Minimoto Garage - Dell'Orto PHBN 17.5mm Carburetor Setup Guide

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Dell'Orto PHBN 17.5mm Carburetor Setup Guide
Big Thanks to Pocketbike.com for this awesome guide!
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 Pocketbike.com TechFAQ 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)

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.

Thanks to Dave in California for these awesome pictures!
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)
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