Classic bike logo

Brilliant Biking Inventions – The CV Carb

Being into the classic side of motorcycling, we love a good bank of carbs, here at CB-Net. And it’s amazing to think they’ve been with us for almost 135 years!

Yes, it was the great Karl Friedrich Benz (he of Mercedes Benz fame) who invented the carburettor in 1885 and this way of fuelling our automotive and two-wheeled passions pretty much lasted an entire century, before we fully fell for the charms of electronic fuel-injection.

Even then, for us motorcyclists, carbs kept our bikes accurately fuelled for longer than they did on our cars – by about 15 or so years. And even when the first FI systems appeared on exotic Italian machines of the mid-late 1980s and (finally) the mass-produced Japanese sportsbikes of the early 2000s, carbs had been so well refined that they showed up the jerky, vibey fuel-injection systems of the day for what they were…

The acronym ‘CV’ either stands for ‘Constant Velocity’ or ‘Constant Vacuum’ and it was this that gave the CV carb it’s real advantage allowing for linear/smooth fuel and air delivery and therefore smooth/linear power delivery…

Earlier carburettors had throttle cables attached directly to slides which opened up as you twisted the throttle and as the slide lifts, the needle moves out of the needle jet thus giving more fuel, but hopefully keeping the ratio the same. A CV carb feeds the engine only as much fuel and air mixture as the engine needs, as the throttle cable is attached to a butterfly valve, rather than directly to the slide and above the slide is a rubber diaphragm with springs which push the slide down to a closed position. The atmospheric pressure differential between the engine and the butterfly creates a vacuum and this sucks against the rubber diaphragm pulling it up, therefore lifting the slide and needle and giving the engine more fuel.

Earlier carbs would stutter/falter following large throttle openings, sometimes at precisely the wrong time if you wanted acceleration or power – and that’s what you wanted when you opened the throttle wide, right? But with the CV carb the venturi effect of drawing fuel and air was constant and consistent.

Obviously, CV carbs weren’t perfect and the early versions were somewhat crude. Experts who would spend time and effort rebuilding carbs would point to the fact that they weren’t actually that smooth in operation and rather than performance, it was economy and emissions that were the driving forces behind their design.

Whatever critics of early CV carbs thought, they were soon refined to become almost perfect – just as the mainstream manufacturers decided to go with fuel-injection.
Don’t believe that CV carbs were almost bob-on by the late 1990s early 2000s? Well, ride a well set-up Thundercat, R1 or Fazer 600, or perhaps an RR-W/X FireBlade or CBR600F final steelie… then try a 929 Blade as a comparison… These early injected bikes were often far from perfect and often hunted low down in the rev-range and were jerky off the throttle in the lower gears.

Weird then that the Italians seemed to get the Weber-Marelli injection system on early Ducati 8-valvers to work very well indeed: apparently the system on the 888 was derived from that used on the Ford Escort XR3i… class! But fuel-injection is for another time!