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

[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  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]

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

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

[Foxx4Beaver]

the jetting that's currently in the 38 will be fine for getting it started for your 1st heat cycle.

when I went with the new 41.5 on my brand new $6K motor, it had a 55 pilot in it...it would start fine, but once I tried to crack the throttle, it would die right out, it was way too lean, there just wasn't enough fuel...I ended up at a 68 pilot. Watch for this...if you can't get it to rev up!.

once you've completed the 1st heat cycle, pull the plug and check it's color....you'll know right away what you'll need to do on the low end of the jetting...if anything at all.
Keep checking the plug after each heat cycle...
I was nervous too that 1st time after spending so much money...but the only way to find out, is to fire it off and read your plug.
you'll be fine...just go real easy until that 1st heat cycle is done....and keep the revs down until you've got at least 4 good heat cycles completed.

[Mike Grant]

I pulled the carb apart today after work........it has no main jet (was told when I purchased it) 45 pilot with NOZH needle...... Bike shop ordered me 168 main , 58 pilot and a throttle cable so I should have everything early next week. I'll let you know how it runs then.  Sure would be lucky if this needle works ok

[Foxx4Beaver]

be sure to put the clip in the 3rd/middle position for safety's sake.

[Mike Grant]

Well I got it running ........what a pain in the arse to get the bike started though. I currently have a 58 pilot and a DGL needle in the 3rd clip . I am assuming this hard starting might be due too the pilot being too small. Going to try a 60 or 62. Also didn't want to idle on it's own either. Throttle response seemed crisp once it was going.

[Foxx4Beaver]

Well I got it running ........what a pain in the arse to get the bike started though. I currently have a 58 pilot and a DGL needle in the 3rd clip . I am assuming this hard starting might be due too the pilot being too small. Going to try a 60 or 62. Also didn't want to idle on it's own either. Throttle response seemed crisp once it was going.

if you used a generous amount of oil during assembly, that would contribute in the hard initial start. The idle issue wouldn't bother me too much, as I don't set either of my bikes to idle...
but be more concerned on getting the pilot circuit jetted good for your heat cycles...then you can go back to the idle issue.

did you check the plug afterwards?....it should be fairly dark if you used plenty of 2T oil upon assembly...but if it's on the light tan-to-whiter side, then yes, definitely go up 2-3 sizes on the pilot.

 

[Mike Grant]

Just replaced the plug with an NGK 8 series instead of a 9 and it starts 3 kicks cold and 1 one warm. Plug I pulled out looked dark tan with no signs of white or damage. I have ran the bike twice now for about 5-6 minutes to heat cycle the engine and I am going to try and borrow my buddy go-pro to get some video's so I can get others opinion on the carb set-up. Might go this weekend if I don't end up working

[Foxx4Beaver]

sounds like you've got it on the "right track"...

after a couple more heat cycles, it'll be time to take it for an easy ride around the block, and slowly determine if the "mid-range" jetting will need to be adjusted...then after a full tank's worth of running, you can start determining how the "upper-range" jetting is by doing some WOT plug chops.
your plug is like an open book, read it's color, and it'll tell you what the motor wants.
video of it running would be cool...can't wait to see video of it roostin

[jcm3]

...then after a full tank's worth of running, you can start determining how the "upper-range" jetting is by doing some WOT plug chops.

The fun stuff!  (WOT anyway, not necessarily the chop)