Scoot F.A.Q.

Q. What other parts or supplies will I need along with a big bore kit?
( 139QMB / GY6 50 / QMB139 / 1P39QMB )

First read the F.A.Q. question regarding big bore kits HERE if you haven't already. Then review the list below.
  • If using a 50mm or larger bore, a crankshaft upgrade is advisable because of premature connecting rod bearing failures frequently associated with large bore diameters and stock Chinese crankshafts.

  • Bores up to 47mm often work alright with a stock cylinder head. Above 47mm may require a "big bore" head to keep compression at a reasonable level. A big bore head is a cylinder head that has a larger combustion chamber. Some big bore heads are just like stock heads, but with the enlarged chamber. Others include larger valves and/or port work.

  • If you will be using a different cylinder head with your big bore kit, you will need to know if it has 64mm or 69mm valves. You will also need to know if your current head uses 64 or 69mm valves. You can use either type of head, but the rocker arms and valve lengths must match. For example if you use a 64mm head, you must use rocker arms or an entire rocker assembly intended for a 64mm head. The same is true for 69mm heads. CLICK HERE for more details.

  • You will need a selection of main jets to help get your carburetor in tune with the new big bore kit. Some advise a main jet as a one-size-fits-all solution. This may work well enough to get by, but I find it's best to go through a tuning process and select the jet that works best in your specific application. Too many variables such as air filter style, elevation, conditions, carburetor, and more won't allow any one jet to work the best for every application. Stock and many aftermarket carburetors for the 139QMB use Keihin 99101-116 small round main jets. You can buy a jet kit that covers most tuning needs at Parts For Scooters (part # 114-49). You can also find them individually or buy kits from other retailers. If you are upgrading to a larger carb from a stock GY6 125-150 you will need Keihin 99101-393 round main jets. Parts For Scooters sells those in a kit as well (part # 114-48).

  • It may not be necessary in all cases, but you may benefit from having a selection of roller or slider weights on hand to get your CVT in sync with your new engine combo. Heavier weights may be needed to control RPM because of the additional torque that the larger displacement generates if the CVT was previously tuned with the stock cylinder. Most 139QMB engines use 16x13mm roller or slider weights. Tuning roller sets that include many weights can be bought cheaper than buying multiple individual sets of rollers or sliders. They're good for helping you get in tune, but these weights are not intended for regular use. They will not last long. If you use tuning roller weights you should replace them with a quality set of rollers or sliders once you figure out what weights work best. If replacing rollers with sliders, buy sliders 1 gram heavier than the rollers needed to achieve similar RPM.

  • I like to change the oil after a BBK installation, and frequently during the first miles, so you may want to have a quart or more of your favorite oil around. I would advise using a standard (non-synthetic) for the first 500 miles.

  • Go over the BBK Installation Tutorial and make sure you have all of the tools to complete the job.

Related Info :
Big Bore Kit Installation Tutorial
Carb & CVT Tuning Video
64mm vs 69mm Valve Lengths

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