YAJQ (Yet Another Jetting Question)

[mandark1967]

I performed a search, read the FAQ's (good collection of information, by the way!) and I read the jetting guide on the just KDX section. However, I still have some jetting questions I need some help with. Specifically, I'm looking for a decent jetting baseline I can reliably run

CLIMATE BACKGROUND:

The area I live in (Memphis, TN) experiences major temperature and humidity changes during the day from Springtime until late Fall. We often have temperature swings that can exceed 50 degrees between the day's low and high temps.

Additionally, in the summer it is pretty common for the temps to be in the range of 98-106 with over 95% humidity one day while the next day has 30% humidity and temps of 85 degrees.

This climate, combined with the mods to my bike, has been playing havoc with my atempts to dial in my jetting, and I need some assistance, please.

BIKE INFO:
1998 KDX220
35mm Keihin PWK35 Carb with "stock" jets & needle
Note: I bought a brand new 35mm from Sudco and have made no changes to the jetting it shipped to me with yet
FMF Gnarly Rev Pipe and Turbine Core II SA/Silencer
Fredette's Torque Ring not installed (but I have one)
STOCK airbox lid (snorkel remains)
Fuel to oil ratio of 40:1 (90+octane and Suzuki CCI)

I will be installing Boyesen Carbon Reeds when they arrive
I have (believe it or not) a completely stock airbox with snorkel, but I plan on it and placing (6) 3/8" holes in the top of the airbox


Without the Reeds and Modified AirBox, the bike seems to run well in the low-mid throttle setting. It runs rich...but not so badly that it sputters.

It "loads up" between 50% and 80% throttle. From about 80% to WOT, it becomes smooth again and accelerates cleanly.

My assumption is that the needle jet/main is most likely the culprit, but I am worried that dialing the jetting in so closely to a "particular" temp/humidity setting would cause problems when we get a large temp/humidity swing...

I am thnking I should, at a minimum, raise the clip 1 notch and go 1 size leaner on the main. Currently, the plug is very black (the part down deep, not the easily viewed tip) so it seems it's far too rich.

Do you think that leaning it out a bit with the temps in the mid 50s would kill it when we hop back up to 80?

Does anyone have a suggested range for jetting that would be reliable, while not fouling too many plugs?

 

["REV"]

Stock is 160 main with a 48 pilot
You should buy 155 main and 45 pilot, as this is a good jetting starting point, but you will likely need leaner jets.

* I currently run that Jetting combo in my 35mm carb, 60 degrees, rev pipe/ fmf Q silencer, and it is way rich, I'll need leaner too.

You will probably end up with 150/40 or somewhere near there. Happy Testing!

 

[mandark1967]

Stock is 160 main with a 48 pilot
You should buy 155 main and 45 pilot, as this is a good jetting starting point, but you will likely need leaner jets.

* I currently run that Jetting combo in my 35mm carb, 60 degrees, rev pipe/ fmf Q silencer, and it is way rich, I'll need leaner too.

You will probably end up with 150/40 or somewhere near there. Happy Testing!

Thanks for the information! I'll try buying some jets in that range tomorrow.

 

[John Harris]

It has been my experience with the KDX that the primary jetting is often too rich and that is where the bog comes from in the mid range because when it gets up in revs it cleans out and pulls good. You have an unusual complicating factor in that you are running 10% larger displacement and different porting than most of us with 200 cc running the 35mm PWK. Never the less the previous suggestions of 45 and 155 are probably good starting points. You may want to just try the 45 pilot with the clip on the needle set in next to top first, and leave the main at 160. With the larger displacement engine, you need to be careful reducing the main jet size. From now at least until cold weather you can probably run with 42 pilot just fine. You will have to use your air screw to fine tune (if you want it that fine) every day or even during the day with temp changes. Carb tuning is work and trial/error, but the best time and very little money spent on your bike. Cheers John

 

[krazyinski]

I am running a 2000 kdx 220 with fredette porting and head work a long with a 36mm carb,FMF woods pipe.
temps start in 50's end up in the 80's in the spring along with changing humidity high to low. winter is worse temps swing from 40's one day to 70's the next. What I have done is dial in my pilot to a 42 and main to a 158. I found going lean on the main makes the power fall flat on top. After this I began playing with a hand full of needles.pats small engine parts for the needles. I narrowed it down to two needles CGM at 3rd from the top for spring 50-80 degree then 2nd from the top in the summer 80-100 high humidity, a DEL 3rd from the top for winter 50 and below as it warms I will run it 2nd from the top. I know it sounds like a lot but I like my bike running spot on ever time. The carb only takes less than 5 minutes to pull out and change the needle.

 

[mandark1967]

thanks for the information!