Fitting Bing Carbs

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Carburettor Modification

Obviously before we start and more to the point, before you start modifying your designed and engineered motor bike, I take no responsibility for any damage or harm caused by you following these instructions. Nor do I take any responsibility for any personal harm to yourself or others if you have followed these instructions and things go wrong!

That said I have had no problems with any of these modifications and would not expect you to have either. You need to be able to operate hand tools and a 4" angle grinder and electric drill is about as technical as we go. REMEMBER: "These machines have no brain, please use your own". ALWAYS WEAR PROTECTIVE SAFETY GLASSES OR GOGGLES WHEN USING ELECTRICAL HAND TOOLS.

Parts Required:

2 off Carburettor Stubbs as per Drawing (See below).

6 off Chang Jiang Fibre Carburettor mounting gaskets (3 per side required for stud length).

1 Pair of R45 BMW 28mm Carburettors - BMW Model numbers (left & right sides – 64/28/303 & 64/28/304)

2 off 130 or 135 Sized Main Jets (the R45 carbs have a 105 Main jet as standard)

2 off 43mm Cylinder head to Carburettor Rubbers (BMW supplied)

4 off 43mm Jubilee Clips

1 BMW Handle Bar Choke Lever and Cables

Standard CJ750 Throttle Cables

2 off Cable nipples (The type with the adjusting screw in the ends)

2 off Ľ” In-line fuel filters

75cm off Ľ” Rubber Petrol Pipe

Once you have had the Carburettor stubs machined from 316SS at your local machine shop they are a direct fit on to the existing cylinder barrel carburettor stud bolts. The two machined flats allow the existing nuts to be used and whilst probable not 100% engineering spec for the nut clearance, it does the job! Remember to use 3 of the fibre gaskets between the barrel and the Carb stub, each side, to help isolate the stub from the produced heat. Don’t use copper gaskets! Although the heat isolation is from the BING carburettors being indirectly mounted to the barrels by the rubber hose. I originally tried to remove the studs to allow shorter ones to be fitted but they were extremely tight and I did not want to run the risk of shearing off the studs so i just left them. Here is the drawing to fabricate the Carb stubs.

Next step is to cut off the original throttle cable lead/alloy nipple at the carburettor ends, then using the replacement adjustable cable nipples, slide the freshly cut cable end in to the adjustable nipple hole (ensure that you have put the nipple in to the carburettor throttle-flap lever first). With the handle bar throttle twist grip in the fully closed position and the cable adjuster wound almost fully in to the carb top, tighten the adjustable nipple such that there is minimal free play in the inner cable. Leave the cable adjuster wound out around 2-3 turns to give you some cable free play later when you come to setting up the carburettors.

If you have purchased the BMW Choke lever you will notice that it comes, as standard, together with the complete BMW clutch lever as well! You need to cut off the clutch lever so that you are just left with the choke lever. You will also have to use a round file to enlarge the inside bore of the part that will slid over the handle bar to allow it to slid over and around the contours of the CJ750 handle bars. Once you have modified the choke lever and fitted it on to the left hand side of the CJ750 handle bars (Pictured below), you can then feed the choke cables around the steering head and under the fuel tank. Try to ensure that you route the cables as smoothly as possible avoiding any sharp bends or kinks. Connect the choke cables to each carburettor choke lever, again ensuring that the choke cable adjusters are fully screwed in to the carburettors. Alternatively to the BMW choke lever you can probably purchase an aftermarket handlebar lever suitable for this purpose.

Next step is to fit the carburettors to the cylinder stubs using the 43mm rubbers and the Jubilee clips (see below).

Now you need to adjust both the choke and the throttle cables so that the is only a little bit of outter cable slack with both the twist grip throttle and the choke lever both in the closed position. (Ensure that the throttle idle stop screws are backed off and not holding the throttle flaps open). BMW recommended cable set-up is for 0.5-1.0mm of outer cable free play with the throttle closed.

Now fit the new rubber petrol hose with a fuel filter in each line to prevent any rust and water from getting in to the carburettors. (Pictured above centre). The more observant of you will have noticed a further two modifications that I had to make, one to the Reverse selector on the left-hand side of the gearbox and the hand gear change on the right-hand side of the gear box (Pictured below). This is to allow the Conical air filters to fit without impinging on the operation of both these levers. In both cases it was as simple as taking a hacksaw to the offending part, I cut off the reverse lever peg and replaced it with a long bolt which is threaded in to the remaining lever (See pictures below). The hand gear lever was shortened to allow it to freely move under the air filter when the carburettor is in place.

Check the basic carburettor settings from the BMW “Haynes” manual or similar workshop manual for an R45 model. Regarding the main jet and needle settings I have found that either the 130 or 135 Main jet provides a brown tinge to the spark plugs at top end which indicates the mixture is correct at full speed. The needle should be set in the 3rd notch from the top with the 130 sized Main Jet or in the top notch if using the 135 Main Jet. However with the 135 main jet installed I found that the mixture is a little on the rich side when running at lower speeds around town. The spark plug electrodes are still golden brown but there is also a trace of carbon build up around the spark plug threaded body. I have ridden my bike both hard on the open road and also easy around town and have had no problems with pre-ignition or overheating, even in heavy traffic. Now you are ready to start your bike for the first time!

Once you have got your bike running you will need to balance the carbs with each other to ensure that one cylinder is not trying to work harder than the other. The best way to do this is with a carburettor balance tools such as made by "Carbtune", which are available from all good bike shops. I used the standard mixture screw settings of 1.0 - 1.5 turns out from rest to achieve good idling and then adjusted each throttle stop screw until both cylinders were running evenly on idle. I also then ran the engine at around 1500 RPM and by adjusting the outer throttle cable adjusters at the carb tops achieved balanced running of both cylinders. Lock off the adjuster lock nuts ensuring to hold the adjuster with pliers to prevent movement during tightening, then re-check the carburettor balance at idle speed. The balancing of the Carbs needs to be checked around ever 6 months or when you notice a change in the idle and take up of the engine from standing still in traffic.

Happy riding folks! Top of Page