17.5mm carb setup guide
Here's an excellent picture of a Dell 'Orto PHBN 17.5mm carb Gordon Edwards of GEM Minimoto sent me. This picture has helped me in so many ways!! (He sells just about ANY pocketbike made at good prices too! not just GEMs, and any odd part for any odd bike you've got!). Gordon's gone above and beyond the call of duty to help me out- I highly recommend buying from him!
Dell 'Orto PHBN 17.5mm
That about illustrates anything and everything important. Go slow on the disassembly and you'll do fine. You should only need to touch the jets and needles. Remember, the carb on the Polini is a PHBN, so look at sub insert H and N. I is the choke lever, and L is the throttle cable illustration.
Gordon will have some nicer illustrations scanned later on too.
Initial settings for the 17.5mm carb (left side of carb):
(*note:
I believe this to be a correct setup on a 50CC race bike- others may differ
somewhat)
Center screw (idle position screw): 4
full turns from fully closed.
Right screw (idle air mixture bleed
screw): 3 and 5/8 turns from fully closed.
Use the tapered needle (A 8), installed
with the clip in the center notch position.
Use the #90 or #92 main jet, depending on
your bike’s level of modification. You
may need to change this jet further, if you are unable to obtain satisfactory
tuning results.
Note: These are rough initial settings
ONLY. You MUST properly tune the
carb to YOUR bike by testing and riding the bike, to properly determine
correct jetting and needle placement. Jetting should be verified by
“reading” the spark plug (see the TechFAQ – http://pocketbike.com/techfaq.htm
for more info on reading your plug).
Failure to verify mixture settings may
result in a lean condition, and cause engine failure.
To lean the mixture at lower RPMs, lower
the needle further into the jet, by raising the clip’s position on the
needle. To richen, raise the
needle further out of the jet, by lowering the clip.
Setup and
operation notes:
The black knob is the choke, you must
pull up and turn the knob ¼ turn to set the choke. Release is NOT automatic, you must release the choke
manually.
The fuel inlet line fitting is
located next to the choke knob. The
small hose dangling from the carb is a vent hose.
You may route this hose to a vent canister if desired, or required for
racing. Do not uncap any vacuum
fittings that come capped, and it is not necessary to cap any exposed vacuum
fittings (they are unused in this application).
The stock 14mm throttle cable WILL NOT
WORK with the 17.5mm carb. If
your kit does not include a 17.5mm cable, purchase the proper 17.5mm throttle
cable (see our online pricelist at http://pocketbike.com/pricelist.htm)
Use the appropriate chrome cable
guide elbow for your application (you will likely need to install the shorter
one). Be careful when installing
or removing them not to put any side-loads on the tubes – they are made of
BRASS, and can bend or break if mishandled.
East Coast Minimoto
stocks a full range of jets to fit the 17.5mm carb should you require
something beyond the scope of the jets enclosed with the kit.
These instructions should explain anything you need to know regarding the odd screws and knobs on the carb itself.
Be careful with the float pivot pin too, as I hear it is damaged easily.
Here's my Dell 'Orto 17.5mm carb and the rubber manifold:
ADJUSTMENTS ILLUSTRATED:
(obviously the jets can't really be adjusted, just replaced. The needle can be lowered into the jet to lean out the low end by moving the 'e' clip)
Email me for additional questions: