Maintenance & Technical > KX500 Original

Jetting guru's......need some advice

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Mike Grant:
Hey guys I was hoping someone could give me a ballpark idea on where to so start with jetting my 1987 KX500. I just purchased a 38mm PWK carb off of a 2003 KTM 300mxc to use on my bike. I do not have the carb in my hands yet but I believe it is an air-stryker model. There is tons of jetting info for the 89+ models with the OEM 39mm pwk but I am not sure how much having a smaller carb is going to change the jetting specs. This will be going on a fresh rebuild so the last thing I want to do is screw up the jetting and seize the engine. I'm assuming the smaller carb will have a stronger vacuum that might need slightly leaner setting compared to a 39mm. Here is as much info on the bike I can think of

-1987 KX500
-Stock porting on cylinder
-Pro circuit pipe with an FMF silencer
-Newer intake manifold with "stuffers" 92+ I believe
-Boyseen reeds
-38mm PWK carb
-Elevation is approx 200 to 1200 ft above sea level
-Temp is around 45-65

Thanks for the help guys

DoldGuy:
Mike,

I have not run a 38 on the KX500 but have run smaller carbs on my KX250's and ran the original jetting & as you know  :-D ended up one step leaner on the main & also one step leaner (clip) on the needle. You should be very close with "stock" K5 jetting, and need very little change.

Hope this helps,

DoldGuy

Mike Grant:
About to order some jets for my bike . Anything you would change before I order parts. Carb is a 38mm PWK airsrtyker off a 2003 KTM 300 MXC

168 main
58  pilot
N82R needle
6.5 slide ......came with carb

Foxx4Beaver:

--- Quote from: Mike Grant on January 22, 2015, 04:30:27 PM ---About to order some jets for my bike . Anything you would change before I order parts. Carb is a 38mm PWK airsrtyker off a 2003 KTM 300 MXC

168 main
58  pilot
N82R needle
6.5 slide ......came with carb

--- End quote ---

have you had a chance to run it yet with those settings?....if not, it's really kinda senseless to order anything until you know what'll need to be changed, if any changes will be needed at all.
you'll have to do some WOT plug chops, then check the plug's color to find out which way to go on the upper end of it....meaning possible needle size change, clip position, and main jet size.
if it's blubbery and smoking excessively down low, you'll know the pilot's too rich(the air screw position will only help so much here)...so you may possibly have to change the pilot a time or two until you get it just right. 
I've got 16 different needles here, let me know if you need some to try, and I'll ship some out to you for a lot less than new ones.

Mike Grant:
I haven't even started the bike yet. I just can't find any other bikes with similar set up as mine. I just want a safe baseline so I don't cook my fresh rebuild.  I was tempted to use the stock mikuni to break the bike in but I bought a newer style intake manifold (89+) and the O.D of the mikuni is way too small  to clamp in the boot

I purchased the carb off E-bay from a KTM 300 MXC and it came with the OEM jets and needle.......not even close to what I am going to need for this bike

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