Fuel consumption running weak

With few of us being able to accurately assess the colour of our plugs, what fuel consumption would be considered weak over a long cruise at say 60-70 MPH? imperial gallons I hear the figure of 65 MPG with a single Mk 1 Concentric and in the past also with a single SU. With twin JS flat slides I get about 52MPG (imperial) on a run. Without a rolling road to test under load, it's all a bit of a guessing game.

Plug colour is usually only used as a guide to main jet size. If you are not using full throttle a lot, it is probably not relevant. Mid-range jetting is more about getting clean. strong acceleration than fuel consumption. However lean midrange jetting probably gives best acceleration and fuel consumption. The trick is to stay rich enough not to burn anything in the head or a piston. When you read a plug - it should be the recommended heat range and there should be a 1mm black ring right down inside on the porcelain where it meets the metal. While the black ring is there, you are safe. However if you don't use full throttle, the black ring is not a reliable indication.

The fuel consumption thing is a very coarse screen, so to speak. Some people ride at parade pace and think they're really wailing on it, and others use WOT a LOT. So, 55MPG to 35MPG is a wide range.

If your carbs are jetted rich on the mains and you are in the habit of just whacking the throttle open, fuel consumption will be bad. If you are jetted lean right down the needle to get good performance, fuel consumption will be better if you feed the throttle open, as you would do when riding a two-stroke. I don't think the bike's speed has much to do with it, except that at higher speeds there is more wind resistance, so the throttle is open wider. On my bikes the main jets are always slightly too rich. so if you open the throttle more than two thirds, fuel economy suffers - but you don't burn pistons or valves.

gripper said:

With few of us being able to accurately assess the colour of our plugs, what fuel consumption would be considered weak over a long cruise at say 60-70 MPH? imperial gallons I hear the figure of 65 MPG with a single Mk 1 Concentric and in the past also with a single SU. With twin JS flat slides I get about 52MPG (imperial) on a run. Without a rolling road to test under load, it's all a bit of a guessing game.

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You really need to study the art of spark plug reading if you want to avoid mayhem inside your engine, i.e. ALWAYS do a plug chop when attempting to go UP a slight incline.
It is absolutely no use to you for checking the main jet, for the pilot /aircrew setting, I always use a Colortune to set it, AT NIGHT for a clear reading :!:

ALWAYS do a plug chop when attempting to go UP a slight incline

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I have read this many times over the years, the engine should be under load

but how does the combustion chamber's fuel mixture, which colors the plugs, know that it is under load in order to change the mixture to be more correct?

and since the motor is not under load most of the time while maintaining steady speeds, why should under load be the preferred mixture for plug reading?

just pondering.....

When you ride the bike uphill the extra load causes you to open the throttle wider, so if you do a plug chop the reading you get is more representative of the metering which is done by the main jet. If you do the exercise on a flat road, you can get the same effect however the bike would be going much faster and you are more likely to feather the throttle. Then the reading you would get would be when the metering is done by the needle and needle-jet. It sometimes takes a fair bit of time to get the plug to change after you have changed the jets. I usually do a few laps of a race circuit then cut the motor dead after blasting down the longest straight. Even that doesn't really give you a true indication. So I always run the mains slightly rich. Very lean to just before detonation gives better performance. You are unlikely to burn a piston or valve by leaning-off the midrange jetting. If it is too lean in the mid-range, you would know it - so don't continue to ride the bike if it is coughing and spitting back through the carbs - stop and raise the needles.

I'm guessing that Jim Comstock with his state of the art fuel injection system would have it set on a rolling road/dyno for ideal mixture throughout the entire range. I know he has other mods including a big engine, maybe he can give me an idea as to what MPG he gets with his setup. As I said in my original post, what would I expect to see at 60-70 cruise? (not riding like the pope or checking out the upper rev limit) Unleaded fuel is not so good at leaving a plug colour.

A state of the art injection system would use a throttle position sensor and a programmable advance curve as the basis for optimisation. With a Commando, you don't usually have that. What I've been telling you only relates to the old-fashioned system where you have the variables - plug heat range, comp. ratio, ignition advance and jetting. I notice that the Indian Enfields are fuel injected - might be a cheaper way forward there. Personally, I have never used a dyno, so I really don't know what can be achieved. What I do know, is that for best performance, there is not much margin for error in the variables I mentioned. In a conventional carn using petrol, half a thou of an inch too big in the needle jet is the difference between flying and sluggish.

If I remember correctly Jim Comstock uses a knock sensor to set his tuning parameters. The distance between knocking and best performance might be an area of interest for people going down that path. Lean in the midrange is good, but too lean is not good. In the end it is a matter of balancing the variables and using a set point which gives best performance - then you need to cope with changes in the weather. I usually start with standard ignition advance, plug type and comp. ratio and jet from there. If the comp.ratio is increased, retard the ignition advance and jet bigger than normal. A different cam should not change the jetting. However if you change the fuel, you start from scratch.

When you are jetting your bike, don't start it up and go screaming off down the highway. When you make changes observe the symptoms which arise, you can always tell when it is too lean in the midrange, however the only time you are likely to cause real damage is if the main jets are too small and you thrash the bike. Even changing the exhaust system can cause a burnt piston to happen, if the jetting was at the optimum before you made the change..

What I'm saying is, If someone tells me he is getting 80 MPG, it's a pretty safe bet that his bike is running weak (presuming he's not going everywhere at 35 MPH) So if someone tells me he rides normally and gets 65 MPG, is that also weak? I understand that if he's taking part in the traffic light sprint then he might get down to 30 MPG

Might depend on how much he uses the throttle. If he has got the needle jet and needle on the minimum size needed to stop engine damage and he doesn't change gear much, he might get very high MPG.

gripper said:

With few of us being able to accurately assess the colour of our plugs, what fuel consumption would be considered weak over a long cruise at say 60-70 MPH? imperial gallons I hear the figure of 65 MPG with a single Mk 1 Concentric and in the past also with a single SU. With twin JS flat slides I get about 52MPG (imperial) on a run. Without a rolling road to test under load, it's all a bit of a guessing game.

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I spent a week riding around Scotland last year on my mk3 850. Mostly 60-70 mph for 1500 miles and bloody brilliant it was. Bike has a single vm34 Mikuni and 19t g/box sprocket. I got 59mpg. So I reckon your 52 mpg on twin carbs at similar speeds sounds about right.