Better boost bottle response with re-jetting carb
This one is for you Skyliner70cc ( At least while you are at high altitude )
Quarkdude's instructions on Re-jetting the carb for better boost bottle response and a better running engine: Mainly for the 1800ft above sea level engines and above.
Step 1: Remove carb from engine or if there is room, loosen the clamp
that holds the carb to the intake and rotate the carb to get
access to the float bowl on the bottom of the carb. Rotating
the carb saves you from having to unscrew and disconnect the
throttle cable.
Step 2: This is a good time to close the fuel petcock so that when the
float bowl is removed, the fuel won't be gushing out. Also
it helps to put a rag underneath the carb so any spillage is
captured. Remove the 2 screws holding the float bowl on.
A little bit of fuel in the float bowl may be present. Remove
the float bowl, there is a gasket on top of the float bowl, be
sure to save it for reassembly. There will be a white plastic
round float inside that can be set aside. Now the brass jet
that we want to modify is exposed. It has a hex head on it
to put a small box end wrench or equivelant on to remove it.
Take the jet out. The one I have screws into anothe brass
piece, so I seperated the two pieces and took the jet itself
to the soldering iron.
Step 3: Here you can take a pair of needle nose vise grips and clamp
the jet in the jaws lightly to hold it while it is being soldered
( gets pretty hot ). Use at least a 60 watt soldering iron to
have enough heat to heat up the brass jet. Use 99% pure
silver solder to melt into the middle of the jet to close up the
hole. Use lots of flux to help transfer the heat of the soldering
iron to the brass so that the solder flows nicely into the middle
of the jet and seals it off completely. Check underneath the jet
to see if the solder has filled it all the way thru. If you get too
much solder, just lift the needle nose vise grips and lightly
bang it to the work surface while the solder it molten to
remove any excess solder. Now let the brass jet cool for at
least 10 to 15 minutes before touching it - Very very hot before
that. Now that the jet is cool, it is time to drill it to the size that
will work with your setup. I find that starting a little small on
the drill size helps to narrow down the mystery of what size
the jet really needs to be for your altitude and perfomance
needs. Start small, reassemble the float and remount or
tighten the carb and try a test ride to see how the bike feels.
Usually if the jet is too lean (which in this case will probably
be the case because we started small ) the bike will either not
go as fast as it did before the mod or it will bog a little. For
long term this would not be what you want, but for testing and
tuning around the block it should be fine. Keep taking the float
off the carb and removing the jet and drilling it up one size
then ride the bike around the block for a test and tune
evaluation. When the bike feels like it has a crisp acceleration
after releasing the cluch and doesn't bog on the top end, then
that is roughly the jetting that will work with your setup. The
idle screw on the side of the carb may need to be adjusted.
Sometimes an adjustment of the needle clip might need to be
done. . I did some research on the web as to what drill size to use for a
50cc pocket bike motor and it was roughly a 0.025 inch drill bit
pretty small. That is actually where I started and stepped the
drill bits up 0.025 then 0.026, 0.027 ( Test and tune between
each one) and then finally 0.028 and this is what worked the
best. The exhaust temperature out of the head at the top of
exhust pipe can be measured using an infrared thermometer.
The hotter the exhaust is getting, the leaner the jetting is
getting.
