Carb Tuning

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Carb Tuning

Postby ArkAngel » 01:52 pm Oct 11 2015

Hi Guys /Girls.

First let me give you a bit of back round. I rebuilt my bike with a new top end and new main and pilots jets ( the tread should still be around somewhere) and new gaskets.
I just finished getting my trailer sorted and toke my bike out for its first ride on Saturday. The bike ran great and it also idled really well.

The problem i had is that i went through 2 spark plugs on Saturday and then on Sunday i decided to do the jetting guide, now this is where is gets interesting.

I warmed up my bike and then put in a new spark plug and started the jetting guide. I put my air screw to 1.5 turns out and started the bike.. It start first time. I then started turning the idle screw all the way in and then out and it didn't make a different at all, i then tried the air screw and it did the same.. i can take it all the way in or out and it does not make a difference to the way the bikes idles.

Any help or suggestions will be appreciated?
Let me know if you need any other info?

Bike is a KDX 250, 1991
with a new piston and gaskets.
Float level also checked.
Running a BPR7ES spark plug, ( because cape town (South Africa) uses them
Fuel Mix is 40:1

Thanks in advance
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Carb Tuning

Postby Keystone » 02:44 pm Oct 11 2015

The idle screw will adjust the idle. The fuel/air screw will only affect your pilot jet. Your pilot jet might be perfect. If your still fouling plugs, maybe your main jet or needle clip position is too rich. That would explain why adjusting the fuel/air screw makes no difference for you.
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Re: Carb Tuning

Postby Jaguar » 08:39 pm Oct 11 2015

it's impossible that the slide stop screw makes no difference.
if the idle air screw makes no difference then probably its passageway is clogged up.
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Re: Carb Tuning

Postby royadams » 08:51 pm Oct 11 2015

Also if the pilot is way to rich the air screw will have no effect. As far as the idle screw if it has no effect your cable probably has no free play.
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Carb Tuning

Postby ArkAngel » 02:31 am Oct 12 2015

Thanks all,

I will try moving the needle position 1 up so its less rich. Then ill give that a bash. Ill also just check my main jet again, but im sure i went leaner before.

Ill keep you updated.
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Re: Carb Tuning

Postby G22inSC » 06:11 am Oct 12 2015

If I were you this is what I would do. Make sure you don't have any air leaks. Check for correct throttle cable "play" and make sure all jets were stock or really close to stock. Then follow the guide and only change one thing at a time. It's a process and will take some time. Good luck.
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Re: Carb Tuning

Postby 6 Riders » 11:52 am Oct 12 2015

Check the needle PN, see if it is stock or not, then set it on the 3rd clip (middle). Make a note of the jet sizes before starting.
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Re: Carb Tuning

Postby ArkAngel » 12:31 pm Oct 12 2015

Hi Guys,

My needle is the second from the top and i am running a 155 / 40. I was browsing around the net looking for some info to maybe check if i am near what my main and pilot should be. I came across something called Jetting guide table ( or correction table) I am not sure if i am reading it right, but it says i should be on a 144 main and 42 pilot. If anyone can assist me with reading one of the tables it might help me getting the correct jets to start over with? I live in cape town at sea level, with temps from 30C to 38C.
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Re: Carb Tuning

Postby 6 Riders » 12:38 pm Oct 12 2015

Use the jetting guide.....Also before you start, put your needle on the 3rd clip.
Every bike is different and different mods, means different jetting, so the jetting guide is mandatory for us "regular" guys. If you follow the first part of the guide, you'll find out if the pilot is rich or lean pretty quick.
Now, don't be lazy!
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Re: Carb Tuning

Postby ArkAngel » 01:01 pm Oct 12 2015

I was doing the jetting Guide but it did not work. I was trying to get it to idle faster or slower and my bike done nothing, no change. I have checked for air leaks also. I changed the position down to the 3rd one and now it managed to foul a brand new plug in 20min and its looks like the plug is burnt black and very wet.
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Re: Carb Tuning

Postby 6 Riders » 01:06 pm Oct 12 2015

Well, something is not right in your carb, probably the float height or needle/needle seat. Since you have to take the carb out to check these things, double check the reeds to make sure that they are flat and have no cracks, tears or dirt and that they are assembled correctly on the reed block. NO JOKE, so many people have problems like yours, just to find out the reeds are wrong in one (or more) of those ways.
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Carb Tuning

Postby royadams » 07:56 pm Oct 12 2015

I would start with a jet block gasket just to be safe, especially since you are fighting a rich condition. In my 250 I ran a 38 155 one leaner on the slide and two leaner on the needle, a br7es plug. Boysen reeds kx head gaskets.where do you live
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Re: Carb Tuning

Postby ArkAngel » 12:50 am Oct 13 2015

Hi Guys,
I did manage to check the reeds last night and they looked fine. They make a nice seal and don't have any cracks or tears.
@Royadams,, ill try get the gasket today and give that a try. I live in south Africa cape town. We have no real humidity but it gets HOT here.
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Re: Carb Tuning

Postby royadams » 04:44 pm Oct 13 2015

If you lived in the states I would have sent you my old carb to try.
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