This is the fourth set of Bing CV carburetors I’ve completely rebuilt. This is the link to the current work on the 1977 R100RS that uses the Bing 94/40 model of carburetors and specifically the 103-104 series used on the 1977 “CFO” engine version of the R100RS.

• 13 BMW R100RS Rebuild & Refinish Bing Type 94/40 Carburetors

The previous rebuilds include the smaller 64/32 series used on the R75 series /5, /6 and /7, and the R90/6 bikes and the larger 94/40 series used on the later R100 model bikes. I documented the procedures of the earlier work in the following write-ups.

• 13 BMW 1973 R75/5 Rebuild Bing Carburetors & Karcoma Petcocks
• 13 BMW 1975 R75/6: Silver Ghost Restoration-Part 17 Carburetor Rebuild, The Good, The Bad & The Ugly
• 13 BMW 1983 R100RS Rebuild Bing Carburetors & Karcoma Petcocks

As is often the case, I found the o-rings were hard and brittle and in one instance I found two o-rings on the idle fuel jet! The internals were pretty clean so I someone cleaned the carburetors and for some reason added an o-ring instead of replacing it on the idle fuel jet.

I decided to shoot some short videos to demonstrate how I understand the way the Bing CV carburetors work. Each video covers one of the four major functions, or circuits, used in these carburetors. The operation of the model 64 and 94 CV carburetors is the same.

Constant Velocity Circuit Operation

Here is a short video showing how the constant velocity circuit works.

Main + Needle Jet Circuit Operation

Here is a short video showing how these components work in the main+needle jet circuit.

Enriching Circuit Operation

Here is a short video showing how the enriching circuit works.

Idle Circuit Operation

Here is a short video showing how the idle circuit works.

Here are some pictures of the completed carburetors.

Refinished Outside

Refinished Inside

Refinished Back

Refinished Front

Refinished Top with Added Rondel

When I pulled the valve covers off, I found loose needle bearings in the left valve cover when I was tearing down the top end. This is not uncommon as the bearing cage lip wasn’t wide enough on the earlier needle bearing cages and can fracture. So I replaced all the needle bearings in the heads. Each rocker has a pair of needle bearing cages so there are eight total.

I removed the valves and cleaned the heads to see what I could see. Some of the valve faces are worn down and the valve springs are sacked past the minimum. I suspect the exhaust valve seats are original so I’m planning on having new exhaust seats installed. I’ll also replace all the valve guides, springs and the valves so these heads, which are dual-plugged, should last for a long time.

Here is the link to the write-up on how I did this work.

• 11 BMW 1977 R100RS Replace Rocker Needle Bearings, Remove Valves & Inspect

Here are a couple of pictures from the write-up.

Left Exhaust Rocket-Bottom Rocker Needle Bearing Cage Damaged

Left Exhaust Lower Rocker-Recovered Needle Bearings, Some Broken, and Pieces of The Cage Lip (Tooth Pick For Scale)

Rocker Arm Bearings in Bottom of Oil Pan

Left Exhaust Top Rocker Needle Bearings-Note Gap Between Needles at Bottom Which is Normal

Ready to Drive Bearing Cages Out of Rocker Arm

Valve Spring Compressor

Valve Spring Compressor Ready To Remove Valve

Portion of Valve Face is 1 mm Thick

Portion of Valve Face is 0.5 mm

I pulled the pistons and connecting rods for inspection. This write-up shows my procedure.

• 11 BMW 1977 R100RS Remove Pistons, Connecting Rods & Inspect

I found a score in one of the rod bearings and I’m going to replace the bearings. The pistons are in good condition, but this bike had the original top end replaced at some point with the later Nikasil cylinders, pistons, rings and gudgeon pin (wrist pin). Unfortunately this reduce the compression from 9.5:1 to about 8.2:1 producing less horsepower and torque. There are European versions of the pistons and rings that provide the original 9.5:1 compression. Hmmm …

Here are a few pictures & a short video or two from the write-up.

“Al-Fin” Cylinder with Steel Liner

Nikasil Cylinder is Aluminum And Does Not Have A Steel Liner

Napa 12 Point “Serrated Wrench” for Rod Bolts (part# 2305)

Piston Rings, Left to Right, Top, 2nd and Two Part Oil Control

Score in Right Cap Bearing Shell

I completed work on the front of the engine when I replaced the timing chain, crankshaft timing gear, front main seal and crankshaft nose bearing. I also replaced the three seals in the timing chest cover.  You can see how I did that work here:

• 11 BMW 1977 R100RS Replace Timing Chain, Crankshaft Timing Gear & Nose Bearing

So the next area of work is the rear of the engine. I removed the clutch and had it refurbished by Southland Clutch. I removed the flywheel, replaced the rear crankshaft main seal and the oil pump cover o-ring. While I had the pump cover off, I measured the clearances of the oil pump gears and found them all good and toward the low end of the allowable wear limit. Here is how I did this work:

• 21 BMW 1977 R100RS Remove, Refurbish, Install Clutch
• 11 BMW 1977 R100RS Replace Rear Main Seal & Oil Pump Cover O-Ring

I added a new full advance timing mark to the flywheel at 25° BTDC. I have dual plug heads and the advice from Tom Cutter at Rubber Chicken Racing Garage is to retard the full advance by 6° by retarding the “S” timing by 3° and limiting the full advance by another 3°. Before doing this, I did some digging to learn more about the flywheel in this bike and the automatic timing unit (ATU) and documented BMW flywheel changes in the /5, /6 and /7-R100 series of motors and the ATU’s.  You can find that information here:

• 12 BMW 1977 R100RS Changes To Ignition For Dual Plug Heads
• 12 BMW 1977 R100RS Install Dyna III Electronic Ignition-Refurbish Automatic Timing Unit (ATU)

Next up is to remove the pistons, rings and connecting rods, see how the rod big end bearings look, remove the rings, clean the pistons and inspect them and then make decisions about what needs to be done.

Here are a couple pictured from the write-up.

M6 x 10 mm Bolt In Alternator Rotor Allen Head Bolt

Front Cover Snug with Gap At Bottom Ensures Flywheel is Blocked Toward Rear

Ready To Remove Clutch

Accumulated Crud on Transmission Shelf

Clutch Removal Bolts-Nut Distance From Bolt Head

Clutch Alignment Marks

Diaphragm Spring with Streaks of Lubricant 🙁

New Diaphragm Spring Height & Partial Part Number (Right)

Added Flywheel Index Marks

Oil Leak From Crankshaft Bolt Hole (1:00)

Groove Cut Into Flywheel By Rear Main Crankshaft Seal

Starting To Clean Bell Housing

Bell Housing After Cleaning

Timing Marks Painted for Visibility

Flywheel Timing Marks with added 25 Degrees BTDC Mark

New Flywheel Hub O-ring

Engine Strapped To Work Bench Before Torquing Flywheel Bolts