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Old 10-03-2008, 10:08 AM   #46
Beastly Adventurer
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Joined: Oct 2007
Location: Blighty
Oddometer: 8,324

If you’re getting poor mileage then the engine is running too rich. Put my needles up one notch, stock jetting and I’m getting 44mpg on a run 60-80mph.
Surely the answer is to have the jetting set up using a dyno. I remember messing around with carbs on dirt bikes, doesn’t matter if the jets are made out of gold if the hole is too big it will run rich which equals poor mpg.
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Old 10-03-2008, 10:19 AM   #47
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Joined: Sep 2004
Location: Main Street, Shedd, Oregon
Oddometer: 2,089
Rich or Lean?

Here's a few carb/efi laws:

If the valves are tight, the mixture will be leaner. Like you've put a different hotter cam in it. When tight, they'll open sooner and close later. This will also give you more topend power and less low end.

If the valves are on the loose end, the mixture will seem richer, it will idle better and you'll have more low speed power. This is subtile, but I like loose valves.

The float measurement has a huge effect on your carb mixture. A higher float (smaller number, like 2.8 mm) will make the mixture richer. A lower number (like 3.2 mm) will make it leaner. This is the first carb setting you must do and is not to be fiddled with later. I like a lower float setting than stock, like about 3.5 mm. It makes the overall running cleaner and consistent on this bike.

If you open up the exhaust you'll generally need more fuel, but mostly on the 1/2 to full throtte, the main jet has the most effect here.

If you restrict the intake with the uni-filter, then you'll need to lean out the carbs, once again mostly on the 1/2 to full throttle. I'd suspect the uni-filter has much more effect than the open exhaust. The stock KTM cans flow quite a bit to start with.

95% of the time you are running your bike with 1/4 throttle or less. Put some marks on your twist grip and mark every 1/4 turn point. Zero to 1/4 throttle is where the fuel mileage lives. The fuel screws effect this range more than anything.

How much choke your bike needs to start is a large indication of the proper mixture. Can you start it cold without using the choke? Then it is way too rich. It should always need some choke to start when cold. It should need choke for about 1 minute minimum, or about 1/2 mile if you ride it immediately.

Work on the carb in this order, float level, main jet 3/4-full, needle height 1/2-3/4, low speed jet 0-1/2 and finally the fuel screws 0-1/8. If you back up and touch the floats, or the pipes, you have to start over again. Everything overlaps and effects other settings somewhat.

Main jet - if your bike pulls hard with full throttle, but pulls a little harder if you back off the throttle to 7/8 throttle, then your main jet is too rich.

I like to jet the low end 0-1/4 throttle on the lean side, very close to an ideal mixture for mileage. And the full throttle on the rich side for power.

All new bikes owners get worse mileage. It's the fun factor. After you get 20k miles, you tend to be more mellow with the throttle.

I get about 40 mph when riding by myself. Only 32 mpg when riding with others on the street.

Good luck with this.
"Dad, can I get a motorcycle when I grow up?"
"Son, you can't do both"
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Old 10-04-2008, 01:11 AM   #48
HappyGoLucky OP
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Joined: Feb 2005
Location: Displaced.ment
Oddometer: 4,311
Hey guys, super posts - thank you

Abyss, I hear you mate. I needed the open exhausts cos everything I have (including my pillion) was getting burnt to a crisp.

The valves are definately concerning me as I can hear a ping when I accelerate in first at very low revs. Nope, it aint chain slap, nor is it the "piston slap".

Thanks for the confirmation on the valves story, I'll be doing that very soon. On that note: my bike has always been down on power compared to others 950 bikes. (roll on tests confirm)

Zoober, thats a super post mate I am going to reset floats back to 3.5mm and try it again. Seems the right thing to do. I also learnt a lot from your post, tx.

To answer some of your q's:
  • cant start without choke, it will die almost immediately - need to run choke for at least a minute
  • there is more power at full throttle than 7/8ths

I am now wondering if an exhaust leak can also contribute to the horrid mileage? I got a sneak suspicion...
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Old 10-13-2008, 10:00 AM   #49
HappyGoLucky OP
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Joined: Feb 2005
Location: Displaced.ment
Oddometer: 4,311
Laugh Problem Solved........

So my mileage has gone up a minimum of 20% per tank - and thats WITH a pillion on, and against a very stiff wind - the speed would hold steady without gaining ,at 100mph in 6th. I'll explain what I did.

  1. New Spark Plugs
  2. Clean Unifilter, and add Motorex Foam Oil
  3. Reset valve clearance - only exhaust valves were slightly out (on the tight side)
  4. Clean out air filter (loads of sand in there, despite Unifilter - roughly 5 teaspoons of sand in the filter, and about 5 table spoons inside the airbox)
  5. Dynamic Flush of coolant
  6. RV on the blanking plates
  7. Recheck IMS for 1.75 turns out, as per H2Wind kit (rear, I opened a smidgen more)
  8. Reset carb floats to 3.5mm - this is the key (imo) to the solving of the problem.
What mileage am I getting?
  • at 90-100mph GPS speed, two up, against a stiff wind, I got a 20% increase in economy over riding at 70mph previously
  • at sub 30mph speeds, two up, and lots of 1st/2nd gear work, with a 3mile burst up to 110mph, I got a 35% increase in economy.
  • I can only imagine that I'll get a 40% increase in economy (minimum), if I sit at legal speeds on the highway/dirt.

Many thanks to ALL who responded to this thread, and in particular to Zuber for really making clear what affects what. I am indebted (and so is my petrol card!)

Interesting to note that my rear float is wonky (despite me pulling it and reseating it) - the left side of the float sits at 2.5mm, and the right portion of it, clears at 3.5mm. And I was setting the floats wrong previously which is likely why my economy went further down, when I set them to 3.5mm (as I had thought)

Thanks! The OC rocks....
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Old 10-13-2008, 10:49 AM   #50
Beastly Adventurer
Joined: Jun 2004
Oddometer: 3,495
Originally Posted by HappyGoLucky
Reset valve clearance - only exhaust valves were slightly out (on the tight side)
In my experience that could explain 10-15% mpg loss by itself.
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Old 10-13-2008, 11:04 AM   #51
Joined: Apr 2008
Location: Isle of Arran, Bonny Scotland
Oddometer: 85
Glad to hear you found a cure
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