Trying to understand shimming, jetting, air filters etc - Kawasaki Ninja 250R Forum
 
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post #1 of 9 (permalink) Old 06-24-2012, 09:38 AM Thread Starter
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Trying to understand shimming, jetting, air filters etc

Hi,

I'm new to motorcycles and want to do some cheap/free mods.

Also, i don't know much about how motorcycle works and have a lot of quesitions. I'm hoping some of the veterans/experienced guys here will take a few minutes out to explain it. I'd really appreciate it

What exactly is "shimming the needles" ? How does it improve the performance? Are there different settings we need to choose when we "shim the needles" ?

Is "shimming the needles" and "rejetting"/"jetting" the same thing? If not, what is "rejetting"/"jetting" ? Do we need to buy some kit to get "jetting/rejetting" ?

People also talk about getting an air filter like K&N or something similar at the same time. What does the air filter do?

Also, i've heard something about stock setting being a bit on the lean side. Is that something related?

What does the intake snorkel do? And how does removing it help?

Also, are there any more similar/cheap mods which improve performance ?


I know this is a lot of questions . But i'm hoping some of the veterans here will take a few minutes out to explain it. I'd really appreciate it.

Last edited by IndianDude; 06-24-2012 at 10:20 AM.
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post #2 of 9 (permalink) Old 06-25-2012, 04:01 PM Thread Starter
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I think my original questions were too broad for anyone to answer. So I've done a bit of research on some of the above topics and i was wondering if anyone could give me feedback as to whether i got it right or not.

Basically, to get better fuel economy and emission standards, the Kawasaki Ninja 250 is designed a bit on the "leaner" end, i.e. a little less fuel goes into the engine. We can fix this by making more fuel enter the engine, i.e., make it "richer", by "Shimming the carb needles". "Shimming the carb needles" is basically raising the carb needles by adding washers to it. This extra fuel entering the engine will make the bike feel more powerful, especially in the low rev range, i.e. 2k - 5k.

Another thing i think i can do is remove the snorkel from the airbox. The snorkel is mainly meant for noise reduction. Removing the snorkel will cause more air to enter and this should make the the bike idles better, throttle response should be improved and the bike should have a better intake roar when revved.

So, to start off my mods,l, i'm thinking of shimming the carb needles by adding 2 washers and removing the snorkel. I think this will make the bike feel a bit more powerful and more responsive to the throttle.

What do you guys think? Are these 2 good mods to start off with? Any feedback would be appreciated. .

Last edited by IndianDude; 06-25-2012 at 04:34 PM.
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post #3 of 9 (permalink) Old 06-25-2012, 04:10 PM
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Originally Posted by IndianDude View Post
.....So, to start off my mods,l, i'm thinking of shimming the carb needles by adding 2 washers and removing the snorkel. I think this will make the bike feel a bit more powerful and more responsive to the throttle.

What do you guys think? Are these 2 good mods to start off with?
Sounds good.
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post #4 of 9 (permalink) Old 07-06-2012, 02:48 PM Thread Starter
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Just one more question guys.

My motorcycle isn't broken in yet. It has only 300 miles on it. Should i wait for it to get broken in, at 600 miles, before i do the shimming on the carb needles + removing the snorkel? Or is it ok to do it now?
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post #5 of 9 (permalink) Old 07-07-2012, 03:42 PM
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Originally Posted by IndianDude View Post
Just one more question guys.

My motorcycle isn't broken in yet. It has only 300 miles on it. Should i wait for it to get broken in, at 600 miles, before i do the shimming on the carb needles + removing the snorkel? Or is it ok to do it now?
Just leave alone until you have your first service done. For now just enjoy the ride.
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post #6 of 9 (permalink) Old 08-30-2012, 05:38 PM
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I will try to answer a couple of your other qestions. A high performance air filter will allow more air into the engine, which lets us add more fuel, which makes more power. Any way we can get more air into the engine or more exhaust out will allow us to add more fuel. Optimal air to fuel ratio (AFR) in a gasoline engine is approximately 14.7 to 1. That means for every 14.7 grams of air 1 gram of fuel is required to create the best mixture. That is where changing carburator jets comes in. If you make an intake or exhaust improvement your engine will be ingesting more air so we need to add more fuel to get the ratio back in check. Installing larger jets in the carbs will allow more fuel into the engine
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post #7 of 9 (permalink) Old 10-26-2012, 05:57 PM
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good info

I just bought a new 2009 250r with 4100 miles rides nice but lacking a little somethen. just reading some of these post espcailly this one has helped me alot with do it urself things. im going to put a new sprocket on the front 15 teeth hoping to lower the rpms on freeway since speed limits are around 70 id like the bike to ride a little easier at higher speed bc i usally ride longer distances but on highway so i want the bike to ride a little easier to help engine life. any other ideas about how to do this please share your knowledge with me. thanks for reading...
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post #8 of 9 (permalink) Old 12-24-2012, 06:46 PM
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I know this is old, but I hate finding forums that have what I was looking for, but no answer, so I'll add my .02 for future readers.

You have the right ideas with shimming and the snorkel. I agree with DaBlue in waiting for the initial service, that was if something IS off that'd be covered, your unrelated handiwork doesn't give a reason for them to blame you. Also, it'll show you how the bike is going to run once it's broken in and settled, and the plugs will show if it's rich lean, rich or perfect.

Shimming is different than jetting. The shims pull the needles up a tad in the way that would happen if the throttle were to be cracked slightly, but without introducing more air into the mix. I recommend using dial calipers or a micrometer to make sure the needles on each side are raising the needles the same amount, regardless if you use 1 or 4 needles.
There are two jets per carb on these bikes. You have the pilot jet and pilot jet. The pilot jet is like a pilot flame in a stove, furnace, heater, etc. in that it is there for the little bit of initial feed; stock on my '09 is a Keihin 38. The main jet controls fuel in the upper RPM range (the specifics miss me at the moment bc I've spent more time on old stand-up jet skis, so I can tell you those!); stock pilot is a Keihin 98, I believe.
I have a full system Yoshimura and K&N pods, so my bike is moving a lot of air. To compensate this, and maintain the air/fuel ration mentioned above, I moved my pilot to a Keihin 40, the main to a Keihin 110, and I have 2 washers for the needles.

When in doubt, go rich. A too-lean condition will add performance due to the quicker combustion (roughly like advancing timing), but can damage the engine bc it'll be burning hotter. I almost toasted a quad this way.


For Kawaninja's topic, adding teeth will aid in the highway, but take away from lower speeds. Another way is to go wit a slightly taller tire when it's time to change. The difference isn't huge, but would aid in what you're looking for.


As stated above, enjoy the ride for a while on a new bike, and for a new used bike, check it over to make sure the PO didn't do bad maintenance, and then enjoy it for a while as-is. The more familiar you are with your bike, the better you'll know what to do to it.

Merry Christmas!
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post #9 of 9 (permalink) Old 02-03-2013, 04:46 PM
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Today I did the shims on my wifes 2009... 2 4mm washers on each needle... WOW!!! huge throttle response gains! She had 2 flat spots.. off idle and another between 6000-7500rpms... both are gone now... I highley recommend ths mod. Might add.. I am 600ft above sea level... Worst part of the mod... getting the diaphram back in the groove....
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