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Air screw effects

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So my bike is running kind of weird in high rpm in gear 1 and 2 but 3 and up is perfect. The other day I ran out of gas and the 20 seconds before it died it ran perfect. So I'm guessing it's a tad rich. If I lean it out a little with the air screw will that make it too lean in the other gears?

It's an 03 kx125

93 octane

Castor 927 at

32/1 sometimes 28/1

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The 'out of gas' power you speak of is true....but so lean that if you could run it like that for more than several seconds, it would detonate it self to death.

 

The air screw is for idle and 1/16 throttle only

 

Jetting is about throttle position in the power band.

 

If it blubbers at max throttle, you are rich on the main

Edited by TheKoolAidMadeMeSick

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I was thinking of running c12 and 93 but I didn't want to rejet. Since it's rich right now I can run race gas without rejeting right?

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I agree with TheKoolAid, the high rpm range you speak of is not handled by the air screw.

Sounds like you're wanting to adjust your jetting with the amount of octane you run. It doesn't work like that. And C12 is a non-ogygenated fuel anyway. And running higher octance than what the engine needs to prevent predetonation actually causes the engine to make less power.

It also looks like you are trying to lean your jetting by sometimes running a leaner premix when switching from 32:1 to 28:1.

You're wanting to go about this all wrong. You need to stick to a premix ratio and fuel octane and physically jet the carb for those and your local conditions, but you say you don't want to do that, so until you come around to accepting its what you need to do, I think we're done here.

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Ok thanks I think I'm gonna go down a size on the main but I don't know how many sizes to go down. Any suggestions? Also why would it only happen in gear 1 and 2?

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Also I ended up buying 5 gals of c12 and mixed that 50/50 with 93 oct

It made the bike run even richer why is that?

Also I didn't really notice any increase in power, do you think I really did lose power if I only ran half race gas half pump?

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C12 is 108 octane if I recall. You don't get a linear mix when mixing 50/50, or any ratio. The difference between 93 and 108 is 15 numbers octane, so its not as easy or accurate as just mixing 50/50 to get 100.5 octane for the bike.

Octane basically determines how easily a fuel vaporizes to become an ignitable mixture for combustion. Running too high of an octane means the fuel is that much harder to vaporize, and in a low compression engine (below around 11:1 or so) that doesn't need it to prevent predetonation, you end up with an inefficient burn going on in your cylinder, exhausting a lot of unburnt fuel, which could come across as seeming running rich.

Have you not tried just straight 93? If you get pinging with just 93 alone, try mixing in your C12 at 10% increments til you find a ratio where the engine doesn't ping.

In 1st and 2nd gears, you can run up thru the rpm range pretty quick, exposing any faults in the fuel circuit transitions. Your description of happening at "high rpms" is vague because we don't know what high rpms is to you. Rather, try to give accurate throttle position amounts of where this is happening.

Before you go swapping out jets, make sure whats in there isn't partially stopped up/dirty. You could probably benefit from a good carb cleaning, jet size and condition inspection, and checking/setting the float to spec if needed. Take a look at the needle clip position too. Get back to us.

And one other thing, by all means, fine tune the air screw for best off idle response. I find the "whack the throttle " method the easiest and fastest.

Edited by Trailryder42
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The carb is clean and there is no pinging with 93, I just wanted to try race fuel to see what it was all about.

By high rpm I mean 80%-100%

It starts at 80 and keeps bogging until 100 and it won't get any better

By buddy had a spare main one size smaller which is a 420 and it helps a lot, it barely bogs anymore on 93 but still does when I run the c12

Here's a clip of first gear on 93

http://youtu.be/9Ku5sXFoguE

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And don't forget, you have another circuit that intersects with the main that you can experiment with, the needle clip position.

If one more size leaner on the main is too lean, either raise the needle a position or put the 420 main back in and drop the needle a position.

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Also I ended up buying 5 gals of c12 and mixed that 50/50 with 93 oct

It made the bike run even richer why is that?

C12 has a much different distillation curve than pump gas. Without going into too much detail, it essentially vaporizes at a lower temperature than pump gas, so it will make your bike run rich when you start mixing it in. It also reduces the alcohol in the overall mix, which further richens things up. The good thing about c12 in a 2-stroke, this lower vaporization temp helps the bike run cooler, it doesn't quite do the same thing for a 4-stroke because you are not pulling all the combustion mixture through the cases like on a 2-stroke.

The higher the percentage, the more rich the bike will get. You have to jet down when you mix it in. I use a 40% C12/60% 92 pump in my bikes and I had to go down about 1-2 main sizes, about 1 on the pilot. Your results may vary.

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I should have the jets today. I really want to make it scream but I don't know when it is too lean. What is pinging and how do I check for it, or any other method to find out how to tell if a bike is lean

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I really don't want to lean it out too much I don't know how to insure that I'm not too lean

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The best way is to run straight low end race fuel (non oxygenated) so you can do a plug chop and read your plug

The other way is to use your EARS. 

 

Lean is 'bowwwwwh' sound accompanied by a loss of power (like a kill button)

Rich is a rapid 'bluuh-bluh-bluh' sound followed by a loss of power (like flooding)

If you get neither, but it still seems underpowered, that is 'below the power band' due to your particular cylinder porting, and pipe, and reeds.

 

Tune the main first, then the pilot, then the needle, each with a plug chop.

 

Or, go to your bike's forum, and copy someone else's jetting with a similarly tuned bike and local.

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