'' wide at the outside depending on the rim width and hub width to allow for the dishing of the rim(concavity), your disk wil get smaller as it is bent down from the hub to the rim if you follow me.
I mount these discs in the same fashion as the sprockets sort of, Well I'll get to that.

Oh and dont use aluminum pulleys , they WILL calf.
I pay about $30 for 2 steel pulleys but I went through enough aluminum ones to realise steel actually is more cost efficiant

You can use a belt or a chain , your choice . I will show how 2 go with iether.

Wheather you decide to go with a belt or a chain, iether way you're going to need a jackaxle.
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The one I'm about to make is almost to simple, kinda like the guy building it!!?!!FPRIVATE "TYPE=PICT;ALT="

k pic 1
is a jackaxle! well its gonna be. what it was was a pocket bike rim until i cut the hub out of the middle . I'm using it because I had it . It has races, bearings, an axle. and even has threaded holes for our drive sprocket which incidently, came off same pb.
You can use rear of front wheel , backs wider but fits and has holes on both ends ,one for sprocket , the other side for disk brake which can be used as an engine brake now that your wheel. has migrated into a jackaxle. The one I'm using is the narrower front axle and the disk brake and rear sprocket have icendtical bolt pattern so bolts right up

pic 2
is a stock 68t pocketbike sprocket

pic 3 is a sprocket that a garnered from a cluster off a mountain bike. Itsa little light duty but will work if you havent anything better

pic 4

shows the drive sprocket bolted onto jackaxle and the secondary drive sprocket sitting inside. this sprocket will be substituted for a pully in a vbelt build. You have to cut the centre out of pulley to slide over jack axle and once we get the engine and rear final drive sprocket attached, we'll know how much of a spacer we'll need between the sprockets on the jack axle.
We'll get back to that later.

pic 5

is a front sprocket of a mountain bike woth a rock ring

pic 6

same sprocket attached to flatstock i cut out of some stuff i got laying around . the inner holes will be used to attach assemble to spokes in same manner as happy time sprokets. I should mention I bolted the rock ring to the steel ring, not the sprocket itself!!??!!
belt drives wil use a 10 or an 11'' pulley instead. I have a couple happy time rear sprockets that I'll be using but if you havent one , this will get you rolling.

Remember everything I show you here is not an idea but a tryed and proven system however they are all my own designs and by no means are they state of the art or are they the best way of doing it. If you think you have a better way of doing something , youre probably right and thats the way you should do it.

You'll be needing some materials to build the motor mounting plate , the rack etc.
Apiece of fairly heavy plate that wont bend from torque to bolt the engine down. try find something that has a second side you can use just to make it more sturdy. will post a pic inna bit. also some angle iron. I use 1 1/2'' angle iron .I get it free from old steel bedframes that are everywheres. the metal folding ones . They work exc. use what you got or what you can get. And unless you're a welder, a bunch of nuts and bolts. I dont weld. I bolt everything. I use 3/4' and 1'' long 1/4'' bolts. for virtually the whole build. don't forgot lock washers.
~There's nothing left on the right side of my brain~
~And nothing right on the left~


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A little about exhaust as per request.
The stinger (pipe after the chamber) has holes drilled in it but runs straight threw the can.
When i want to run quiet, I just open the can and surround the pipe with insulation. and throw the can back on. really quietens it down and gives it a deeper tone but you do lloose a little top end speed..

I've been known to adjust my exhaust daily , depending on what type of riding I,m doing, even changing up pipes depending if I want more bottom end torque or a higher range speed band.

You can adjust what rpm your power band kicks in at by adjusting the length of the pipe between the manifold and chamber..

Also , by adding back pressure with the insulation, your going to improve your bottom end performance where as for top end power , you want it to breath as clear as possible, no back pressure.

I'lll post more on exhaust as time allows.

Theres a link below to a working model of a 2 stroke expansion chamber that I posted online.. you can see how it changes from a positive to a negative pressure and back again and draws unburnt fuel back into the cyllinder. This is what creates what is known as a power band .
This effect only happens in a small rpm range which is why 2 strokes have such small powerbands
When you hear the term Tuned Exhaust , this means the length of the header pipe aswell as the length, width, and angles of the exhaust chamber are tuned to excact measurements that are determined by the size of intake, the exhaust port , the cylinder size etc. aswell as the rpm range the rider wants his peak power at.
Cheers .


A little about gear ratios. si you can get a couple sprockets or pulleys together.
Now your not going to believe how easy this is to figure out although it took me a while to snap at first.

Because were using the pb front sprocket (pinion) aswell as the pb rear sproket , we know that up to the jack axle we have a correct ratio to move the bike the circumference of the pocketbike tire which is I believe approx 32 ''.

Unfortunately, using the same rpms and load, we have to make the bike travel the distance of the bicycle tire circumference which , for a 26'' is approx 78''.
These measurements might be alittle off as I'm goingfrom memery which isnt the best these days.

So we have to travel 78'' instead of 32' which is approx 2 1/2 times as far and theres your ratio.!!!
2 1/2 to 1
What I took into effect also is the fact that I dont need to scream around corners running 10.000 rpm or beat any body off the line and I have pedals to help out the bottom end , therefore I can safely gear it a little higher and stay within an allowable tolerance.

I find 2.2 to 1 nicely allows for these considerations giving me a nice balance betwwen upper and lower power .
The pb uses a centrifigal clutch. I have 2 clutch bells. One has a 6 tooth pinion, the other, a 7 tooth. IT is4 small bolts easily accessed to change the gearing 15% higher or lower in under 2 minutes. I switch up quite frequently if i'm pullig a load or hilly terrain and so forth.

For a belt drive, the best combo is a 5 '' or 5 1/2 '' and an 11 or 12 inch .pulley. You can use larger or smaller aslong as you keep the same ratio and noteffect it but these sizes make for asy installation both on the jack axle aswellas the wheel..
Simply apply this ratio to whatever sixe sprockets you have but they should be smaller. I think that having the large sprocket at about 7 or 8 nches is ideal.

This is not set in concrete , a straight 2 to 1 works real well but much over 2.2 to 1 is no good. you'll have not enough power to let the engine climb to its higher rpm range and gearing it higher will actually make you slower both in the bottom aswll as the top end
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Alright , Lets make a rack. This is a short note I wanted to make to you builders before I forget. When I first started building, the one thing I always took into consideration and worked everything else into was wieght .
After many builds its at the bottom of my priorities.. ( to an extent).

I found that as my builds proggressed as did my workmanship and knowledge aswell as hands on knowhow .I was regressing!!

Instead of getting lighter and streamlined , I was going the opposite direction!!!

But heres a fact, , While my builds were getting bulkier and heavier. so were they getting perportianately faster, more stable , requiring less maintenance and I was replaceing worn parts much less frequently..

This was a hard lesson to learn as( in my mind anyways) gravity and aero dynamics rule. They may be proffesionals at holdin ya back but I'll tell ya what, vibration and friction whoop em hands down .
~~Just a side note~~



alright were going to put a rack together but first , I'v e got to mention Blow Down Time and dont look at me like that!!! I've a couple fellas eager for some exhaust but this has to be taken into consideration before hand
***WARNING**** Not for the squeemish!!!!
This is for the 1 % 'ers who NEED the POWER HAAAHAAAHAAA


Blow Down Time is the distance in degrees between the opening of the exhaust port to the opening of the transfer ports. BDT (in degrees) controls the time that a cylinder has to empty itself of exhaust before the transfer ports open and allow the fresh fuel-air mixture into the cylinder. In order for the fuel air mixture to move into the cylinder, the pressure inside the cylinder must drop below that of the crankcase. For example, a RC engine may have peak cylinder pressure of approximately 750 psi and that pressure reduces as the piston travels downward. At 15,000 RPM's the engines BDTiming has only .000244 seconds to empty the remaining cylinder pressure to less then approx. 20psi .in order for the transfers to start flowing fresh air mixture into the cylinder when they ports open. As RPM's increase, shorter BDT's reduce the possibility that the exhaust will have enough time to leave the cylinder before the transfer ports open and increase the possibility of the exhaust and fresh charge mixing in the cylinder and decreasing the engines power potential. Increases in BDT extends an engines RPM potential by allowing more time for the exhaust and pressure to leave the cylinder before the incoming fresh charge enters the cylinder.

Stock Blow Down Times on a GoPed vary from 12' on the GSR 40 to 22' on the G23RC engine. BDT greatly influences the performance of tuned pipes. Engines with low BDT's, like the GEO, GSR and LH all have their transfer ports open when the return wave from the tuned pipe arrives at the exhaust port. When a return wave comes back too soon it may cause a back-flow or delay through the transfer port or increase the mixing of the fresh fuel air mixture with exhaust. Engines with more blow down time experience the same problem, usually at 1/2 the engines peak horsepower RPM. The return wave comes back early at BDC and causes the a dip in the power at that time.

Knowing this you would think that increasing the BDT would be the easiest way to increase your power. The new power comes at a cost though. Increases in BDT by raising the exhaust port, decrease the power stroke. During dyno testing of the GEO engine I found that raising the BDT from 18' to 22' (a mere .030") was the difference from decent power to just mediocre. That particular engine needed that .030" of power stroke for decent power. The power stroke is the distance from (peak pressure) TDC to EO (exhaust port open). The power stroke is the pressure from expanding combustion, pushing down on the piston, creating the torque transmitted by the crankshaft to a GoPed's spindle.

Reducing the power stroke by small amounts at first may help the power. On engines that produce very little power to begin with small changes to BDT can have big repercussions. Reducing the BDT a little more and you may loose bottom end power. Raising the exhaust port even more may decrease the power through out the RPM range.

Our small engines have their own range of BDT & power stroke #'s that work, BDT & PS #'s for larger 2-stroke engines generally do not. The secret is to find that perfect balance between BDT and PS and many other factors to produce the best power.

Controlling Factors: changes in bore and/or stroke, compression ratio, changes in squish clearance, changes in exhaust port timing, air density changes

Compression is the product of the compression ratio and the current atmospheric pressure. Standard air pressure at sea level is 14.7 psi @ 65 degrees Fahrenheit. Even though that is the standard, the actual pressure may be a little higher or lower then that at any given day. Increases in altitude, temperature and humidity change the atmospheric pressure and is calculated to give a corrected altitude. The point of all this is to show that the compression measured along the coastline at sea level will be always be more than it is in the mountains. The deserts on a hot day will be the same as being in the mountains, even if it is at sea level or below.

Compression is a good tool for gauging engine wear and octane requirements and should be checked with a high quality "Snap-On" compression gauge. Many other popular brands will read low, often by 20 to 30 psi. If you don't know what your compression should be, you can get a rough idea by multiplying the engines geometric compression ratio x 15. Compression is checked with the throttle held wide open and strong pulls on the starter rope.

Crankcase (primary) Compression Ratio
Controlling Factors: bore, stroke, type of crank (pork chop, full circle, etc.), crankcase volume.

Crank case compression ratio is measured in a similar fashion to the secondary CR. All the area under the piston crown at TDC / the area under the piston crown at BDC. The primary compression is responsible for pushing the fresh fuel air mixture up through the transfer ports when they open to approximately BDC. Depending on the compression ratio, blow down time and exhaust port area, the cylinder pressure may be greater than the crankcase pressure. In these circumstances, it will cause a delay in the scavenging charge entering the cylinder. An increase in crankcase compression ratio, exhaust port area and /or blow down timing would help this. Too much crankcase compression can hurt the intake ports flow of fresh fuel - air mixture from the carb, when there is too much intake port timing and /or insufficient port velocity to overcome the back flow from the crankcase.

Crankcase Pressure Time (CPT)
Controlling Factors: intake port duration, transfer port duration, crankcase compression

CPT is the amount of time in degrees, that the engine has to build up enough pressure to send the fresh fuel-air mixture through the transfer ports and into the cylinder when the ports open. CPT is measured from the "closing of the intake port" (IC), to the "opening of the transfer ports"(TO). Depending on how the engine is ported and what type of stroke is being used, an RC engine may have as little as 30 degrees and a stock GEO can have up to 65 degrees to build up the pressure to force the scavenging flow out the transfer ports when they open. Since our piston port engines have symmetrical timing on the transfer port and intake port. Any increases of either intake or transfer port timing, will decrease CPT. Examples of this would be, cutting the intake skirt or installing a stroker crank will decrease CPT. Normal ranges for CPT is 38' to 48', lower degrees can give more peak power and the opposite will produce more torque.

Compression Ratio (Geometric) and (Trapped)
Controlling Factors: bore, stroke, combustion chamber volume, squish clearance or deck height, exhaust port height, type or # of piston ring(s), type of piston crown (radius or flat),

There are 2 types of compression ratios: Geometric and Trapped. Geometric compression is measured from bottom dead center to top dead center. Trapped compression starts when the exhaust port closes to top dead center. The geometric compression ratio can be measured calculating the volume of the cylinder + the volume of the combustion chamber at TDC / combustion chamber at TDC. The trapped compression ratio can be measured in the same way, except you need to calculate from where the exhaust port closes to TDC + combustion chamber \ combustion chamber.

As listed in C.F.'s increases in bore size or stroke will increase the compression ratio. Decreases in the engines exiting squish clearance or deck height will increase the compression ratio and increase the compression @ approximate. 1 psi for every .001" removed. Combustion chamber volume is normally measured in cc's, but on GoPed's they have been simplified to differences in geometric compression ratios (12:1, 14:1, etc.) Decreases in cc volume will increase compression and increases in cc's will lower compression usually at approximately 7psi / cc. When machining a combustion chamber to increase compression, you can use the approximation of 1 psi for every .002" removed.

Raising the exhaust port will also lower the compression ratio and psi. Because the pressure inside the cylinder rises exponentially small changes in exhaust port height may only change the compression slightly. Piston rings don't actually change the compression ratio, but there can be up to a 30 psi difference in compression between thick dual rings and a thin single ring.
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OK fellas . lol . who was it wantd to make thier own tuned exhaust??
Quite abit to take into consideration and more considerations to come concerning exhaust. K lets builds a rack.