[dANbOT]

Hello

I ride an 07 Honda CR250 modified for woods riding. I have recently purchased a 36mm Keihin PWK to replace my 38mm PWK. I have read that a smaller bore carb should provide better performance at lower rpm / throttle settings at the expense of some high rpm power.
After installing the carb, I have noticed some strange tendencies from my motor. It is blackening plugs and smoking like a very rich tune, but acting lean in some situations.

My question is what jetting changes would be expected from this 38mm to 36mm carb swap. My knowledge leads me to believe that a faster velocity / stronger signal would make the same jetting specs run richer on the smaller carb. Any thoughts?

I'm going to dive in tomorrow morning and pressure test the engine to be sure there are no outside factors messing with my results.

 

[dANbOT]

Well, I have made some progress. Since nobody was able to offer suggestions, I'm happy to share what I have learned thus far in hopes it will help someone down the line.
My leakdown test was a pass, so I moved onto tuning on breaks during long rides.

My suspicions were partially incorrect.
-I seem to need a richer idle circuit setting than I did with the 38mm.
-the needle... set exactly as it was in the 38mm is about the same, maybe a touch richer
-main settings are similar to the 38mm

I started with what came in it (new Sudco carb) which was:
165 main
55 pilot
DGJ needle middle clip
5.0 slide
-Ultra rich on the pilot
-needle very lean from 1/8th throttle to 3/4 throttle
-no torque at low rpm
-plug still looks dark even with the lean symptoms

Round Two
165 main
40 pilot
DGJ needle 4th clip from top
#5 slide
-much better starting pont
-pilot circuit in range now a/s 1 turn out, idle is perfect at about 1600
-needle still feels "zingy lean" from 1/8th throttle to 3/4 only when im clutching it climbing technical stuff instead of all the time
-more torque, but still missing a lot of low rpm performance

Round Three
165 main
40 pilot
Honda 715/289 needle (stock 2000 CR250 PWK) 3rd clip from top
#5 slide
-much richer needle, runs alot better at mid throttle. it will actually load up if I run 1/4 throttle for more than a couple minutes
-low rpm improved greatly but not what it used to be yet
-Close enough now to start fine tuning, I'll cal this a base point for now

Round Four
#7 slide from my 38mm PWK
-even more improvement in low rpm torque, can't quite lugg down where i need it but closer
-pilot seems lean now: increasing idle speed, and less crisp sounding just off idle. a quick air screw adjustment couldn't tune it out. Possibly whats holding back my low rpm performance now.

I plan to richen the pilot, 42.5 or 45, and the needle will need to be one or maybe two clips leaner. Once I have the idle circuit and needle dialed I will do a couple WOT chops to check the plug on the main, but it seems safe for now.

So.... although I'm estimating a little still, here are the changes needed to be made from 38mm PWK to 36mm PWK carb:
Pilot
38mm- 35
36mm- 42.5 or 45
I expected the opposite, but a richer pilot was necessary

Needle
A slight rich condition with the 38's settings, one or two clip positions should cure it.

Main
Is the same or very close

Slide
Is the same


Of course this is on my 2007 Honda CR250
3000 - 6000 feet
FMF Gnarly into Turbinecore2
Boyesen RAD Valve with 3 extra gaskets as a spacer
Modified power valve stopper to allow it to close a little further

I am still hoping to get back my super low rpm lugging power that I had with the 38mm. The 36 was supposed to be an improvement in this area, so I still have hopes! Mid range response is alot better than the 38mm at this point, feels very "peppy" without having too much hit.
Here's a pic of my girl:

 

[Ecojbr]

Did the same thing to my kx 250. Jetting will need to be real lean as there is more "signal/vacuum" or whatever term you use. Surprisingly there was very little difference between the two carbs

 

[jaguar]

Since idle and main jets are easily replaceable then the main differences between carbs is the clearance area between the needle and its jet, and the taper of the needle. Of course you can save yourself a ton of time and money by just figuring out what is needed by using jetting software : dragonfly75.com/moto/carbjetting.html