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Old 9 Sep 2005
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640 '04 Adventure Fuel Efficiency

Fellow travelers,

I have a few questions about the KTM 640 Adventure 2004.
I have fitted a custom made lighter exhaust which is very well made and light but I have doubts about its suitability. During my previous trip to Tunisia, my riding partner who has the same bike as me but fitted with a standard KTM aluminium supermoto exhaust could run for 70 km more than I did.
Could it be the exhaust? Mind that I weigh 10 kg more than him but can that make such a difference? Can anyone recommend an exhaust that would be more fuel efficient, if that’s the case?
Regarding the fuel tank. In the beginning I thought it was the same as previous models with a capacity of 28 lt. The manual however states that it is 26 lt. Anyone aware of its actual capacity?
The reason I am asking all this is that we are planning to make rather long crossings in Morocco this October, of about 350 km with out refueling stations in between so fuel capacity is critical. Any other tips about fuel efficiency are more than welcomed.

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Old 9 Sep 2005
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So many factors affect fuel consumption: fuel quality, riding style, weight of luggage and rider, terrain, tyre type, tyre pressures, state of tune, general state of the engine, wind strength and direction to name a few.

My experience: 2002 640 LC4-E with Adventure tank, side tanks, rally engine guard, Ortlieb panniers, engine in normal state of tune, KTM SXC silencer fitted, riding in Africa, generally with '90' rated fuel: 18km/litre on the piste, 21 km/litre on sealed roads.

The actual capacity of the Adventure tank? The only real way to find out is to measure it. I would think factors like ambient temperature might affect it, and that no two tanks are exactly the same capacity.
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Old 10 Sep 2005
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Thanks for the tip. All this is true, I agree. But all the other parameters apart from the exhaust where identical since we had the same luggage (wind resistance), same fuel (same fuelling points) same carb tune up, same tyres, both bikes with identical mileage, and we were riding side by side most of the time. The only difference was the exhaust and me being 7 cm taller.
Actually because I had suspicions this was happening it sort of became like an experiment to see which bike was more efficient. So we refueled and drove on asphalt until one of us switched to reserve. I switched after 450 km and he kept on for another 70 km.
That’s why I ask, exhaust??
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Old 10 Sep 2005
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I agree with Ian.

I suggest you swap bikes. Then you'll know what effect your height,weight and riding styles have on both bikes.

By being taller you are increasing the drag and by being heavier the work the motor has to do up every hill.

speed probably has the greatest effect:

100 km/h, 4,000 rpm 17/42 200m 19.0 km/l
120 km/h, 5,000 rpm 17/42 200m 16.5 km/l
140 km/h, 6,000 rpm 17/42 200m 13.5 km/l

I only ever get to 20 km/l if I do 4,000 rpm, so now im changing back to original exhaust system from full Ackro and reducing main jet from 165 to 157.

Good luck.



[This message has been edited by Jerome (edited 10 September 2005).]

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Old 11 Sep 2005
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Please forgive me if this shows my ignorance!
I would think that using the original carb set up in terms of jets etc. and a more freeflowing exhaust would increase fuel efficiency? The bike would presumably run at lower revs/throttle for a greater power output? I'm intrigued by what appears to be the black magic that revolves around air/fuel mixtures and fuel efficiency/power output and admit to only the loosest grasp of the principles!


*Disclaimer* - I am not saying my bike is better than your bike. I am not saying my way is better than your way. I am not mocking your religion/politics/other belief system. When reading my post imagine me sitting behind a frothing pint of ale, smiling and offering you a bag of peanuts. This is the sentiment in which my post is made. Please accept it as such!
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Old 12 Sep 2005
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I am pretty ignorant too about these matters too. I think it goes something like this:

Fuel consumption will increase when accelerating, the faster and more often one accelerates the more fuel is wasted. This is hard to calculate and is how riding style comes in.

I am interested in the consumption while cruising at a steady speed for example of 100 km/hr.
Basically the engine has to produce enough power to overcome the air resistance plus any friction from tires and mechanical parts. The aim is to produce this power with the least amount of fuel possible (efficiency). I would think that having a lean mixture set up (more air less fuel) will do that more or less but will heat up the engine too.
I don’t know what the contribution of the exhaust is.
How does an open exhaust (followed with the necessary main jet change) compare with a more restricted one?
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Old 13 Sep 2005
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Hi Yuri,
You ask a good question. Just traveled 4K miles in the USA on my new to me '02 LC$ Adventure. As I traveled my fuel mileage got continually worse. I was back in my home state this weekend and attended a BMW rally at Wisconsin Dells. At the rally site the overflow tube from the carb started to drain a large stream of fuel. I tapped on the carb body without results. Off came the carb by unbolting and pivoting the subframe backwards to make more room. I opened the carb and replaced the needle and seat on the float and that cured the overflow(I had parts with me). The rubber tip on the needle was very hard.
Immediately the bike ran much cleaner and more evenly. Vibration was much less at speed. Almost a new bike.. I would say the problem was developing for many miles and finially showed itself. I am not sure if you could be having the same experience. I have not ridden the bike thru a whole tank of gas since replacement but will very soon and I will report back if my mileage improves. At the end of my trip I was getting 40 miles/gallon but earlier got almost 50 miles/gallon at the start of the trip. Possibily alcohol in the USA fuels could cause this problem or maybe it was just coincidence. When I see fuel mileage results for the next tank I will know more.
BTW the bike was trouble free except for this problem.
Getting 50 mpg again consistently. Been thinking about gas mileage. Bike is running great, maybe a little lean on the pilot and needle, fine on the main. My exhaust is stock. Not sure why a traveler would want to change from the stock exhaust since the bike is powerful and gets good mileage and is quiet.
Bill Shockley, Tomahawk, WI.

[This message has been edited by Bill Shockley (edited 22 September 2005).]
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Old 16 Sep 2005
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Answer for Matt Cartney:

If everything is stock and you put on a freer flowing exhaust, fuel economy often goes down. With a restrictive exhaust, less of the exhaust actually leaves the motor each cycle. This is just like "exhaust gas recirculation." The second time the cylinder fires, there is still some unburnt fuel from the first time it fired, etc.

To get the same mixture as stock, an opened up exhaust needs to be made richer, to make up for some of the fuel not getting a second chance to burn. Even without being made richer, more unburned fuel is leaving out the exhaust than in the stock condition. While you have the potential for making more power, and perhaps that would make the difference in high wind or up steep hills, for regular riding the wasted fuel outweighs the better breathing of the motor and fuel economy goes down a little.

This is one of the reasons that economy cars get little tiny exhaust pipes. The car would perform better opened up, but they want the restriction because it helps fuel economy.

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