Dell'Orto jetting Aprilia 125 H₂O (Motor Hiro)
The factory jetting of the Aprilia 125 H₂O (Motor Hiro) (1981) with the Dell'Orto PHBH 28 is: main jet 102, pilot jet 55, throttle slide 40, jet needle X 27 (clip position 2 from top), atomiser 266 BN. All figures come from factory documents and are source-checked.
| 1981 Dellorto PHBH 28 | |
|---|---|
| Carburettor | PHBH 28 AS |
| Dell'Orto reg. numberfactory calibration number, often stamped on the carburettor flange | 3349 |
| Throttle slideDell'Orto slide designation (e.g. 60/1) | 40 |
| Jet needle | X 27 In shop |
| Needle clip position (from top)position of the needle clip | 2 |
| Atomiseralso called needle jet or emulsion tube | 266 BN |
| Main jetcontrols the mixture at full throttle | 102 In shop |
| Pilot jetcontrols idle and the lower part-throttle range | 55 In shop |
| Starter jetjet of the choke (starter circuit) | 70 In shop |
| Pump jetjet of the accelerator pump (PHF/PHM only) | – |
| Float needle valveneedle valve of the float chamber | 200 In shop |
| Float (count – weight)as printed in the source, e.g. “1 – gr. 8.5” = one float, 8.5 grams | 2 – gr. 6,5 In shop |
| Pump deliverycc ±0.5 over 20 pump strokes, as given in the factory table | – |
“–” means: not fitted on this model, or not documented in the source. Source: Dell'Orto application catalogue, edition 12/1990, p. 4/5, row “Motociclo 125 H₂O (Motor Hiro) 1981” (Aprilia-section, reg. 3349)
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Important: figures apply to standard condition
These factory figures apply to standard condition with the original air filter and original exhaust. Any modification (open bellmouths, different exhaust, camshaft …) changes the airflow and calls for a revised setup.
Compiled from several sources – chiefly official Dell'Orto application catalogues and original workshop manuals – and cross-checked against factory tables and community knowledge. Details: Sources & method.
Matching jet set
A set around the factory value (main jets in steps either side) covers the range you need while dialling in – only a test on the engine shows the final size. Ready-made sets per model are in preparation.
Matching parts for Dellorto PHBH 28
Parts that are usually replaced when overhauling this carburettor range.
Further reading
Frequently asked questions
The spark plugs are black and sooty — what should I do?
Sooty black plugs show a mixture that is too rich. Which circuit is responsible depends on where the bike runs rich: at idle and steady speed it is more likely the pilot jet and mixture screw, in the mid range the needle position and atomiser, at full throttle the main jet. Also check for a sticking choke, the float level, a leaking float needle valve and an oil-soaked air filter. Change one step at a time and check the plug colour after every change.
Discussion: guzzisti.de
Air filter or exhaust modified — what jetting do I need now?
There is no blanket figure, and nobody can name one honestly without testing. More open intakes and exhaust systems lean out the mixture, so the setup has to go richer, usually starting with the main jet and often at the needle/atomiser too. Real-world reports show: the factory value is your anchor, and from there you test in steps (plug colour and how it rides, better still a lambda reading). Concrete community examples with full context are on the carburettor pages under “Real-world setups”.
Discussion: groups.google.com
In what order do I tune a Dell'Orto?
The proven order: full throttle first (main jet), then part throttle (needle position, atomiser if needed), and idle last (pilot jet and mixture screw) — and above all: only one change per step, tested on a warmed-up engine. The reason: the circuits overlap, and a wrong main jet falsifies any part-throttle judgement. The detailed procedure is in the guide “Tuning a Dell'Orto step by step” (German).
Discussion: caferacer-de
Do both carburettors need the same jetting?
Yes. Both cylinders need the same set of jets, and the factory figures always apply per carburettor. After every job the pair belongs synchronised (same slide lift, same mixture screw setting), otherwise one cylinder runs richer than the other and the plug colour becomes unreadable. Unequal jetting is almost always a sign of a previous owner tinkering.
Discussion: 2-ventiler.de