Keihin 22mm carburetor rejetting

[John in PA]

I've replaced stock exhaust with NHRC unit (great build quality, fit and finish) and modified air intake for improved flow (air intake pipe and Uni pod filter tucked inside modded factory airbox. EPA crap removed and vent/overflow  hoses rerouted.  Stock jets are 110 main and 35 pilot.  Parts seller (Scootertronics) supplied 130 main and 45 pilot.  Ran like crap!!  I talked to a local bike shop where I've dealt for years.  They suggested a 117.5 main and 40 pilot.  Runs much better, but no improved top speed at WOT.  Also some mild popping on decelleration/throttle overrun.  I read that lean bottom end can not be improved with jet alone, that some passage in the carb needs to be drilled.  Anyone have more info on this??

[lifeisabeach]

I could be wrong here, but I think an increased top speed will only occur with an increased RPM or a change of rear end gears

[lifeisabeach]

Curious as well,  what size uni filter did you use?

[John in PA]

I lied when I said it was a UNI.  It's an EMGO conical filter, stainless mesh over gauze.  Used it with the chromed air intake from NCY.  Had to use the cut off end of the stock airbox tube as a bushing where the intake tube mounts on the carburetor as the intake tube was too large to fit stock carb.  Nice tight fit now, though.

[swiftscoot]

I've found the best way tune a carb is to buy a set of ten main and ten idle jets, and go up or down by two to five thousandths.  The other way is to use drill bits to slowly increase your jet size.  Sometimes going from (for example) a 117 to a 120 is enough to put it too rich.  It's a balancing process between the idle and the main jet.  You get one where you think it's good, then adjust the other until it's just right; then adjust your air fuel mixture and get your idle screw set.  Often after you get your main somewhere good, then change your idle, you'll realize your main needs more work.  Keep track of each move, and eventually it will work out.  Plus there's the needle which can be leaned or richened.

You can definitely increase top speed on a Gy6, and yes RPMs are important, but it's all about getting these engines to breathe.

You need only to jet and adjust to tune a carb.  Any popping indicates you're running too lean, or if you go wide open and it feels like a complete loss of power.  Avoid running lean at all costs.  Hesitations in acceleration indicates it being too rich.