The Puch E50 is the default US tuning platform: single-speed chain drive, centrifugal clutch in the bell, piston-ported cases, and a deep aftermarket for pipes, kits, and clutches. Maxi, Newport, and Magnum badges share the same engine math — identify HP rating and Bing carb part number before you copy jet sizes.
Build comparison
| Tier | Goal | Key parts | Realistic top speed | Budget |
|---|---|---|---|---|
| Mild | Sorted stocker | Derestrict intake/exhaust, correct Bing jets, timing 14–17° BTDC | 28–34 mph | $50–$200 |
| Mid | 70cc daily | Cast 70cc kit, port match, pipe, blue clutch springs, 15/15 Bing | 38–45 mph | $400–$750 |
| Wild | Race reed | Reed case work, ported 70cc+, 3-shoe clutch, full-circle crank, race pipe | 45–55+ mph | $900–$1,800+ |
Know the platform
- Transmission — Single-speed centrifugal clutch, not a variator. Tune shoes and springs in the bell.
- Timing — E50 ignition targets 14–17.5° BTDC with a timing light. Kitted aluminum jugs often need 14–14.5° to avoid heat soak seizures.
- 1 HP bikes — Extra-thick head gasket, restricted intake port, and smaller Bing jets (#48–#52 range on many 12 mm carbs). Derestrict before chasing speed parts.
- Port math — 43 mm stroke, 90 mm rod in the calculator.
Build 1 — Mild
- Compression test and seal check — tired E50s read ~90–110 PSI; compare to a known runner.
- Points gap ~0.014", plug B6HS gapped 0.020"–0.024", timing verified with light (not by points crack).
- Remove intake restrictor disc and exhaust baffle; jet up from stock (a 1/12/294 1 HP bike starts around #52 — a 2 HP 14 mm Bing may stock #64).
- Fresh clutch shoes if glazed; stock springs OK for commuting.
What you learn: One-horsepower restrictions hide in the head gasket and intake port, not just the exhaust.
Design pipe for this tier — broad peak, stock 193.7° exhaust (21.5 mm port height per Puchforum chart). Measure port width for accurate header D₁.
Build 2 — Mid
- 70cc cast kit with matched transfers and exhaust port height measured into the calculator.
- Performance pipe sized for 70cc E50 — jet +2 to +4 minimum after install.
- Upgrade to Bing 15/15 if airflow demands; plug-chop every change.
- Clutch: blue springs minimum on piped builds; lighten shoes carefully (under ~55 g risks slip). Target grab ~5800 RPM on well-tuned street setups.
- Premix 40:1 during break-in on fresh kits.
What you learn: E50 mid builds are clutch-limited as often as carb-limited — glazed shoes waste pipe gains.
Design pipe for this tier — balanced peak, Airsal 70cc 170° exhaust, ~9.5k RPM target. Log your port map and update port width in the designer.
Build 3 — Wild
- Reed conversion requires case machining — separate project from a bolt-on pipe.
- Aggressive porting with logged dimensions; aluminum kits want conservative ignition timing.
- 3-shoe clutch, pivot braces on high-RPM builds, full-circle crank where port timing demands it.
- Race pipes favor top-end — plan on narrow powerband and richer premix (32:1–40:1).
What you learn: Wild E50s are ignition- and clutch-sensitive; more degrees BTDC is not free power on a kitted jug.
Design pipe for this tier — sharp peak, Airsal 72cc ~190° / 10k RPM per kit summary. Narrow band — match clutch and jetting.