Motobecane Engine

1976 Motobecane Logo

Contents:  1. Motobecane Mobylette Engines 
Contents:  2. Model 40 and 50 Engine Parts

 

 


AV7 engines up to 1970

1. Motobecane Mobylette Engines

These are the Motobecane (AV) or Motoconfort (AU) Mobylette engine models and the bike models they came on. US bike models are red. Not included here are the alias brands Wards Riverside, Raleigh, Suvega engines with their models and MB equivalents. 

 approx.  engine  xxx   bike model xxxx Gurtner    comp
x
years     model xxxx models xxxxxxx carburetor  ratio   drive

Engine serial numbers for each year 1951-75

1949-51 AV3 xxxxxx AV3 Mobylette x   A10-512   6.5  Standard clutchless
1952-56 AV3.5 S xx  AV or AU 31,33     000-516   6.5  Standard clutchless

1952-59 AV7 S xxxx AV or AU 32 xxx   B10516   6.5  Standard clutchless
1956-59 AV7 Sb xxx AV or AU 41 xxx   B10516   6.5  Standard clutchless

1952-56 AV3.5  xxx  AV or AU 31,33    0000-516   6.5   single automatic clutch
1957-66 AV7 De xxx AV/AU 85-65-76 BA10-540   7.5  Dimoby dual auto clutch
1966-69 AV7 De xxx AV/AU 85-65-76 BA10-610   7.5  Dimoby dual auto clutch
1965-66 AV7 Db xxx AV or AU 42 xxx BA10-540   6.5  Dimoby dual auto clutch
1966-69 AV7 Db xxx AV or AU 42 xxx BA10-610   6.5  Dimoby dual auto clutch
1960-61 AV7 Dbe X  AV or AU 43,44 xBA10-540   7.5  Dimoby dual auto clutch
1966-68 AV7 Dbe X  AV or AU 43,44 xBA10-610   7.5  Dimoby dual auto clutch
1958-60 AV7 Dbg xx BG or CG 43,44 AR8.7-525   6.5  Dimoby friction drive
1967-70 M1 Isodyne X1,Cady M1,C1 AR110-674   8.0  Dimoby dual auto clutch
1970-72 M1 Isodyne Cady M1,M1PR  AR110-691   8.0  Dimoby dual auto clutch
1973-75 M3 Isodyne Cady M3PR x  AR111.5-753   8.0  Dimoby dual auto clutch
1978-79 AV7 7 xxxx  7, 7 Ranger xx  AR112-884   7.8  Dimoby dual auto clutch
1978-79 AV7 7  xxx  Model 7 30mph AR112884   7.8  Dimoby dual auto clutch
1978-79 AV7 7?xxx  Model 7 20+25  AR110-919   6.5  Dimoby dual auto clutch
1968-77 AV7 Dc 0xx 40, 50 25+30   AR212-705   7.8  Dimoby dual auto clutch
1974-80 AV7 Dc xxx 40, 50 25+30   AR212-729   7.8  Dimoby dual auto clutch
1974-80 AV7 Dc xxx 40, 50 20mph   AR210-910   6.5  Dimoby dual auto clutch
1978-79 AV10 xxxxx 41TS, 51S, 51T  AD13-976  0.0   Dimoby, case reed valve

1958-59 AV7 V xxxx  AV or AU 68-79  BA10-549   7.5  Variator + dual auto clutch
1959-70 AV7 V2 xxx  AV or AU 88  xx AR13-488   7.5  Variator + dual auto clutch
1968-77 AV7 Vb xxx  AV or AU 48  xx BA10-549   7.5  Variator + dual auto clutch
1966-71 AV7 V2.5 x  AV89, SP50  xx-   H13-569   9.0  Variator + dual auto clutch
1966-68 AV7 V2.5 S  SP50R xxxxxxx    H13-569   9.0  Variator + dual auto clutch
1966-71 AV7 V2.5 B  AV98 xxxxxxxx    H14-626   9.0  Variator + dual auto clutch
1968-71 AV7 Vb  xxx 40V, 50V xxx   AR212705   7.5  Variator + dual auto clutch
1972-77 AV7 Vb  xxx 40V, 50V xxx   AR212729   7.5  Variator + dual auto clutch
1974-75 AV7 Vb  xxx 50V 25 & 30    AR212705   7.5  Variator + dual auto clutch
1976-77 AV7 Vc  xxx 50V 25 & 30    AR212705   7.8  Variator + dual auto clutch
1977-80 AV7 Vc  xxx 50V 30 mph    AR212873   7.8  Variator + dual auto clutch
1977-80 AV7 Vc  xxx 50V 25 mph    AR210737   7.8  Variator + dual auto clutch
1977-77 AV7 xxxx 99Z   xx x xx    AD13858  0.0   Variator, supercharger piston
1978-79 AV10   xxx   92GT,51V,51SU AD13-976  0.0   Variator, case reed valve
1982-83 AV10   MBK M16, 41V, 51V   AD13-971 -976 -138 Var, case reed valve
1984-98 AV10   MBK 51V, many others AD13-976  0.0 Variator, case reed valve

Sources:
1. AV7 Engines Chart from Mobylette Repair Manual, official, but only goes up to 1969.
2. motobec.fr for their awesome library of sales brochures for each year, and parts manuals
3. Dating Your Motobecane from Ets Mauger for their excellent grid of years and models
4. Motobecane Club of France for illustrations and information
5. mobyjoost on The Mobylette/Motobecane Forum UK, a Mobylette expert and enthusiast
6. La bleue pour les bleus, on Le Monde en Bleue, another Moby expert
7. Mobylette Mopeds – Owners Workshop Manual by Haynes, covers 1965-76 UK models

 

1949-51 AV3

1949-51 AV3

AV3 Motobecane had been making motorcycles since 1922, but after WWII they switched to making motorized bicycles. Europe’s roads were bombed out, gas stations were gone, bridges were gone, so people had to walk or climb across streams and canyons carrying their bicycles and cargo. In 1949 a lightweight moped was born out of the ashes, called Mobylette (little mobility). It’s 50cc two-stroke engine had an aluminum cylinder with a steel sleeve. It had no crank seals. Both transfers were on the right side, and the piston had transfer windows and a ridge across the top. It was not at all symmetrical left to right. That is why the exhaust exits to the left.

 

1952-56 AV3.5 S

1952-56 AV3.5

AV3.5 This also had the early scavenging design (how to get the intake in and exhaust out in one piston stroke, without mixing or wasting any). Both transfer ports were on the right side. Improvements were 1) it had a hard aluminum cylinder with a hard chrome lining, 2) it had crank seals, 3) some models had a new automatic running clutch and automatic starting clutch. It was simply called “embrayage automatique”, not “Dimoby”. But it was the first Dimoby. Once the rear wheel was moving over 5 mph, the automatic starting clutch starts the engine. Once the engine rpms get above about 2000, the automatic running clutch begins to propel the bike forward.

 

1957-66 AV7 De

1952-59 AV7 S

AV7 S, AV7 De  The AV7 S and AV7 De (both 1.5 hp) had a improved, completely different scavenging design, known as Schurle Loop. Intake from the crankcase was forced upward around the left and right sides of the piston. Two streams were directed towards the intake side by the left and right transfer ports. They then merged and looped around, past the spark plug, towards the exhaust side, forcing out any exhaust gases. This was more efficient. In the 1950’s all two-stroke engines adopted this design. The piston changed to a flat-dome top, but the cylinder remained all aluminum with hard chrome lined cylinder wall.

The AV7 De exhaust had a new design, called blunderbuss. Inverting the expansion chamber made the exhaust short, allowing the engine to swing, while still allowing the exhaust to expand. The new compact exhaust design allowed a new swinging engine mount design. The AV7 De V-belt was held at a constant tension automatically by a spring, instead of bolted solid and adjusted periodically. 

1958-59 AV7 V

variator illustration

 

AV7 V The AV7 V (1.5 hp) had a variator, which is a belt drive with one or both pulleys that vary in size. The belt is a V-belt that grips from the sides of the V. It sinks down deeper between the V-shaped sides of the pulley, as they move apart, and raises up higher between the V-shaped sides as they come together. A set of 4 or 6 steel balls roll outward when the engine rpms increase. That forces the pulley sides together, causing the V-belt to ride higher. The result is the engine rpm stays the same as the bike accelerates.

The blunderbuss short exhaust allowed the engine to swing about 20 degrees. Below 15mph the engine is forward, in low gear, and above 25mph it is rearward, in high gear. Amazing!

 

1960-68 AV7 Dbe

1959-70 AV7 V2

AV7 V2, AV7 Dbe The AV7 V2 (2.0 hp) and AV7 Dbe (1.5 hp) are 95% the same as the preceding AV7 V and Db. Only the crankshaft, crank bearings, and piston changed. The old crank axles were 15mm everywhere, and took standard bearings 6302 (15 x 42 x 13) MB#00518. But installing the press-fit bearings meant dragging the clutch side bearing over the clutch clip groove and needle bearing surface, sometimes getting stuck or causing harm. The new crank was thicker at the bearings, 16mm. The bearings no longer damaged the crank during installation, but were not standard. They were special 6302-QR (16 x 42 x 13 with rounded inner edge) MB#16906. The AV7 V2 illustration does not show the 15-to-16 step in the crankshaft, because it is the older AV7 V illustration, but it does show the new crank and bearing part numbers. 

The piston design did not change. But the piston was offered in many slightly different sizes or “sorts”. The AV7 V parts diagram said 14820 for the piston, but the AV7 V2 said nothing. That is very subtle. There was a separate sheet for pistons with over ten different part numbers and sizes. Because not all of the plated cylinders are made to the exact same diameter, before this you had to sand the piston if it happened to be too tight for that cylinder, or settle for a loose fit like a worn engine has, if it happened to be too loose. 

 

1966-71 AV7 V2.5

AV7 V2.5 review

 

AV7 V2.5 This was a late-1960’s 2.5 horsepower version, instead of 2.0 or 1.5 hp. It had noticeably different head fins that run at an angle instead of straight back. Both the cylinder and head fins were wider.

 

 

 

 

Haynes Manual

1966-75 M1

M1, M3 These late-60’s and early-70’s moped engines had the Dimoby dual clutch with drive belt, and magneto behind it, both on the left side. They were called Isodyne engines, which roughly means “same side”. Wierd, like Solex!

 

 

 

 

 

1968-77 AV7 Vb or Dc

1968-77 AV7 Dc

AV7 Vb, AV7 Dc These 1970’s models most notable differences are the square shape of the cylinder and head cooling fins (instead of round) and the all-chrome exhaust (instead of painted rear section). Many things became square in the 70’s.

AV7 Vc This was the late-70’s AV7 (2.0 or 1.5 hp) on US-models 50V series.

 

 

AV7 Z cutaway

 

1977 AV7 Z

AV7 Z This 1977-only engine was a unique design. On top it has big fins and a piston-controlled reed valve. So the piston has an intake window. But look under the crankcase and you will see a second cylinder and piston. It pressurizes and depressurizes the crankcase every revolution. It is timed to be lowest pressure during the intake process, and highest during the transfer process. It is a supercharger for a two-stroke engine. But the production did not last long.

 

 

1978-98 AV10

AV10 In 1978 Motobecane came out with an improved and  modernized engine design. Case reed induction is where the carburetor feeds the crankcase through a reed valve, instead of the back of the cylinder and under the moving piston. This changed the back-and-forth flow of piston-port induction to a unidirectional stop-and-go flow. The result was better performance, like 20% more low-rpm torque.

In 1981 Motobecane became bankrupt. For 2 years nothing was produced. Then they were bought by Yamaha, and their name changed to MBK. Production resumed in 1982.

By the mid-80’s pedals were no longer mandatory in most countries. So many of the mid-80’s MBK mopeds have kick-start with foot-pegs instead of pedals

 

 

 

 


2. Model 40 and 50 Engine Parts

under construction

Motobecane model 50 figure 1

  #   part#  qty  price   description  1. Engines, gaskets      
01   53444    1  N/A   engine 1-speed (20mph   6V ’76-77) 
01   53949    1  N/A   engine 1-speed (25mph   6V ’76-77) 
01   52341    1  N/A   engine variator (25mph   6V ’76-77) 
01   53744    1  N/A   engine 1-speed (30mph   6V ’74-77) 
01   53745    1  N/A   engine variator (30mph   6V ’74-77) 
01   54146    1  N/A   engine 1-speed (20mph 12V ’78-80) 
01   54147    1  N/A   engine 1-speed (25mph 12V ’78-80) 
01   54148    1  N/A   engine variator (25mph 12V ’78-80) 
01   54149    1  N/A   engine 1-speed (30mph 12V ’78-80) 
01   54844    1  N/A   engine variator (30mph 12V ’78-80) 
02   18568    1 $15.0 case bolt set 2 #2.1, 2 #2.2, 3 #2.3
2.1  15276    1 $0.90 bolt M7-1.25 x 45 flange (10 hex)
2.2  14936    1 $0.80 bolt M7-1.00 x 33 (10 hex)
2.3  00366    1 $0.90 bolt M7-1.00 x 48 (10 hex)
2.4  00366.1 1 $1.50 bolt M7-1.00 x 40 (set has 48)
2.5  00366    1 $0.90 bolt M7-1.00 x 48 (10 hex)
2.6  14936    1 $0.80 bolt M7-1.00 x 33 (10 hex)
2.7  15276    1 $0.90 bolt M7-1.25 x 45 flange (10 hex)
2.8  00011    5 $0.70 nut  M7 (10 hex)
03   14854    1 $9.00 gasket cylinder head 
04   14895    1 $8.00 gasket cylinder base
05   16910    1 $7.00 gasket crankcase
06   52646    1  N/A   gasket intake restricted (20mph)
06   17073    1 $7.00 gasket intake (25 and 30mph)
07   16912    1 $25.0 gasket kit

Motobecane model 50 figure 2

  #   part#  qty  price   description  2. Engine mounts      
01   19886    1 $6.00 bolt engine mount M8-1.00 x 140     
01   20453    1  N/A   bolt for up-exhaust
M8-1.00 x 145?  
02   20534    1 $4.00 spacer tube Ø8 x Ø14 x 18
03   17994    1  N/A   Flexibloc right Ø8 x ??
04   19888    1 $3.00 ground cable, engine-to-frame
05   21014    2 $3.00 stepped spacer Ø8 x Ø12-17 x 10.5
06   20789    1  N/A   Flexibloc left Ø12 x ??
07   17523    1 $2.50 nut locking M8-1.00
08   16541    1 $0.20 washer Ø8
09   20790    1 $7.00 spacer tube plastic Ø8 x Ø12 x 23
10   17661    1 $3.00 bracket for variator cover
11   00011    4 $0.70 nut  M7 (10 hex)
12   01527    7 $0.50 lock washer Ø7
13   15276    2 $0.90 bolt M7-1.25 x 45 flange (10 hex)
14   14936    2 $0.80 case bolt
15   01526    2 $0.50 nut M6 special?
16   01172    2 $0.20 washer Ø6
17   17120    1 $5.00 bracket spring retainer variator
17   21282    1 $6.00 bracket spring retainer 1-speed
18   00968    2 $0.70 bolt M6 x ??
19   15933    2 $7.00 belt tension springs (each)
20   00366    2 $0.90 case bolt M7-1.00 x 48 (10 hex)
21   00010    2 $0.40 nut M6
22   15934    1 $9.00 Flexibloc lower Ø6 x Ø?? x ??
23   00050    1 $0.70 bolt M6 x 25? 
24   21709    1 $4.00 lower engine mount plate right
25   21710    1 $5.00 lower engine mount plate left
26   17354    1 $3.00 bracket variator cover

Motobecane 50 figure 3

  #   part#  qty  price   description  3. Cylinder and head
01   53505    1 $140. cylinder E with piston J (20 mph)
01   53505    1 $140. higher top deck, lower compression

01   23460    1 $180. cylinder AA w/piston AA (25 & 30)
01   23460    1 $180. cylinder AA w/piston AA (25 & 30)
01   23460    1 $180. cylinder C with piston C  (25 & 30)
01   23460    1 $180. cylinder C with piston C  (25 & 30)
01   23460    1 $180. cylinder C with piston C  (25 & 30)
01   23460    1 $180. cylinder E with piston E  (25 & 30)
01   23460    1 $180. cylinder E with piston E  (25 & 30)
01   23460    1 $140. cylinder C with piston C good-used
01   23460    1 $140. cylinder C with piston E good-used

02   14721    1 $12.0 decomp valve kit
03   14821    2 --N/A- piston ring (pre-6/76) Ø39 x 2.0 FG
03   22148    2 $12.0 piston ring (6/76-on) Ø39 x 2.0 GI
04   15094    1 $12.0 piston pin Ø13 x 30
05   00658    2 $2.00 piston pin clip Ø13
06   14854    1 $9.00 gasket cylinder head 
07   19885    1 $35.0 cylinder head complete
08   00053    2 $1.00 rivet for ID plate
09   21294    1  N/A   engine ID plate   
10   00059    1 $1.00 spark plug gasket ring
11   01547    1 $3.00 spark plug NGK B6HS
12   15025    1 $8.00 decomp valve bare
13   14919    1 $7.00 mounting lug
14   14855    1 $7.00 guide
15   14897    1 $7.00 gasket
16   14987    1 $4.00 washer
17   15007    1 $4.00 spring 
18   15099    1 $6.00 valve link
19   00018    1  N/A   cottor pin, use small bent nail
20   14895    1 $8.00 gasket cylinder base  
21   22156    1 $3.00 wire guide

Piston (at skirt) to cylinder clearance: new is 0.005 to 0.01 mm (0.0002 to 0.0004″), max is 0.05 mm (0.002″).
This is a very small clearance, measured at the bottom of the piston, allowed by the all-aluminum cylinder. 
Usually pistons are measured at the bottom skirt, called “lower” below, because when cold, that is the widest part.
The bottom is slightly wider than the top, because there is less material, and therefore less thermal expansion.
When the piston is hot, the bottom and top are the same. The whole piston expands, as does the cylinder. 
Motobecane measures just below the rings, called “upper” below. On these pistons lower is 0.02 mm bigger.
Motobecane original pistons have no marks on top, listed below as “(M)”. Inside has the M logo (some also AJ3). 

< Some vintage replacement pistons have this logo stamped on top, listed as “(Θ)”. Inside says FRANCE.
They also have a size letter stamped on top. These were original replacements from Motobecane USA.
Other vintage aftermarket have no logo on top, listed below as “(O)”.

Motobecane 50 figure 4

  #   part#  qty  price   description x upper/lower    4. Piston
73    22273       N/A    piston(M) W  38.92/38.94 for 38.95 cyl
74    22274      $75.0 piston(M) AA 38.93/38.95 for 38.96 cyl
75    22275      $75.0 piston(M) A   38.94/38.96 for 38.97 cyl
75.1 22275.1   $75.0 piston(Θ) A   38.94/38.96 for 38.97 cyl

76    22276      N/A    piston(M) C   38.95/38.97 for 38.98 cyl
77    22277      $75.0 piston(M) E   38.96/38.98 for 38.99 cyl
78    22278      $75.0 piston(M) G   38.97/38.99 for 39.00 cyl
79    22279      N/A    piston(M)  J   38.98/39.00 for 39.01 cyl
79.1 22279.1   $75.0 piston(Θ)  J   38.98/39.00 for 39.01 cyl

80    22280      N/A    piston(M)  L   38.99/39.01 for 39.02 cyl
81    22281      $75.0 piston(M) N   39.00/39.02 for 39.03 cyl
82    22435      $75.0 piston(M) P   39.01/39.03 for 39.04 cyl
82.1 22435.1   $75.0 piston(O)      39.09/39.11 for very worn

 

Motobecane 50 figure 5
left is for center-mount carb, right is side-mount

   #   part#  qty  price   description  5. Intake pipe
01   16673    1  N/A   intake pipe short (variator 25mph) Ø16
01   16619    1 $18.0 intake pipe short (var. 30 pre-3/77) Ø17
02   01176    3 $0.90 lock nut M6
03   01172    3 $0.25 lock washer Ø6
04   52646    1  N/A   intake gasket restricted (20mph)  
04   17073    1 $7.00 intake gasket (25 & 30 mph) 
05   00796    2 $2.50 stud M6 x 25
06   53496    1  N/A   intake pipe thin long (1-sp 20mph) Ø19
06   21201    1 $13.0 intake pipe long (1sp 25&30mph) Ø19
06   22899    1 $16.0 intake pipe long (var. 30 3/77-on) Ø19
07   21202    1 $6.00 support bracket
08   23018    1 $1.50 carriage bolt M6

 

 

Motobecane 50 figure 6

  #   part#  qty  price   description  6. Crank and case
01   19608    1  N/A   crankcase with seals (25 & 30)
1.1  19608.1 1 $15.0 left case half bare (25 & 30)
1.2  19608.2 1 $15.0 right case half bare (25 & 30)
1.3
  19608.3 1  N/A   crankcase (20mph) with a ridge that
1.3  19608.3 1  N/A   only allows a 20mph cylinder to fit

02    
03    
04    
04    
05    
06    
06    
06
07
08   19609    1 $150. crankshaft Ø16 Ø15 pre-’80 for 6302QR
08   19609    1 $150. less threads for dome square-hole nut
8.1
  19609.1 1 $150. crankshaft Ø16 Ø15 ’80-on for 6302QR
8.1  19609.1 1 $150. more threads for large hex flywheel nut
8.2  19609.2 1 $150. crankshaft Ø15 Ø15 ’80-on for 6302
8.2  19609.2 1 $150. machined to take regular bearings

09
10
11
12
13

Crankshafts: top 19609, bottom 19609.2

Motobecane Crankshafts: are the same for variator and non-variator pulley-clutches.
Top is used, pre-1980, shorter right side threads. Takes special 16mm bearings 6302QR 16x42x13.
Bottom is new 1980-later original, machined to 15mm at bearings, to take ordinary bearings 6302 15x42x13.

Myrons offers new pre-1980 crankshafts, that take special 6302QR bearings.

Myrons offers a new original 1980-later crankshaft that has been machined from 16mm to 15mm at the center where the bearings press on. The press fit is precise and correct. The piston pin and needle bearing are new, included, and factory sealed in wax, that peels off. Two 6302 bearings, and two 15x21x4 seals are included. Price is $180.

It is long enough to take the 1980-later hex nut, sold separately at $10. But it will also take the pre-1980 square-hole nut, if the extra threads are ground off.