BBK installed, unsure how to properly tune carb

[msuscooternewbie]

Yesterday I finished the install of a 100cc bbk. Took it outside and turned it over and it started wonderfully. Had good throttle response to a little revving and it seemed to be running decently at idle. Then it started to bog a little and now it won't start at all. I'm guessing the fuel mixture is wrong for the build but I'm new to everything like this so I'm not sure exactly how to properly tune a carb or if I need to replace the jets (local repair shop giving me advice led me to this site). Maybe adjust the needle a little and see what I can make work with the a/f screw, or just buy a new aftermarket carb for it that is better suited for the job.

Can anyone help me? I'm not completely lost but as of right now I have a bike that doesn't run without solid information to act upon to get her back on the road. It's a 50mm kit I believe

[badpumpkin]

I would suggest a 34,35 idle and 90,to 95 main. I have a couple of tao taos with 74cc kits. Also look at other members scooter specs at the bottom of there posts.

[badpumpkin]

I would suggest a 34,35 idle and 90,to 95 main. I have a couple of tao taos with 74cc kits. Also look at other members scooter specs at the bottom of there posts.

[niz76]

Welcome!

The stock carb will work just fine but installing a larger main jet/tuning carb is a must!

Pretty cool that your local shop sent you here.

[eclark5483]

Tuning the idle mix is super easy, even easier if you have a tach.

Typically, you start out by turning the air screw all the way in, then back it out 1 and 1/2 turns. Start the bike, make sure it's warm, and set the fast idle speed as low as it can go without killing the bike. Next, make half turns and wait about 8-10 seconds between turns and listen for speed changes. The bike should start to speed up a bit. If the bike's idle speed starts to drop, turn the mix back in 1/8th of a turn and you should be just right. If you keep turning and the speed just keeps increasing, then you will know you are too rich on the pilot. After you have it adjusted to the best mix, go ahead and turn the fast idle screw in till you get to the bike's spec idle rev. If you don't know what that is, turn it in slow till the back tire just starts to spin but can still be stopped with your foot or hand.

Here's a pretty good tutorial video:

[msuscooternewbie]

Feb 25, 2017 15:12:05 GMT -5 eclark5483 said:

Tuning the idle mix is super easy, even easier if you have a tach.

Typically, you start out by turning the air screw all the way in, then back it out 1 and 1/2 turns. Start the bike, make sure it's warm, and set the fast idle speed as low as it can go without killing the bike. Next, make half turns and wait about 8-10 seconds between turns and listen for speed changes. The bike should start to speed up a bit. If the bike's idle speed starts to drop, turn the mix back in 1/8th of a turn and you should be just right. If you keep turning and the speed just keeps increasing, then you will know you are too rich on the pilot. After you have it adjusted to the best mix, go ahead and turn the fast idle screw in till you get to the bike's spec idle rev. If you don't know what that is, turn it in slow till the back tire just starts to spin but can still be stopped with your foot or hand.

Here's a pretty good tutorial video:

Thanks for getting back to me quickly, should I be worried about anything else right now because the bike won't start with the current mix? As I said before, when I first finished the build it did start right up and it ran at idle decently well but now I can't get it to pop. Just start the carb tuning process and see where that takes me?

[eclark5483]

Yep, if you can't get it dialed in using the standard tuning method, then you are probably too lean and need to up the pilot jet. If the idle keeps going up the more your unscrew, and never goes down, you are too rich. You should go no more then 3 turns out, any more then that, is more then likely too rich. Look at your plug as well. The color should indicate where you are at.

[msuscooternewbie]

Feb 25, 2017 16:19:20 GMT -5 eclark5483 said:

Yep, if you can't get it dialed in using the standard tuning method, then you are probably too lean and need to up the pilot jet. If the idle keeps going up the more your unscrew, and never goes down, you are too rich. You should go no more then 3 turns out, any more then that, is more then likely too rich. Look at your plug as well. The color should indicate where you are at.

To up the pilot jet I'm going to need to take apart the carb and buy a new jet, correct? Sorry I have so many questions like I said I'm new to this and I need to learn as quickly as I can because I was hasty and decided to install a bbk without proper training.

If that's what I need to do, where is a good place for me to order that jet and should I also up the main jet as well? Or just get an entirely new carb

[msuscooternewbie]

Feb 25, 2017 15:02:05 GMT -5 niz76 said:

Welcome!

The stock carb will work just fine but installing a larger main jet/tuning carb is a must!

Pretty cool that your local shop sent you here.

So I'll need to for sure take apart the carb and install new, larger diameter jets for both the pilot (the smaller one for idle) and the main jet (for when I'm giving it some throttle), I'm just making sure I'm understanding correctly because I'm so new to this. Would you have a recommendation where to buy these jets and what sizes would you recommend? How about just buying an entirely new carburetor that has these jets installed?

[eclark5483]

Scrappydogscooters scooters sells carbs that are already up jetted, and also sells jets

[eclark5483]

Here is a link to the performance carb on scrappydogscooter's website:

www.scrappydogscooters.com/139QMB_50cc_Performance.html

If you scroll down a bit, you will see this:



It comes with a #35 pilot, and #85 main jet. This should suit you well for the most part.

There are other sources available, you might wanna look around for a better price, but I can't speak for the quality of the other offerings.

Depending on the scooter, you might not have any other choice but buy a new carb (which I would recommend anyway), unless you drill out the screws to change out the jets. Many of them are like that stock. Brent has a tutorial video on here that covers how to do it. Myself personally, I say piss on that, get a better carb instead of mucking around with one of those damn junk stock ones.

[msuscooternewbie]

Thanks for the advice. I installed a new carb and still can't get it to light up so I'm going to have to take it in. Thanks for the advice guys.