Dell'Orto VHSB 38 – jetting & tuning
Flat-slide sports range of the late two-stroke 125s; jet needles of the K series. Fitted from the factory to 2 documented models from Aprilia, KTM.
| Make | Model | Year | Version | Designation | Reg. no. | Details |
|---|---|---|---|---|---|---|
| Aprilia | AF1 250 Replica | 1987 | Standard | VHSB 38 GD | 9714 | view jetting |
| KTM | 250 MX 88 | 1987 | Standard | VHSB 38 AD | 9706 | view jetting |
| KTM | 250 MX 88 | 1988 | Standard | VHSB 38 DD | 9716 | view jetting |
| Make | Aprilia | KTM | KTM |
|---|---|---|---|
| Model | AF1 250 Replica | 250 MX 88 | 250 MX 88 |
| Year | 1987 | 1987 | 1988 |
| Version | Standard | Standard | Standard |
| Designation | VHSB 38 GD | VHSB 38 AD | VHSB 38 DD |
| Reg. no. | 9714 | 9706 | 9716 |
| Details | view jetting | view jetting | view jetting |
| Model | Year | Main jet | Pilot jet | Starter jet | Needle (clip) | Atomiser | Slide |
|---|---|---|---|---|---|---|---|
| Aprilia AF1 250 Replica | 1987 | 210 | 58 | 60 | K 52 (2) | 268 DQ | 40 |
| KTM 250 MX 88 | 1987 | 185 | 50 | 60 | K 46 (2) | 268 DQ | 40 |
| KTM 250 MX 88 | 1988 | 185 | 50 | 60 | K 46 (2) | 268 DQ | 40 |
| Model | Aprilia AF1 250 Replica | KTM 250 MX 88 | KTM 250 MX 88 |
|---|---|---|---|
| Year | 1987 | 1987 | 1988 |
| Main jet | 210 | 185 | 185 |
| Pilot jet | 58 | 50 | 50 |
| Starter jet | 60 | 60 | 60 |
| Needle (clip) | K 52 (2) | K 46 (2) | K 46 (2) |
| Atomiser | 268 DQ | 268 DQ | 268 DQ |
| Slide | 40 | 40 | 40 |
“–” 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 VHSB 38 the documented factory main jets range from 185 to 210. 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 VHSB 38
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