Why fuel economy matters - motorcycles and air-fuel ratios
Many motorcycling enthusiasts simply don’t care about their bikes fuel economy or not, particularly those who ride performance machines. To them, the fuel consumption of the bike is merely a necessary cost associated with enjoying their bike. For others though, fuel economy is a characteristic which influences how desirable a bike might be relative to other alternative models. After-all, fuel is one of the primary costs associated with using your pride and joy and if there are two bikes you like equally and one costs less to run, why not go with that instead?
Whether or not you care about the economy of your bike relative to other motorbikes, you ought to care about the fuel consumption of your particular motorcycle, compared to other copies of the same model.
Why? Well, although there are always going to be a number of variables which influence fuel economy, it serves as a very useful proxy for how well an engine is performing, particularly for older motorbikes. Take for example our CBR250RR MC22.
When we first got it, it was returning an abysmal 6 – 6.5L/100km. Objectively, that is a horrendous amount of fuel for a 250cc bike to consume (even if it’s a four cylinder), being 50% more than our XSR700 ever consumed, even when ridden very hard - a bike with almost three times the displacement. While not entirely unheard of for these bikes, it is well above the official figure (yes, we know 90’s economy figures are dubious) and still towards the high end of what others had reported for economy.
After the bike had its carburetors rebuilt and was re-tuned on a dyno, it started returning 5 – 5.5L/100km.
Subsequent to that, we had the bike dyno tuned again, as we had identified that it was running quite rich below 10,000RPM. Following that, the bike started returning 3.9L – 4.2L/100km, all the while making double the torque at 6,000RPM and about 50% more horsepower at 12,000RPM. Just check out the dyno graph below.
What does that mean? Well, in our case, it demonstrates that the bike had been in a very poor state of tune and was drowning in fuel, such that it was costing us more money to run and making far less power and torque than it ought to (costing us enjoyment).
However, excessive fuel consumption isn’t only an indicator of a poorly tuned bike, it may also indicate some or a combination of: poor tune, vacuum leak, poor compression, underinflated tyres, poor spark, or myriad other issues.
It’s not uncommon for older bikes to return worse economy as they age for two related reasons: 1) they simply aren’t as efficient at combusting as they were when they were newer, due to ageing parts such as piston rings and 2) they are not been maintained to the standard which they should be.
Another thing to keep in mind is that manufacturers err on the side of caution (and emissions laws) and tend to run bikes in a richer state of tune than might be optimal for power and fuel economy.
Air-fuel ratios, tuning and performance
So, what does a rich or lean state of tune actually mean?
A rich condition refers to where the engine receives a higher ratio of fuel to air than is optimal, while a lean condition refers to the opposite: the engine is receiving a high amount of oxygen relative to fuel. This is known as the air-fuel ratio (AFR). The ideal AFR for your bike will depend on what your objective is, the conditions you’re operating in and, as ever, who you speak to.
As a general rule of thumb, an AFR of 14:1 or greater is considered to be lean (despite being the theoretically ideal figure which you’ll often see cited), promoting greater efficiency, while 12.5:1 is ideal for creating power, and 11:1 or lower is rich and is likely to decrease fuel efficiency. However, the ideal AFR for your application will depend on 1) the particular motor your running and 2) what your objective is (e.g reliability, fuel economy, power).
Manufacturers generally tend to tune engines to be slightly rich. There are two important reasons for this:
to make the engine run cooler than it would in a lean state of tune, providing an additional safety margin to avoid overheating and destroying your engine; and
reduce the level of noxious emissions produced for a given amount of combustion cycles.
Interestingly, some motorcycle engines will have particular cylinders run richer to compensate for the inherent thermal limitations of their positioning in relation to the other cylinders. For example, the two outer cylinders on the CBR250RR (#1 and #4) run slighter larger (richer) main jets at #105, compared to cylinders two and three in the middle, which run #102 main jets.
of the cooling effect of running rich is that some cylinders will run richer than others to compensate for their positioning,
Running a bike that’s on the rich side of the ideal AFR will tend to cause the following characteristics:
a. decreased fuel economy (increased fuel consumption);
b. reduced throttle response, making your engine feel ‘doughy’ or lethargic; and
c. may decrease the overall horsepower and torque your engine could produce.
By contrast, running an engine leaner will tend to cause the opposite effects, including increasing the likelihood of overheating. Once the engine becomes lean enough, your overall horsepower and torque will also decrease as the engine becomes starved of fuel. Instead of lethargic throttle response, you may experience stuttering, find that the engine ‘hunts’ (RPM does not stay constant) at idle, and the bike may be more difficult to start.
Keyster’s carburettor tuning ‘Tornade’ kits, include a useful diagram (see below) for how changing each part of the carburettors impacts on the AFR, and lists the symptoms associated with being too rich or lean.
As you can see, changing to their largest needle, #118 from the standard seize of #105, increases fuel supply by a whopping 23%! Given these bikes, in their standard tune, run slightly rich, I can’t envisage a situation where, absent serious engine work, you would ever need to increase the main jet size by anything close to that.
What does that mean? Well, if you have a stock bike, it’s highly unlikely that you’ll benefit from running larger main jets. More fuel only = more power, if you also have a corresponding increase in air. Without that, you’ll just be decreasing fuel economy, throttle response, and power and torque.