I have dismantled my 1971? 3800 and notice that the inlet manifold pipe is only about 7.5 mm. Did the later Motobecane engines use a larger diameter inlet pipe?

I ask this as their inlet port in the cylinder is 9mm. I cant see the point of having a large inlet port if it is restricted by the manifold?

Cheers

MattO

The Solex equipped mopeds voluntarily restricted the power output to limit the maximum speed of the vehicle to in between 30 and 35 Km/hr. due to varincas in production dimensions however there is a wide spread im performance between different bikes produced in the same production runs.

Although Solex claimed to have a well developed Quality control system the effectivess in some aspects was not to good, especially in the mapping of the ports of the cylinder

If I bored out my carb to 9mm can I purchase a 9 mm inlet pipe?

If you try to bore the carb please use a modified drill, especially suited for Brass type of materials. Else the drill will wedge itself into the soft material of the carb and fully destroy you and the carb.

Do not exceed 8.5 mm ( 8 is best suited) or you have the chance that the eccentricity of the internal bore and the casting will result in the drill coming out of the side of the carb.

After drilling you will have to remove the burrs, especially out of the venturi tube else the petrol cannot get through.
If you know the trick it is possible to remove the venturi tube in advance of the drilling and put it back afterwards.

Hi Peter

If the piston is made after March 1986 by MBK (46.98 mm long) does it already have the 3mm removed?

Cheers

MattO

It is easy to see if the piston is already shortened
an original one leaves some of the piston skirt visible when the piston is at TDC a shortened one leave about 2 mm free just above the inlet port at TDC ( so it needs to travel 2 mm back before you can see the skirt of the piston appearing in the inlet port)