Why does MPG go down when all cylinders aren working?

[GabrielGoes]

Properly that is.. My buddy who broke down on 2 cylinders 20 miles from home rode home on 2 and the bike drank an entire tank ful o' gas. I forgot to screw my condensor on properly so i had a lot of power loss etc. so it wasnt really getting good spark, my bike took 2 tanks of gas in a heartbeat, i figured it should burn less gas? why more?

[simon kuether]

I'd have thought that all 4 pots will still be sucking the same amount of fuel . The trouble is that the 2 that are firing are "fighting" the two that are not , so they are working harder , so they will use more fuel , so they work harder , so ..........................
I had the same thing happen recently . Two pots not firing and the fuel consumption went through the roof . I thought about taking the plugs out of the offending pots , but then you'd get an unburnt gas mixture being pumped up under the tank . Probably not a good idea . I've been told that the unburnt fuel washes all the lubricant out of the "faulty" pots , so it's not good to ride to far without all 4 cylinders banging away .

Cheers , Simon .

[81GS750]

I agree, My guess is that the firing two have to work harder to compensate for the non-running two, consuming more fuel. Plus, just because two aren't firing doesn't mean they aren't consuming fuel.

[GabrielGoes]

hmm that seems about right, i thought about it before but i couldnt put my finger on it, thanks guys

[LarryA_Texas]

I suppose if you could shut down the fuel to one or two cylinders, like some newer cars are doing when they don't need them all for acceleration, you could save gas. The problem is in how fuel gets into the cylinder in most piston engines. It's sucked in by the down stroke. So even if it's not going to be burned it still gets sucked in. What blows out the tailpipe is a fuel/air mixture...not burned exhaust. And as others have said, when two are down the other two are pulling the entire load plus the two dragging pistons so they have to work harder.

[MisterCinders]

The two cylinders are not working harder, but you are getting half power. So it takes roughly twice the energy to go a mile on two cylinders as it does on four. Of course the fuel system is still drinking for the dead cylinders because those carbs don't know better.

[KEITH KRAUSE]

The firing cylinders aren't working harder.
The non-firing cylinders are still drawing in fuel but without combustion the fuel is unburned and simply passing through and out the exhaust. Almost like a leak. To add to the loss of mpg the bike is now using approx' 375cc to move the same weight.

[Flyboy]

What was said above is correct, and not so correct, but for the purpose of explination to your question, it is fine, what do I mean?

Let us have a look at the basics, to propel a motorcycle forward, we need something to do work, on a bicycle, that something would be our legs, turning the pedals, on a motorcycle, it would be the engine turning the front sprocket.

Now in order for our legs to do the work of turning the bicyle pedals we need energy, so to the motorcycle engine requires energy to do work.

Where does this energy come from? Fuel.
In the case of the bicycle that fuel is in the form of food, our body burns the food, creating heat, and the heat is released in the form of energy.
Just so, the motorcycle fuel, in the form of gasoline is burnt in the engine, creating heat, that heat is again released as energy, which we harness to do work.... turning the sprocket.

So now we can see the simple chain, Fuel -> Heat -> Energy -> Work.

So it stands to reason that the more fuel consumed, the more heat can be generated, thus more work can be done.
This is why, engines that produce a lot of power (put out high levels of work) consume more fuel than engines that put out lower power (low levels of work)
Another thing we need to keep in mind is the efficiency of the engine, how good is it at turning heat into energy?
The more efficient the engine is, the more work you will get from a given amount of fuel, an engine that is not very efficient will produce less work for the same amount of fuel.

When we have two of the cylinders down, they are not producing any heat, therefore no work either, but they are still consuming the same amount of fuel, which is just being pumped straight out the exhaust, in effect their efficiency is Zero, and the motor's efficiency as a whole has dropped by 50%, therefore it is going to do 50% less work for the same amount of fuel used.

It is a given that to travel 20 miles down the road, we need X amount of energy to acieve that, but now that we have lost 50% of our efficiency we only have 1/2 X, clearly we now don't have the reqired energy to do the work needed to complete the task.

So the only way of increasing the energy, is by burning more fuel, thereby increasing the energy back up to X, now we have enough energy to complete the work required.

So it is plain to see, that not only are we consuming fuel, which is not producing any energy on the two dead cylinders, but we also have to increase the fuel supplied to the two good cylinder, to increase thier energy output to get the job done, and by so doing, we also increase the fuel delivered to the two dead cylinders as well.
Now you can see why the fuel consumption goes through the roof when a cylinder or two goes down.

One last thing, it is not quite correct to say that the motor has to fight against the compression of the dead cylinders, because whether all the cylinders are working or not, any time a piston is on its compression stroke it is not producing power, it is only producing power on its power stroke, which follows.
So whether or not it produces a power on the power stroke or not, it still goes through the compression stroke which works against the motor, whether it is alive or not.

Now that I have totally confused the heck out of everyone, including myself, I am off to have a headache tablet and a rest, my head hurts.....

[tejasmud]

In simple terms.

It all comes down to having to turn the throttle MORE to get the bike to go with dead cylinders.

MORE throttle = MORE fuel.

The ride home was achieved by holding the throttle "wide open" with 2 dead cylinders.


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