Post 50mm BBK install questions

[scootzute]

Hey guys first time on the forum and had a few questions. I have recently Purchased a tao tao 50cc moped that was beat to hell and back. I drove it for the last month and the piston rings were so shot i was getting really bad blow by at idle. So i did the most logical thing and ordered a 50mm top end for it and installed it last Thursday. 

Before anyone asks i have already installed several bbk's in the past but only for other people who had already done all the basic mods you can get. 

Now my questions are this what are your guys suggestions for ventilator weight tuning along with rollers vs sliders. Also i am aware of my need to rejet to a bigger main jet but since it is the factory carb am i better off buying an aftermarket and if so what would you suggest, if there are any other modifications that are recommended for adding to the bike along with the bbk please feel free to let me know i know how the 139qmb engine works but i am no guru.

[90GTVert]

Sliders are my choice for improved durability and possibly better performance. My suggestion for tuning with them is to go about 1g heavier than what you need with rollers. If you don't know where you'll need to be with the weights, buy a couple of sets maybe 3g apart. If you have a scale with resolution to 0.1g and some aluminum foil, you can get a lot of combos out of that. Here's an example :

Let's say you buy 5g and 8g sliders. You can add at least 1g of foil to each slider if needed. Here are some possible combos, and of course it can be fine tuned within the range for 0.5g increments if needed.

5g
6g (5g + 1g foil)
7g (6g stuffed + 8g)
8g
9g (8g + 1g foil)

So you can see that you get 5 different weights of sliders (just moving 1g at a time) for the price of 2 sets. It doesn't have to be those weights, but you get the idea and actually I think that's a pretty decent range for the 139QMB. Use trial and error tuning to dial it in.

[scootzute]

Thank you for the advice vert. I am well versed in the engine modifications i'm the neighborhood grease monkey but the whole idea of the ventilator was new too me so ill do the tuning as soon as i can afford it and let you know how it goes.

[dexameth]

I like using the shorter drive boss in the variator. I don't know the exact specs but I do know they are 1mm shorter than the stock variator resulting in higher top speeds and lower cruising rpms. I recommend using stronger clutch springs as the shorter boss will cause the belt to ride a little lower in the clutch at idle, making the clutch spin faster and it will want to engage sooner.

Also, if you are using the stock variator... people are gonna hate this... but look into those "performance" variators on eBay. Normally around 25 bucks. They come with 5, 7, and 9 grams rollers for tuning, blue clutch springs and the shorter boss. People tend to hate them, I don't really like them but only because they wear fast. For the price, easy to replace.

[memturbo]

I just got a 50mm kit for my 139qmb and have had nothing but trouble .
First the rings were not marked in any way except one is slightly darker than the other

I had to guess at where they go on the piston so this might be the problem ,So here is my problem.

My scooter top speed was 32mph after the kit it's still 32mph changed everything including a A9 cam and upjetted the carb + new rollers that came with the kit only difference is it has a bit more low end .

So my question is having the rings wrong causing my problems?

the kit was purchased at Amazon no: CPW 88807

Thanks in advance.

[90GTVert]

Welcome memturbo!

I'd put the darker one in the 2nd groove. Usually they'll be silver and black and silver goes up top. That said, I don't think that's likely to be your issue.

Any idea what the actual compression is (compression test)?

[memturbo]

just checked compression it's about 55 psi ,I checked rings and they were fine

Replaced head with stock and it's around 60psi

[90GTVert]

You may have a compression tester sealing issue. It shouldn't be starting and running at 55-60psi.

[memturbo]

Yeah The tester I rented was fubared , I got an email from the company and the are going to send me a new kit at no cost.

Very nice of them to do so So I recommend them highly anyway when I get the new kit Ill post my findings for you .

Again thank you for the help.