The Mobylette AV89 series was infrequently imported into the US during the 1960s. Though production continued in France beyond the 60s, the 50 series took over in the US. Parts for the AV89 series are therefore much more common in Europe than in North America.
Though both the AV89 series and the 50 series use the AV7 engine, the AV89 series differs in several important ways.
1. The carburetor is a 14mm Gurtner, running a pretty lean jet.
2. The intake port is larger. About twice the area of the stock port. Piston port kits also use this wide intake.
3. The case is ported to match the transfers of the cylinder.
4. The sidewall ports are a little taller and maybe a little wider.
5. The stock compression is 9:1.
6. The stock timing is 1.5mm BTDC.
7. The exhaust has about the same flow as the seventies version.
1. The engine head is different so it cannot be mounted on a 50 series frame and a 50 series engine cannot be mounted on an AV89.
2. The AV89 requires a longer belt (Ventico D243CR) because of the larger pulley. A NAPA 5L380 belt is a close match. 4L350 and 4L360 belts have also been reported as U.S. equivalents.
3. The AV89 has 18" wheels instead of 17" wheels.
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4. The AV89 uses an 8mm petcock, vs. the 10mm petcock used on more recent models. The exception to this rule seems to include all the top-tank AV89's (like the Speciale 50 and SP-50) which take the 10mm petcock. All motobecane petcocks are outside threaded, for internally threaded tanks.
Most AV89s have dates marked on the engine that can be misleading, usually a date from 1960. Though this is the date of manufacture for the engine, it is not the year of production for the moped. A more accurate estimate of the moped's year can be found on the original Gurtner carburetor.