Chinese 4 stroke GY6 139QMB 50cc with 100cc BBK. Can't exceed 35MPH


New Member
Hi everybody.
I'm new to gy6 cvt style engines/bikes. I recently purchased a Chinese 50cc Metro scooter from Baodiao (TaoTao ATM50-A1 clone). This scooter had several limits/restrictions put in place to prevent it from exceeding 30mph. I have removed or replaced all parts that I could imagine having any restrictions including the fuel valve, belt, variator, clutch, carburetor (100 port), intake and insulator gasket (was a strange restriction in stock insulator at 14.5mm opening now 19mm), removed emissions PAIR system, changed valve cover to non pair cover, upgraded engine to SMS Performance's 100cc big bore kit, performance A9 cam, rear gear 17t/49t, performance CDI, preformance spark cable, NGK spark plug, new unrestricted performance straight exhaust and new pipe and I even put new high performance Pirelli tires. Wow can I go over board or what? =)
So why am I writing this post? I still can only go 30-35mph!!!

Ok here's my actual question; does upgrading to a BBK change the stock engines normal power band? I spoke to the manufacturer's rep and they could only tell me the stock engines peak power band should run between 6000-7000 rpm's. I have spoken to people with two different opinions on how to properly tune a gy6 engine. Non techie's response seems to always be add weight to go faster reduce weight to get more launch power. The other opinion (techie) is watch your rpms and increase weights until you cruise between 6000-7000 rpm's with a wot at about 7500. That statement can be found in multiple forum posts but non that seem to be referencing a bike with a 100cc BBK. I'm not sure what the BBK might do to the power band's peak range.
I started at 5g = wot about 9000, went to 6.5g = wot 8300 and I'm now at 8g = wot 7480 cruising at 7000-7200.
At this point I feel I'm actually getting slower. Should I still order 9g and 10g variator weights to try getting to a power band range of 6000-7000 rpm's or do I now have a higher power band threshold because of the BBK I installed? Why can't I exceed 35mph?
Advertisement - register to remove this


Staff member
You are sure the CDI is working correctly? How is your spark plug color looking?


New Member
I actually have two new CDI's. I upgraded the CDI about two months ago and then one of my performance kits also came with a new one as well. I just swapped them and I am still having the same top speed. I installed the bbk about 200 miles ago. Just checked my spark plug and it still looks fine.
Last edited:
Advertisement - register to remove this