30-40mph???

The easiest way to hit 40 mph is to get a rear hub adapter with a 40T sprocket, a billet head & a SBP expansion chamber pipe.

After all that's installed (& the engine's been broken in properly), you can re-jet the carb for best performance.

Jet sizes are more of a what Your engine runs best on so it's not always easy to recommend a one size fix... it depends on your engine as well as what mods have been done. These engines tend to be set up over rich from the factory, but adding a pipe, doing port work, or swapping to a free flowing air filter can all affect how much fuel it needs to run at its best. The carb may be jetted too rich for a stock engine but it could be just right or even too lean for a modified engine, also a jet size that works on a 14mm carb won't work so well in a 18mm carb... typically a size 64 to 66 jet will work out well in a stock engine once it's broken in, but if you got a better flowing pipe you may need to go up a size. Same if you got some port work done it may require just a bit more fuel.
Still most the CG engines will be a bit on the rich side even after upgrades, but the best way to determine exactly what your bike needs is by doing plug chops...

Now for going over 30mph, there are a few ways to get there... smaller rear sprockets, more power, more rpm... the easiest way to get there is by swapping the rear sprocket, but this will be at the cost of acceleration and hill climbing ability, getting a pipe and matching it to your engine's power band will help bring some of the lost acceleration back, and doing a mild port job to give the engine more torque will help the acceleration even more.
The other method is to make the engine run at higher rpm's, this let's you keep the 44 tooth sprocket but tuning for high rpm also takes away from low rpm performance if too much duration is given to the engine, it will make excellent power at higher rpm's but can take away too much from the bottom end. Reed valves can cure this problem on the intake side, but a well set up piston port will run very well and a higher compression ratio can bring up the power from off idle all the way to max rpm, and a good pipe set up for a wide power band will also help. The better matched the port work and the pipe are to the desired rpm range, the more power and torque the engine will make.
A high rpm engine with more power will accelerate better and have a higher top speed if set up right.
Regardless of whatever method you use, the bike is going to be at its best when the engine is set up to run with the gearing you choose, too much miss match between the engine's power band and the gearing will make for a very sluggish ride or one that can accelerate very well but not have a decent cruise or top speed.

Ha! I would like know that the answer to this too! I have a SBP Exp pipe, 40 tooth sprocket with sprocket adapter, Dellorto Clone (RT) Carb, a thin BMX chain, I even got a Bottle Boost (useless) and I still can't break 30mph on level ground. I weigh 197lbs and my bike weighs 68lbs with the rack, lock, and tools I carry. Nope, I still top out at 29mph. I haven't tried the Fred Head yet, I wonder if it will finally put me over 30.

I'm doing 42mph with the 44 tooth rear sprocket and the stock nt carb... it's all in the port work and getting the engine to rev up to 10,000 rpm and still have plenty enough power to push the bike.
My combo is this... ported cylinder, fred head, nt carb, ktm50 pipe modded to fit the bike and 4" added to the belly section, not the header. The stinger outlet is nearly an inch in diameter, compared to the stock pipe's 3/8" outlet, and most performance pipe's 1/2" outlets.
The key to high rpm is getting the exhaust out of the engine... first, getting it out of the cylinder before the transfers start to open, then getting it out of the engine... quickly. A straight pipe tuned to the right length will also cause a scavenge effect at a certain rpm which will allow it to make power in a certain rpm range, this works, but some of the intake charge is pulled out from this scavenge effect and this is why expansion chambers can nearly double the power of a 2 smoker when tuned properly. To go even further, the length of the stinger also comes into play for even more fine tuning.
Raise the roof of the exhaust port by 1mm and you'll get an instant increase of usable rpm, by 2mm will give an even more noticeable boost in rpm, but go a little further and kiss that power good bye since the effective power stroke of the engine is from the top of the cylinder to the too of the exhaust port. To eliminate this problem ya gotta make the port wider, but you also need the exhaust flange on your pipe to be the same width as the port or the port work won't have any effect after a certain point.
There is a LOT more to porting than just opening up the ports and the info is in here, some of it from me and some of it from others, but it don't matter who you get the info from as much as it matters that you get an understanding of how and why it works.
You can also buy ported cylinders from Fred, Arrow Racing, Jake's, and a few others for an instant increase in power and speed, and some of these are sold for a very reasonable price, or you can try some simple port work yourself and enjoy some new found power that costed you just a few hours of your time.
I would recommend getting a new cylinder off eBay and doing some basic work like raising the exhaust roof 1mm, widening the exhaust and intake ports 1mm on each side, cleaning up that casting ridge on both transfer roofs, and lowering the intake floor by 5mm or cutting 5mm off the bottom of the piston skirt on the intake side, then install that jug with a new set of rings and you'll have a bike that can accelerate really nicely with a top speed around 35 or better. Now you can decide whether to do more extensive porting or to go with a 40 tooth rear sprocket since now you got the rpm And the power to get there...
If you got the budget for it, you can buy a top end kit from the sources I've mentioned above, but you can get similar results on your own with a dremel and some reading... I just gave you a basic port job that'll make your bike a lot more impressive too... especially if you already got an sbp pipe.

Going down to a 40t sprocket let me gain a few mph from 30-33. then i bought a torquer pipe and an rt head and tested them around the same time and get 36-37 mph, 38-39(maybe even 40) on perfect conditions which only happened a few times. I wonder if I would have bought a snake pipe if it would let me go 40 mph all the time.

I'm fine with my speed, but worry about the drivers. I literally have to go under the highway or overpass to get to the main part of the city, and both those streets are major intersections/streets.

I don't weight much either, maybe thats why I get those speeds.

Davezilla Thanks for the quick explanation on porting. I have a dremel as well as a drill press if needed and I would love to learn how to port correctly. I'm kinda afraid I will over due it and, ummm, you know, F-it-up! haha. I would think that taking as little as only 1-2 mm off can be tricky, no? Perhaps I will search this website and the rest of the internet for information. A video would be helpful too. Also, my exhaust header is about 17" long, this gives me a pretty good low end acceleration and torque but I was wondering, is this length too long? Will a shorter header give me more top end? I'm running a Dax F-80 69/70cc, balanced crank, 2 stroke. Also, I got a bottle boost but it doesn't seem to do anything, I mean, I can't notice any real increase in performance, but also, no decrease in performance either. Do you recommend dumping it or leaving it on? I already have an un-taped stock intake I could put on.

Boost bottles don't work. That's a proven fact by everyone who's ever used one, that has the abilility to admit it. There are a few people who claim to get more hp with them, but they're the same people who think putting a loud muffler on their car will automatically increase it's horsepower by 10-15%, buy splitfire spark plugs, and a giant trunk wing.

There's basics involved with going faster. One of the most important, yet most neglected thing is the bike itself. You need to fine tune it. Even an 80lb cruiser can go fast if it's well built, everything rolls smooth, doesn't vibrate, drag, orlumber under it's own weight.

Many people have hit 35, even 40 on a mildly tuned engine, with all stock parts, a smaller sprocket, the right mixture, etc.

Port matching the intake and exhaust is the most important thing for a stock engine. Anyone with any brains knows that the smaller ports on the intake and exhaust don't line up with the engine, and a few minutes with a dremel can fix that.

After you learn the basics, then you can start buying parts. 'Cause there's a lot of "race" parts out there that are as useless as a carbon fiber hood on your honda.

There are a lot of other factors. Altitude, rider's weight, lack of aerodynamics (a beach cruiser and an upright riding position has the same wind resistance as a mack truck,) and sometimes, actually, a lot of times, you just got a crappy engine, and nothing will help it.

Find the guys on the forum who really go fast (proven fast, not "oh yeah? My huffy does 55!" Fsst) and follow their advice.

little porting work will do wonders.

im running 48tooth on 26".
Port match to jug and NT Speed carb. intake and exhaust ported.
Stock exhaust w/ rod cut completely and baffles punched out.
and a Cortez angle 6cc head.

and thats still running stock jets.

cant seem to find the video atm. there is one on YouTube.

if u take off the muffler cap, there is a long rod attached to it.
most ppl cut it down to 2-3" and place the cap back in.

i took it completely off.. only about 1/2" remaining, and punched out the baffles inside the large cylinder exhaust. (the rod goes through the baffles inside)

Definitely, the best bang for the buck is dong the port work, just don't go too far with anything unless you have a real good undersrtanding how and why it all works... Matching everything up like Bairdco mentioned is also a critical part of the whole mix as even the meanest port job is worthless if there's an exhaust flange in the way or the intake is so small it can only let in about half the air and fuel etc... The best stock intake is the short straight one, it's too small to perform out of the box but it can be opened out considerably. I bored mine out on my lathe from 13mm to 16mm, but now I'm running an 18mm intake witrh an 18mm Mikuni carb, and the new exhaust I'll be using is a KX65 dirtbike pipe, the stinger on that one is 1" diameter compared to the 3/8" stock pipe. My last bike was using a KTM50 dirtbike pipe and it has a 22mm stinger, that one I just welded a stock header to the pipe inlet and matched it to the exhaust port flange, that combo got me up to 42mph on the stock carb and 44 tooth sprocket, and 43mph from a 40T sprocket. I spent a lot of time with the porting and port timing as well as squish clearance to get the engine to rev past 10,000 rpm and still have good enough power down low to be able to take off from a dead stop. I think my static compression ratio on that engine was like 12.7:1 which was kinda pushing my luck on 93 octane pump gas, but it didn't detonate so I got away with it somehow. I was going to mix in about 30% methanol into my fuel to help keep things cool and detonation free but it worked without the need to go there.
As for jetting, these things are jetted pretty fat out of the box and usually need to be leaned down some, but anytime you free up the intake or exhaust, you gotta jet higher to compensate for the extra air in the mix so the stock jetting was working for me, just a little 4 stroking between 30 and 35mph, then it cleaned up and took off really good.
The ONLY way to determine the right jetting for your engine is by doing a plug chop and checking for the dark tan color. If you go too lean at full throttle for too long it will drastically shorten the life of your engine so always start out rich and work your way leaner keeping an eye on the plug color, using a new plug anytime you do a plug chop for an accurate reading. I prefer the Autolite #275 plug for this, it's way better than a stock plug but they cost around $2 each so they're expendable, but reliable enough to run on daily as well. I'll switch to a platinum plug after the jetting is correct for better longevity. No need for an irridium plug since they won't give a performance gain, they just last a really long time and a $3 double platinum plug will last the life of one of these engines easily.

I'd remind anyone putting a few bucks in these low cost 2 strokes to disassemble the engine completely replace the hollow steel connecting rod journal https://youtu.be/fDaXq8PXMYUwith a solid forged piece and find a forged piston no need mention machining and matching new keys or removing as much of the reciprocating weight fastened to the crankshaft as possible. These are the very basics the engine does work I don't have a reed valve that I've found recommended.

BOYGOFAST said:

I'd remind anyone putting a few bucks in these low cost 2 strokes to disassemble the engine completely replace the hollow steel connecting rod journal https://youtu.be/fDaXq8PXMYUwith a solid forged piece and find a forged piston no need mention machining and matching new keys or removing as much of the reciprocating weight fastened to the crankshaft as possible. These are the very basics the engine does work I don't have a reed valve that I've found recommended.

Click to expand...

The stock crank has been proven up to about 10hp, no need to add unneccesary weight there by putting in a solid journal... MAYBE, and I mean a big MAYBE weld the crank pin in place IF you got the crank out AND if the crank is perfectly true. If one is to take an engine this far apart it's only necessary if building an all out race engine as it's cheaper and much more practical to just buy a balanced bottom end from someone like Dax or Venice Motor Bikes. Others in here also have balanced bottom ends but these are the guys I recommend to buy from since they're friendly to everyone and they sell reliable balanced engines. Fred sells a really nice balanced bottom end as well but he's a bit expensive, I'd buy one of his if I was building a race engine, but for the street, the other 2 I mentioned have really nice engines and bottom ends.

For $10, anyone can buy a new piston, no need to waste money on a forged piston, and good luck finding one that's the right dimensions for these engines... The stock piston works great, just don't go too lean or into detonation... that'll ruin even the best pistons on short order.

There's nothing fastened to the crank that can be removed, just buy a balanced bottom end, but if someone does have a crank out and wants to balance it, there are a few threads that cover the right way to do this... Again, it's faster and cheaper just to buy a balanced bottom end for $69, one would spend $40 on new bearings, $10 on seals, and another $5 on gaskets doing the job themself...

The video... How does that have ANYTHING to do with this topic, it's the same video you've been showing in here for the last 6 months or so... Make one of the bike on the road running if you want to impress anyone... Anyway, does that thing even run yet?

Reeds... The Arrow reed works really well once the rest of the mods to support having a reed are done, this reed has been proven to over 10hp on these engines as well...

Sounds like a good plan.... For the 12.7:1 compression ratio, that's the static compression ratio but the actual is a few numbers lower, probably more in the 9:1 range give or take a few tenths... The actual compression ratio is from the top of the exhaust port to TDC, but it can also change due to volumetric effeciency increases and such that raise it back up at a certain rpm range, there's some crazy voodoo math to figure out what it is exactly at it's peak power and efficiency rpm. The other part of the equation for no detonation is a really tight squish band to keep the combustion away from the edge of the piston crown. I'm sure you've read or seen posts in here where others are dead against ramping the piston because it disrupts the squish band, it does but keeping the squish as tight as possible and not ramping too deep into the piston helps maintain a tight squish, but it will exceed 1mm in the ramped areas.

The NGK plugs are the best ones for these engines and that's what I use as well, but I do switch out to cheaper plugs during break in and tuning where a plug chop requires a new plug for the most accurate reading, that's why I use the Autolite #275 plugs during this period, they're cheap, but way better than the stock plug these things come with. I'm sure I could cross reference the Autolite number with the Bosch number to get an even less expensive plug that's more reliable than the factory plug, but at the same time more expendable than an NGK, There's nothing wrong with using the NGK plugs for tuning and plug chops either since you can use the plugs after you get a reading off them but once they're darkened by a rich mixture, they won't lighten back up quick enough to get accurate subsequent readings. Eventually I'd like to get a Morgan Color Tune setup so theere's no wasting new plugs during tuning, this setup uses a special plug with a clear glass insulator so you can see inside the combustion chamber with the engine running and it's the color of the flame that determines if you're running too rich, too lean, or just right. We've used the colortune setups on everything from 2 stroke dirtbikes to Harleys and it works great, but the colortune setup is hard to find and a bit pricy for this hobby.

The port work and piston ramping is a good way to get extra power and speed, some use a ramped piston to experiment with before doing any actual cutting on a cylinder and others just leave the ramped pistons in place, both ways work, just if you plan on running a really tight squish band it's best to do all the cutting on the cylinder and just using a ramped piston for experimenting to make sure you're not cutting too much out of the ports in the cylinder, then putting an un cut piston back in after the final porting is done. I personally don't see anything wrong with keeping thet ramped piston installed permanently as long as there's no detonation issues and the compression ratio is where you want it to be since ramping does take away from the compression ratio, but very slightly as you're only cutting out maybe 1/2cc worth of metal that adds to your chamber volume so it would be like having a 6.5cc head installed instead of a 6cc head by using a ramped piston. Now if building a race engine where every tiny bit counts then an un cut piston would be the way to go, for the street you'll never miss it. Others are dead against ramping pistons for these reasons, but the real choice belongs to the bike's owner.
I also use blue locktite on everything, the red is just too strong for most the hardware on these engines and will snap a stud before letting go if disassembly is needed later on, it can be released by heating but the blue holds just fine. And it's always a good idea to pre lube the bearings and rings during assembly so that first start isn't so rough on them. Motor oil works just fine but STP works better if the engine isn't going to be started for a while longer like if it has to sit for a week or so before starting, STP will do a better job, otherwise, motor oil works great.