Fuel consumption?

[Barry]

Not to mince words but unless you are really thrashing the bike 32 MPG US (38.4 Imp) is truly appalling. While you are waiting to have the carbs tuned I would be inclined to try a a few simple adjustments.

First and most important thing is check the float levels as there is no point in attempting to tune carbs unless the fuel level in the bowl is correct and the same both sides.  The official method is set the floats to shut off when parallel to the carb flange. This probably works well enough with new floats but I wouldn’t trust it if they are a few years old as they are known to get heavier and not necessarily by the same amount or at the same time. Unless they really are sinkers that doesn’t mean you have to put in new floats every few years but it does mean the parallel method won’t work accurately as this is an inferred method of setting fuel level that relies on identical weight and volume of the floats.

For older floats remove the float bowls quickly and measure the depth of fuel above the centre well. 22-24mm is the accepted target figure. Rather than use a ruler I find  it best to scribe a line on the inside of the bowl. I use a vernier depth gauge to measure the overall bowl depth at 36.35 mm and subtract the target fuel depth to determine the distance from the bowl edge.

For 24mm depth of fuel scribe 12.35mm from top
For 23 mm depth of fuel scribe 13.35 mm from top
For 22mm depth of fuel scribe 14.35 mm from top

To scribe the lines I use a good old fashioned woodworkers marking gauge to ensure accuracy and more important to mark both bowls at exactly the same depth.

The next thing I would do is mark the position of the idle mixture screws with a dab of paint and then turn them clock wise in small increments the width of the screw driver slot alternating between the two carbs.  Keep doing that for as long as the idle speed remains stable. Once the idle speed slows or starts to sound rough back the screws out until it runs smooth again. As long as you mark the original position of the screws you can't go wrong doing this and it could easily net you several MPG if they were set rich which many are.

After this it will depend how comfortable you are going into the carbs and whether or not you want to do a full overhaul and tune up. A lot more can still be done though with the carbs mounted on the bike. Checking the needles  jets for wear and the needles for correct position and wear  would be the next steps.  Also check  and clean the air space around the emulsifiers which depending on use could have started to block up after only a few years. These are the main things that have an impact on MPG.  Only the needle jet wears to any significant degree so I wouldn't worry about the others provided they are correctly sized.

[Dustybin]

Barry can I bring my bike over for a tune up?

[Barry]

You'd be very welcome Tony but somehow I think you have more than enough experience to do it at least as well as me if not better.

[montmil]

Interesting disparity in the fuel usage of each bike. Those owners reporting very low mileage really should consider trying a few of the easier checks and adjustments to improve performance and fuel burn. With a bit of study and research, the overhaul and tuning of the CV Bing carburetors is not really a black art and can be accomplished by most anyone.

Besides bringing the carburetors back up to specification, there's also the need to balance the throttle cables while the engine runs at idle and then with the revs held at approx 4K rpm. The enrichener cables will also need to be confirmed for full off with the engine running.

Even the position of the carburetor in relation to an X-Y axis can effect carb function. Stand behind the bike and eyeball the Bings. If one or both are tilted too far off the vertical, the floats can be hindered in operation and float bowl fuel levels may be at odds.

There are many small details outside of the carburetor's body. One may not be an issue but several can leave you scratching your head. And in the course of 30+ years, previous owners most certainly may have changed any number of Bing bits. If you decide to do a full overhaul, replace everything that is not marked as to the OEM number for your bike's carburetor model number.

I'll leave you with a reminder that plays a large roll in reported fuel mileage. Most of us ride the R65. There's an R45 amongst the crowd with an engine two-thirds the size of the 650cc R65. Avoid comparing apples to oranges. My full-size Ford truck does not nor will it ever get the same fuel mileage as my Cabriolet.

[Barry]

There's an R45 amongst the crowd with an engine two-thirds the size of the 650cc R65. Avoid comparing apples to oranges.

I wouldn't let em off that easily Monte. An R45 is exactly the same weight and has the same frictional and aerodynamic  losses as an R65 so should need the same amount of power to accelerate or cruise at any given speed. The only benefit I have is better part load efficiency but to balance that I lose out on overall gearing. Even the factory consumption figures are near identical at 61.4 MPG Imp For the R65 and 62.8 for the R45.  If I can match the factory figure so should an R65.

[Dustybin]

You'd be very welcome Tony but somehow I think you have more than enough experience to do it at least as well as me if not better.

Barry mate you are doing yourself down there, my experience is more associated with ECUs and individual coil packs and injectors none of which you find on an R65