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

[DeeJae]

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 SMM 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 powerband? 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?

[sc00ter]

Top speed ultimately comes down to final drive gearing. The gear box that transfers the rear pulley power to the rear wheel is controlled by a primary and secondary and final gears. Not sure what a GY6 is geared for on its final drive ratio, but counting the teeth on each gear and calculating the rotation ratio will tell you the approximate max mph. My riding friend knows how to do it, I dont. It doesnt hurt to go with different variator weights first to see if that works.

[DeeJae]

I replaced item 14 in this diagram with the 17/49. It was originally 51/15 I haven't changed item 12 yet. I believe it is 52/16.
www.partsforscooters.com/scooter-parts-diagrams/50cc-4-stroke-QMB139-Gear-Box-Parts_2

I have seen a formula to calculate a GY6 ratio of clutch rotations to tire rotations. So I can calculate the improvement from my previous and current ratio but I don't know where to go from there. How do you convert that ratio to actual MPH limits?
Here's what I think you would need. Basically I take 51 and divide it into 15 which gives me my old secondary gears ratio of 3.4 and then I take my primary gear ratio of 52/16 = 3.25 and times the two ratios together for an overall previous ratio of 11.05 rotations to 1 tire rotation... I think.
So if I use the new gears ratio in place of the previous gears we should see 49/17 = 2.8823529412 x the primary gears ratio of 3.25 = 9.3676470589 to 1 tire rotation.
I have removed almost 2 rotations from my work per tire rotation. I should be seeing a speed increase...?

So if I am currently roughly 9.4 to 1, how do I calculate that into the bikes top speed using a 10" rim with a Pirelli 3.50 x 10 tire. I think the tire circumference would be calculated 3.5"(height) x 2 (top&bottom side wall) + 10" (rim) = 17" diameter x 3.14 = 53.428571429". I should be able to take the amount of inches in a mile and divide it by the tires circumference to see how many revolutions the tire must make to go one mile. So 63360" / 53.428571429 = 1185.8823529 tire rotations per mile?
Ok thats about as far as I can figure. What the next step to calculate my top speed?

[sc00ter]

You got way further than me with the math! I just know for my scooters (Yamaha Zuma 2 strokes) 15/50 will work with stock or cast iron bbk and 14/42 works best with aluminum bbk's. 15/50 is good for 50-60mph and 14/42 will do 60-70mph and is much faster out the hole. You still have to set everything up to maximize performance. Roller weights, torque spring, clutch springs all affect results. Also, if the bbk does not put out enough power to push the gears nothing will help. I put the 14/42 gears in my cast iron bbk by mistake and I could not get over 50mph max! Figured it out really fast and put the 15/50's in and got back to 55mph.

[tpryde1]

You may be missing the forest for the trees. You have all the stock gearing in that scooter for 50 mph. You are wasting time and money re-inventing the wheel. I suspect that what you are missing out on is H.P. what are you running for a main jet? What is your actual displacement in CC's? You can't get 100cc on a 50cc block/QMB 139. Advertising hype. Check for Cylinder Diameter in mm. Be aware that beyond 47mm/72cc you are running very thin cyl walls. A recipe for disaster. See my thread on 72cc BBK. That's almost entirely stock including cam/small valves. 44MPH is optimal and what I get at 4.5 HP. You have the exact same stock gearing. Best of luck

[tpryde1]

Actually I failed to mention you have the exact same scooter as my Roketa Maui. Just different vendor.

[DeeJae]

Originally Posted by tpryde1

Actually I failed to mention you have the exact same scooter as my Roketa Maui. Just different vendor.

yes I've seen your bike, my is a clone of many Chinese and Taiwanese 49cc bikes. The stock gears in this bike were an initial problem but I have changed the secondary gears currently to 17/49 which many clones of my bike successfully run 50-55 miles with here in Maui. The local dealer in my area have several bikes that have this gear set and have no issues what so ever. I missing something, I was just hoping it was something obvious to others that perhaps maybe they had overcome previously.

[Roscoe]

What comes to my mind. Accuracy of of your speedometer, weight of the rider, weather conditions and terrain. My old 81.3 cc scooter does 45 mph and that was with me when I weighed 200 lbs. I put in the heavier weights that came with the big bore kit and I can feel it bog down when the belt moves to the higher position. Next time I have the cover off the transmission I'll go back to the original lighter weights. Maybe if I keep losing weight though I won't have to. You are working with 4 maybe 4.5 horsepower and that's not very much. Also premium 100% gas may help, especially if the engine tends to run hot. I use a bug catcher metal mesh after market air cleaner, pretty much stock carb with 90 main jet (might be a 92) and after market exhaust. 35 mph is the fastest I care to go with the roads and drivers around here but sometimes 40 if traffic demands it.

[Redtangsoo]

I would up jet to a 100 main, for starters with the 100cc bbk.