Jetting is not universal. A main jet that works on a stock Puch Bing will fry a plug on a piped Tomos SHA if you copy numbers blindly. Use starting points, then tune by plug chop at wide-open throttle (WOT).
The Plug Chop Method
- Install a fresh plug, ride 1–2 miles at WOT, kill the engine at speed (don't idle home).
- Read the plug center insulator color: tan/light brown = close; white/chalky = lean; oily/black = rich.
- Change one jet size at a time. Re-test. Patience beats guessing.
Puch Maxi — Bing 14/12 & 15/15
Most US Puchs run a Bing. The middle number in a Bing part code is bore size (12, 14, 15 mm). Stock main jets vary widely by model — verify your carb stamping before copying a number:
| Bing (examples) | Typical application | Stock main jet |
|---|---|---|
| 1/12/294 | Maxi / Luxe 1 HP | #52 |
| 1/12/293 | Maxi 1.5 HP | #60 |
| 1/14/152 | Maxi 2 HP | #64 |
| 1/15/46A | Aftermarket 15 mm | #82 starting point |
Typical progression after mods:
| Mod | Starting direction |
|---|---|
| Stock airbox, stock exhaust | Keep OEM main; fix air leaks first |
| Remove intake restrictor disc | +1 to +2 main jet steps |
| Performance pipe (Bi-Turbo, etc.) | +2 to +4 main — plug chop required |
| Pod filter | Often +2 or more; midrange needle matters |
| 70cc kit | See cylinder kit docs — not a main-jet-only change |
See Puch Maxi tuning guide for clutch and exhaust pairing. Never open the exhaust without checking jetting.
Tomos A35 / A55 — Dellorto SHA & PHVA
SHA and PHVA jets use different numbering than Bing — do not transpose numbers. Stock starting points: A35 SHA 14/12 often around #53 main; A55 PHVA 14 around #55 main / #25 pilot. After air filter or pipe changes:
- Start with a fresh plug and known-good fuel mix (see oil ratios).
- If you lean out: step main jet up one size, re-chop.
- Bogging off idle often implicates idle circuit / choke / air leak, not main jet alone.
See Tomos A35 & A55 guide for chain and gearbox basics.
Motobécane 50V — Gurtner / Encarwi
US Mobys often run Gurtner or Encarwi carbs with jet numbering that does not match Bing or Dellorto. Fix intake leaks and clean the carb before chasing jets. A crushed stock exhaust caps RPM on variated AV7 bikes — jetting cannot fix a corked pipe. On 50V models, variator roller weights and contra spring tension change how much RPM the engine actually uses — pair carb work with variator basics.
Batavus M48 — Encarwi
Laura M48 bikes (HS-50, VA, MoBat, etc.) often use Encarwi carbs — jet numbering does not match Bing or Dellorto. Float height drives part-throttle mixture: start with the float pin tip about 12.5 mm below the float top on an S25 carb, then plug-chop at WOT. Roughly 1 mm of float adjustment equals 1.5–2 main-jet sizes at wide open — use float tweaks for idle-to-half throttle, jets for WOT.
Peugeot 103 — Dellorto SHA
The 103 family uses a variating reed-valve engine with Dellorto SHA carbs on most examples. Seal the carb to the intake with a proper O-ring under the clamp — a leaking SHA mimics jetting problems. Stock jetting varies by sub-model; after pipe or cylinder changes, plug-chop at WOT. Roller weight and spring changes are as important as main jet steps on variated Peugeots.
Minarelli V1 — Dellorto SHA
Horizontal V1 bikes use SHA 14/12 or 15/15 with jet numbering unrelated to Bing. The V1 is single-speed centrifugal clutch — not variated. Derestrict intake and exhaust, then step the main jet gradually. See the Minarelli V1 build paths article.
Dellorto SHA — universal notes
- Use an O-ring between carb and intake; the split clamp compresses the stack — it does not seal by itself.
- SHA jets are 5 mm × 0.8 mm thread — common size across brands.
- Midrange is needle clip and slide cutaway; WOT is main jet — change one circuit at a time.
Honda Express / Hobbit
NC50 and PA50II use a Keihin-style carb with stock jetting tuned for oil injection and mild exhaust. Most reliability issues are fuel system cleanliness and oil pump delivery — not "+3 main jet" tuning. See Honda guide.
Rules That Save Engines
- Fix air leaks at intake manifold and crank seals before chasing jets.
- Rich is safer for test passes; lean seizes pistons.
- Elevation matters — bikes jetted at sea level run leaner at altitude.
- Ask in our live chat with your carb model, pipe, and cylinder state.