139QMB GY6 50 : 72cc Big Bore Kit & A9 Cam : Before & After Acceleration & Cruise : Chinese Scooter

Here's a quick video showing the difference that installing a 47mm big bore kit (72cc) and A9 camshaft made on a 49cc Chinese four-stroke scooter. 0-30MPH acceleration is timed and wide open throttle cruising speeds are shown before and after the work. All modifications are listed at the end of the video.

Please note these two things. First, I am 300 pounds so acceleration will be much quicker with lighter riders and speed may also increase. Second, these are indicated speeds. I did not test with GPS, but I would expect this scooter to go at least 40-42MPH. There is always some error in stock speedometers, but this one appeared to be close.

47mm cylinder kits are commonly called 80cc kits by sellers. The larger 50mm+ kits are often advertised as 100cc. I prefer the smaller 47mm bores for reliability and longevity concerns. All modifications that increase horsepower, torque, RPM or speed are likely to have a negative affect on components due to additional stress. Larger bores may also have heavier pistons. They have been known to cause or hasten crankshaft big end bearing failures. I owned one with a brand new stock crank and 50mm BBK that ruined the connecting rod bearing and left me stranded at 2,003 miles. It seems that many people using 50mm and larger kits try to limit RPM with heavier roller weights to reduce their risk of failure.

If you are considering a big bore kit, I recommend going with a quality brand instead of the cheapest kit that you can find.

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#BigBoreKit #ChineseScooter #ScooterPerformance

[ccscoot]

Hey. For my regular viewers; I've been away for a bit because I was sick for about a month. Just a cold and sinus stuff, but I couldn't talk for 3 weeks. I don't save videos to post later, because I like to keep you up to date with what I'm doing instead of seeing what happened months ago. Thanks for your patience and I really appreciate your support. This is a short vid, but I hope you enjoy seeing the before and after of what I think are a couple of the best bang for the buck mods for these scooters.

[Soknik]

You know it doesn't matter if you're talking about a small block V8, 49cc scooter engine or the little nitro methane burner in an RC car- getting some extra power from a motor just plain feels good!

[Lord_Verminaard]

Damn that is a lot of gains when doing basically the bare minimum in a big-bore job, pretty amazing. Pretty close to a stock 150 performance wise. Nice work!

[stilianivanov]

Usually I didn’t take customers also, because was pretty busy with my own projects and I wasn’t really confident.

The funny thing is that over the years I became quite decent mechanic and I accumulated a bunch of tools, so was quite capable of fixing anything on a 2stroke engine….
So I decided to start wrenching for others in order to pay my garage rent and I ended up being an outboard engine mechanic with my own bigger and better shop.
Funny how your life can take another direction without even realizing it.

[scotthartwig]

I would like to mention I've been running a 90cc naraku BBK for 2 years now with a flow big valve race head and have had no issues. I did also upgrade the crankshaft to a naraku stock stroke heavy duty crankshaft and connecting rod assembly for longevity. I've got the carb set very safely rich and the cvt weights set on the heavy side to get more longevity out of it. Just did a check up this winter to find I had a bent the exhaust valve but the piston was fine besides a slight scratch and indentation from the valve. So order a new set of gaskets and seals along with a new head. Just got it up and running last night and nothing seems wrong. Absolutely love this thing and couldn't ask for a more fun of a toy

[jeffmee]

I love the videos like this on 139QMB 50cc well done on the work you done I would love see more on these scooters from you put I know your into your two strokes thanks.

[noahthepetrolhead]

Great little video and sorry that you have not been well

[Stoneforth]

i miss my chinese scooter. It's been years

[Josemanc]

Good job man. Hope you are feeling better

[jamest.]

It's good to see something more familiar, i finally got the, well some of the bugs out of my scoot, ya wouldn't belive what was causing the shut-off at speed, it a slightly leaky filter, on the tank side of the pump, it got bad enough for a drip every minute or so, before it was just a drip or two over night, I wasn't sure even what it was, I guess it was pulling g air, and getting bubbles in the fuel, causing float issues, or running the jet dry, who knows, but that much is fixed, I got clutch issues now, I got a bit of oil blown out the valve cover, into the CVT/clutch, I have a Molly additive in the oil, even after cleaning everything, replacing the new belt, and cleaning with sand paper, the clutch is still slipping, I'm going to swap clutches, but the springs or weight of the other clutch, it has a 5k rpm grab with stock springs, the clutch on it is about 3400, with aftermarket springs the lightest, it's around 6500, rpm, so it's stock springs for me, but I think it has gears, because it has no low end TQ, I need to check that out, but I have a new head for the thing, with slightly oversize valves, insted of 16/18's, it has 18/21mm valves, I have to port it, it's kind of a rough cast ports, needing porting for any decent flow, I guess a semi finished ports, hopefully it will get the compression up a bit, and make more power with the extra flow, it's definitely not like the naraku head I have for the 52mm build, I'm hoping to get this 50mm bore engine running a bit better before swapping the engines later, maybe winter, or spring, i hope to break 55-60 mph cruise, with the 95cc engine,

[xinixxagnix]

`Welcome back brother, i hope you are all fine (your knee to :(, thanks for your great content ;)

[Tomijus]

I have the same scooter, 0- 30mph 11seconds and top speed 40mph. Bigger carb and jet, cone filter. Malossi belt and 5gram weihts.

[gnorm]

Have a gy6 with a 72cc kit in the garage, clutchwork is a must to make it go. Way harder springs. Otherwise its like having a 1500rpm converter on a top fuel dragster( may be a bit exaggerated, but you get the point ).

Decent gains with larger carb and slighter larger exhaust also

[Manwichman]

I wish you'd work on my stupid ice bear trike. It's a 49cc, now with a 52mm piston and does almost 50. 47 cruise actually but I get a whirring sound from the cvt area ever since I replaced the kickstart assembly. If I pull out the kickstart lever a little it dissipates but after I let go it sinks in a little and makes a noise🙁
You are really talented at this stuff and knowledgeable as heck. Great videos too to help me learn🙋👍

[MischaGielen]

Very nice video, exactly what I was searching for. You said it was for somebody else, but did you get any feedback regarding fuel consumption? A gut-feeling tells me bigger bore = more consumption, but I don't really have a clue about the numbers.

[fenceman]

Sorry to hear you were feeling bad hope all is well. Do aftermarket variator help with these scoots if so what brand ?

[ivannielsen]

Good recovery. And nice update. Will also try to port match my manifold and spacer tommorrow. Did you use a piece of cardboard to make an template?

[nikesan]

I put a bbk on a kymco agility 50. it was a airsal 80cc kit. big differece, got it up to 90kmph. took off a lot faster also. Thing is, u have to put heavier rollers on, I had 7g on and bigger jet otherwise it would bog down.

[MrSandRune]

I ended with 0.40mm main-jet.Nobody who had lower.And engine couldnt take, stalled givin gas.That was fixed by changing from 6g to 8g roller-weights.Spark knocking returned changing from original cdi, to adjustable.47 mm bore.Studied cdi, and nobody who works.The 2 connector are skipped.4 connector Pin 1 trigger signal, Pin 2 Ignition coil Pin 3 and 4 ground, sometimes Pin 4 +12 v.Also work with cdi`s with 6 connector.Not tried, but its spanish mc repairshop.

[blazeorangeandcamo]

Nice to see new content about the good old 139qmb. How significant is the A9 cam? How does it affect performance? Is it for more top end or off the line speed?