Converting the R60/6 to CV carburetors

As standard the R60/6 bikes were equip with Bing 28mm sliding carburetors.

These were probably adequate when new, but 40 years and a few bodged fixes later, this bike was difficult to tune and not responsive to the throttle.
The decision was made to convert it to the later CV carbs.

It has been reported that this conversion was popular with the Greek police. Not for performance issues but because of vapour lock, caused by the hot weather and the fact that the sliding carbs were not well insulated from the hot cylinder block.
The conversion they used, was to fit Bing CV 32mm carbs from the R80 bikes and so this was the modification chosen for this R60.

Carburetor reconditioning

The carburetor service kit contents are shown here.
These days it is worth sourcing a kit compatible with E10 petrol.

The enrichers parts are shown here.

During reassembly - check that the tapered slot of the enricher disc is at the top as you fit it to the main carburetor housing. If it isn't you may have left and right enrichers mixed up.

This image shows the enricher gasket.....

....and the enricher installed with stainless screws.

When attaching the enricher to the carb body - check that the dot on the enricher shaft is pointing towards the inlet channel of the housing. (See arrows).

This image shows the correct location of the O-ring on the throttle spindle.

The butterfly position is important to ensure it fully closes to avoid a fast idle. Note that the edge is chamfered so that it only seals properly in one orientation. Some fiddling is required to get it to sit right. Shine a light from behind to check for a good fit.

The mixture screw and spring. The initial setting is to screw it fully home and then back out 3/4 of a turn.

Correct stack of main jet parts.

Note that above the mixture tube there is often black gunk which can be mistaken for the remains of an O-ring, but there is no O-ring there. The O-ring shown fits into the recess of the mixture tube.

Installing idle jet with O-ring.

Float needle assembly with retaining wire/clip.

The float arm tab needs to be carefully bent so that needle valve to shuts just as the float top and carburetor housing ware parallel.

The float bowl has a jet for the enricher circuit. The hole (arrowed) must also be clear.

New main jet needle was installed by pushing and rotating clockwise into the spring clip. The needle top has 90° notches so you can feel the 4 height positions.

A main diaphragms are secured to the piston with 4 screws. Note the location tab that gives the piston its orientation.

Likewise there is a feature on the outer rim to locate in the carburetor body.

The carb top with spring is held with two screws. After installation the free motion of the main piston must be checked to ensure no tight spots.

CV Conversion

To fit the CV carbs to the cylinder, new inlet stubs were machined from 303 stainless. These had a thread of
32 x 1.5mm and a shoulder of 43mm to connect to the carbs.

The full dimensions are given here (click for PDF).
It was found necessary to remove the end thread, to get the inlet to pull up fully into the cylinder head.

The old inlet stubs were a tight fit in the cylinder. The bike was run to warm everything up and then a large stilson wrench was used to free up the stub.
Plumber's pliers (shown here) were then used to unscrew the inlet.

The new inlets were screwed into place using the plumber's pliers again.

To connect the carbs to the air box the parts shown on the left were used. These were R80 parts. Specifically......
Left intake tube. BMW no:13 72 1 258 059
Right intake tube BMW no:13 72 1 258 060
Stepped joining collar (x2)
BMW part no. 13 72 1 257 395

The carbs were attached to the cylinder head with BMW part: 13 72 1 254 654.
However these parts were shortened by 5mm to make everything fit without a strain.
To cut the tube length square, it was hose clamped to a bar in the lathe and then scored with a knife (on the underside so the knife was dragged aware from the operator).

The original choke cables were reused with a suitable cable clamp on the end to join to the enricher lever.
The clamp was BMW part 13 11 9 057 338
However new throttle cables had to be ordered to give the correct fitting to the new butterfly lever assembly.

5x Large hose clamps (40mm -> 60mm) were used on both sides to make connections air tight.
Large hose clamp BMW Part No.
07 12 9 952 123

The bike was started by setting the mixture screws out by 3/4 of a turn.
Following a ride to warm the bike up the carbs were balanced using a manometer between the vacuum ports to measure relative performance. Information on this process can be found here.