Dell'Orto PHBH 28 – jetting & tuning
Dell'Orto range without accelerator pump; jet needles of the X series. Fitted from the factory to 21 documented models from Aprilia, Benelli, KTM, Laverda, Moto Guzzi.
| Make | Model | Year | Version | Designation | Reg. no. | Details |
|---|---|---|---|---|---|---|
| Aprilia | 125 H₂O (Motor Hiro) | 1981 | Standard | PHBH 28 AS | 3349 | view jetting |
| Aprilia | 125 S H₂O | 1983 | Standard | PHBH 28 AS | 3377 | view jetting |
| Aprilia | AF1 125 Replica 89 | 1989 | Standard | PHBH 28 RD | 4077 | view jetting |
| Aprilia | AF1 125/88 | 1988 | Standard | PHBH 28 RD | 4022 | view jetting |
| Aprilia | AF1 125/89 (PHBH) | 1989 | Standard | PHBH 28 RD | 4055 | view jetting |
| Aprilia | AF1 stradale | 1987 | Standard | PHBH 28 ND | 3481 | view jetting |
| Aprilia | ETX 125/89 – Pegaso | 1989 | Standard | PHBH 28 RD | 4071 | view jetting |
| Aprilia | Tuareg 125/88 | 1988 | Standard | PHBH 28 RD | 4015 | view jetting |
| Aprilia | Tuareg 125/89 | 1989 | Standard | PHBH 28 RD | 4049 | view jetting |
| Benelli | 125 Jarno | 1988 | Standard | PHBH 28 BS | 4027 | view jetting |
| KTM | Cross MX 80 | 1986 | Standard | PHBH 28 GS | 3445 | view jetting |
| Laverda | 125 LB | 1983 | Standard | PHBH 28 BS | 3372 | view jetting |
| Laverda | 125 LB | 1984 | Standard | PHBH 28 BS | 3387 | view jetting |
| Laverda | 125 LB Uno / 125 CU Ride | 1984 | Standard | PHBH 28 BS | 3400 | view jetting |
| Laverda | GS 125 Lesmo | 1986 | Standard | PHBH 28 LS | 3452 | view jetting |
| Moto Guzzi | V35 Custom | 1985 | Standard | PHBH 28 BS/BD | 3443/44 | view jetting |
| Moto Guzzi | V35 Florida | – | Standard | PHBH 28 BD (rechts) / PHBH 28 BS (links) | – | view jetting |
| Moto Guzzi | V35 Imola II | 1984 | Standard | PHBH 28 BS/BD | 3389/90 | view jetting |
| Moto Guzzi | V35 Nevada | 1991 | Standard | PHBH 28 BS | 4129/30 | view jetting |
| Moto Guzzi | V35 NTX | 1986 | Standard | PHBH 28 BS/BD | 3468/69 | view jetting |
| Moto Guzzi | V50 III | 1981 | USA | PHBH 28 BS/BD | 3351/52 | view jetting |
| Moto Guzzi | V50 Monza | 1980 | Standard | PHBH 28 BS/BD | 3339/40 | view jetting |
| Moto Guzzi | V50 Monza | 1981 | USA | PHBH 28 BS/BD | 3351/52 | view jetting |
| Make | Aprilia | Aprilia | Aprilia | Aprilia | Aprilia | Aprilia | Aprilia | Aprilia | Aprilia | Benelli | KTM | Laverda | Laverda | Laverda | Laverda | Moto Guzzi | Moto Guzzi | Moto Guzzi | Moto Guzzi | Moto Guzzi | Moto Guzzi | Moto Guzzi | Moto Guzzi |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Model | 125 H₂O (Motor Hiro) | 125 S H₂O | AF1 125 Replica 89 | AF1 125/88 | AF1 125/89 (PHBH) | AF1 stradale | ETX 125/89 – Pegaso | Tuareg 125/88 | Tuareg 125/89 | 125 Jarno | Cross MX 80 | 125 LB | 125 LB | 125 LB Uno / 125 CU Ride | GS 125 Lesmo | V35 Custom | V35 Florida | V35 Imola II | V35 Nevada | V35 NTX | V50 III | V50 Monza | V50 Monza |
| Year | 1981 | 1983 | 1989 | 1988 | 1989 | 1987 | 1989 | 1988 | 1989 | 1988 | 1986 | 1983 | 1984 | 1984 | 1986 | 1985 | – | 1984 | 1991 | 1986 | 1981 | 1980 | 1981 |
| Version | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | Standard | USA | Standard | USA |
| Designation | PHBH 28 AS | PHBH 28 AS | PHBH 28 RD | PHBH 28 RD | PHBH 28 RD | PHBH 28 ND | PHBH 28 RD | PHBH 28 RD | PHBH 28 RD | PHBH 28 BS | PHBH 28 GS | PHBH 28 BS | PHBH 28 BS | PHBH 28 BS | PHBH 28 LS | PHBH 28 BS/BD | PHBH 28 BD (rechts) / PHBH 28 BS (links) | PHBH 28 BS/BD | PHBH 28 BS | PHBH 28 BS/BD | PHBH 28 BS/BD | PHBH 28 BS/BD | PHBH 28 BS/BD |
| Reg. no. | 3349 | 3377 | 4077 | 4022 | 4055 | 3481 | 4071 | 4015 | 4049 | 4027 | 3445 | 3372 | 3387 | 3400 | 3452 | 3443/44 | – | 3389/90 | 4129/30 | 3468/69 | 3351/52 | 3339/40 | 3351/52 |
| Details | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting | view jetting |
| Model | Year | Main jet | Pilot jet | Starter jet | Needle (clip) | Atomiser | Slide |
|---|---|---|---|---|---|---|---|
| Aprilia 125 H₂O (Motor Hiro) | 1981 | 102 | 55 | 70 | X 27 (2) | 266 BN | 40 |
| Aprilia 125 S H₂O | 1983 | 102 | 55 | 70 | X 27 (3) | 266 BN | 40 |
| Aprilia AF1 125 Replica 89 | 1989 | 132 | 52 | 70 | X 40 (2) | 264 T | 55 |
| Aprilia AF1 125/88 | 1988 | 125 | 55 | 70 | X 56 (3) | 264 T | 55 |
| Aprilia AF1 125/89 (PHBH) | 1989 | 120 | 58 | 70 | X 4 (3) | 266 CF | 50 |
| Aprilia AF1 stradale | 1987 | 120 | 50 | 65 | X 42 (3) | 264 DT | 40 |
| Aprilia ETX 125/89 – Pegaso | 1989 | 130 | 62 | 70 | X 56 (3) | 264 T | 60 |
| Aprilia Tuareg 125/88 | 1988 | 115 | 62 | 70 | X 53 (2) | 262 EF | 55 |
| Aprilia Tuareg 125/89 | 1989 | 128 | 65 | 70 | X 53 (2) | 262 EF | 55 |
| Benelli 125 Jarno | 1988 | 125 | 55 | 65 | X 31 (2) | 262 EF | 45 |
| KTM Cross MX 80 | 1986 | 150 | 70 | 70 | X 24 (2) | 270 T | 40 |
| Laverda 125 LB | 1983 | 100 | 60 | 65 | X 4 (3) | 268 CF | 60 |
| Laverda 125 LB | 1984 | 100 | 60 | 65 | X 4 (3) | 268 CF | 60 |
| Laverda 125 LB Uno / 125 CU Ride | 1984 | 100 | 66 | 65 | X 7 (3) | 264 CF | 40 |
| Laverda GS 125 Lesmo | 1986 | 122 | 45 | 65 | X 38 (2) | 264 DG | 40 |
| Moto Guzzi V35 Custom | 1985 | 112 | 42 | 60 | X 19 (2) | 262 CE | 30 |
| Moto Guzzi V35 Florida | – | 112 | 42 | 60 | X 19 (2) | 262 CE | 30 |
| Moto Guzzi V35 Imola II | 1984 | 112 | 45 | 60 | X 24 (2) | 262 CE | 30 |
| Moto Guzzi V35 Nevada | 1991 | 108 | 45 | 60 | X57 (2) | 262 EH | 30 |
| Moto Guzzi V35 NTX | 1986 | 112 | 42 | 60 | X 19 (2) | 262 EH | 30 |
| Moto Guzzi V50 III (USA) | 1981 | 118 | 45 | 60 | X 22 (3) | 262 T1 | 50 |
| Moto Guzzi V50 Monza | 1980 | 118 | 50 | 60 | X 6 (3) | 268 T | 50 |
| Moto Guzzi V50 Monza (USA) | 1981 | 118 | 45 | 60 | X 22 (3) | 262 T1 | 50 |
| Model | Aprilia 125 H₂O (Motor Hiro) | Aprilia 125 S H₂O | Aprilia AF1 125 Replica 89 | Aprilia AF1 125/88 | Aprilia AF1 125/89 (PHBH) | Aprilia AF1 stradale | Aprilia ETX 125/89 – Pegaso | Aprilia Tuareg 125/88 | Aprilia Tuareg 125/89 | Benelli 125 Jarno | KTM Cross MX 80 | Laverda 125 LB | Laverda 125 LB | Laverda 125 LB Uno / 125 CU Ride | Laverda GS 125 Lesmo | Moto Guzzi V35 Custom | Moto Guzzi V35 Florida | Moto Guzzi V35 Imola II | Moto Guzzi V35 Nevada | Moto Guzzi V35 NTX | Moto Guzzi V50 III (USA) | Moto Guzzi V50 Monza | Moto Guzzi V50 Monza (USA) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Year | 1981 | 1983 | 1989 | 1988 | 1989 | 1987 | 1989 | 1988 | 1989 | 1988 | 1986 | 1983 | 1984 | 1984 | 1986 | 1985 | – | 1984 | 1991 | 1986 | 1981 | 1980 | 1981 |
| Main jet | 102 | 102 | 132 | 125 | 120 | 120 | 130 | 115 | 128 | 125 | 150 | 100 | 100 | 100 | 122 | 112 | 112 | 112 | 108 | 112 | 118 | 118 | 118 |
| Pilot jet | 55 | 55 | 52 | 55 | 58 | 50 | 62 | 62 | 65 | 55 | 70 | 60 | 60 | 66 | 45 | 42 | 42 | 45 | 45 | 42 | 45 | 50 | 45 |
| Starter jet | 70 | 70 | 70 | 70 | 70 | 65 | 70 | 70 | 70 | 65 | 70 | 65 | 65 | 65 | 65 | 60 | 60 | 60 | 60 | 60 | 60 | 60 | 60 |
| Needle (clip) | X 27 (2) | X 27 (3) | X 40 (2) | X 56 (3) | X 4 (3) | X 42 (3) | X 56 (3) | X 53 (2) | X 53 (2) | X 31 (2) | X 24 (2) | X 4 (3) | X 4 (3) | X 7 (3) | X 38 (2) | X 19 (2) | X 19 (2) | X 24 (2) | X57 (2) | X 19 (2) | X 22 (3) | X 6 (3) | X 22 (3) |
| Atomiser | 266 BN | 266 BN | 264 T | 264 T | 266 CF | 264 DT | 264 T | 262 EF | 262 EF | 262 EF | 270 T | 268 CF | 268 CF | 264 CF | 264 DG | 262 CE | 262 CE | 262 CE | 262 EH | 262 EH | 262 T1 | 268 T | 262 T1 |
| Slide | 40 | 40 | 55 | 55 | 50 | 40 | 60 | 55 | 55 | 45 | 40 | 60 | 60 | 40 | 40 | 30 | 30 | 30 | 30 | 30 | 50 | 50 | 50 |
“–” means: not fitted on this model, or not documented in the source.
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.
Tuning guide: running lean / running rich
First narrow down the load range in which the problem occurs — then it is clear which part is responsible. The mapping follows Dell'Orto tuning literature and the Moto Guzzi workshop manuals: from idle to roughly 1/4 throttle the pilot jet and mixture screw are in charge, from roughly 1/8 to 3/4 throttle the jet needle (clip position) and atomiser, and from roughly 1/2 throttle to full load the main jet.
| Symptom | Load range | Responsible | Direction |
|---|---|---|---|
| Pale, greyish-white plugs; flat and gutless at the top end; overheating | from ~1/2 to full throttle | main jet | try one size larger |
| Runs better on choke; popping/banging on the overrun | idle to ~1/4 | pilot jet + mixture screw (check for air leaks first) | richer: larger jet or screw further out |
| Hesitation/flat spot when opening the throttle | transition idle → needle | needle position, pilot jet | richen, check function |
| Surging/leaning out at steady speed and mid throttle | ~1/8 to 3/4 | jet needle (clip) and atomiser | raise the needle one notch (clip lower) = richer |
| Sooty black plugs; burbling running; smell of fuel; high consumption | narrow it down: idle ↔ mid ↔ full | depending on range: pilot jet / needle + atomiser / main jet | leaner: smaller jet or needle lower |
| Gets worse the hotter the engine runs | usually idle/part throttle | overall setup too rich; also check float level | lean it out, check the float height |
Always change in single steps (“one or two jet sizes, then check the plug colour”), judge only on a warmed-up engine and make just one change per test ride. Your anchor is always the factory value for your model (table above) — on the PHBH 28 the documented factory main jets range from 100 to 150. Work from there in single steps in the diagnosed direction.
These notes do not replace a proper check. When in doubt use a lambda meter, a dyno or a specialist workshop – an engine running lean for long periods can be damaged.
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
What does the atomiser do and why does it matter so much?
The atomiser (also called the needle jet or emulsion tube) is the sleeve the jet needle runs in. Together with the needle it governs the mixture in the part-throttle range — exactly where you ride almost all the time. Experienced tuners rate it at least as important as the main jet: a single step (say 262 to 264) changes how the bike rides noticeably. With Dell'Orto the number is the bore and the letters are the design (e.g. AB, AR).
Discussion: caferacer-de
What does the needle clip position do?
The needle hangs from the throttle slide and its taper opens up the cross-section in the atomiser. Move the clip one notch down (needle sits higher) and the part-throttle range gets richer, one notch up (needle lower) leans it out. The factory figure counts the notches from the top. The needle position acts roughly between 1/4 and 3/4 throttle — not at idle and barely at full throttle.
Discussion: caferacer-de
How do I set the mixture screw correctly?
As a starting point the Moto Guzzi workshop manuals for the carburettor models consistently give 1.5 turns out from lightly seated. Fine tuning on a warmed-up engine: turn the mixture screw on each cylinder slowly until the engine runs at its smoothest and fastest, then correct the idle speed and synchronise both carburettors. If the engine only runs cleanly at well over two turns, the pilot jet is usually too small or the circuit is dirty.
Discussion: 2-ventiler.de
The engine hesitates when I open the throttle — why?
When the slide is opened quickly the mixture leans out briefly. PHF and PHM have an accelerator pump for exactly that — if the engine hesitates, its pump jet is often blocked, the diaphragm is damaged, or the idle-to-needle transition is too lean. On carburettors without a pump (e.g. VHB, PHBH) only a clean transition setup helps. Check the mundane causes too: float level, fuel supply, worn slides.
Discussion: 2-ventiler.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