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RB carb CEK, lean (not fault of carb)

Posted: 09:40 am Oct 09 2020
by spencerdiesel
Hey all I have put about eight hours on the bike since I installed the RB carb and I have also put about three hours of tuning into the carb. I’m having an issue with the needle (CEK) needing to be in the richest position so that the bike behaves correctly.

Bike is a 1995 kdx 200 that someone at some point in time has ported for “all bottom end“. I believe this port job has something to do with the odd tuning that this bike has taken.The bike has the following mods: Platinum 2 pipe, V force3 reads with 22 hours on them, Stock silencer, low end oriented porting (done before I got the bike) and the RB carb.

The conditions I ride are: Western North Carolina 2000-2500 ft, humidity usually 50%-80%.

My current jetting is as follows PJ is 38, needle is CEK in the richest position (lean at 1/8 throttle still), And a 150 main jet. PJ is great as is the AS. My best 0-1/8 throttle performance is with the AS 1/4 to 1/2 turn richer than peak idle.

At first I thought the needle needed to be in the richest position because my main was too small. But 2 back to back plug chops at wide open throttle reveal my main is actually still rich. (For my WOT plug chop, I hold WOT uphill in fourth gear with the rear brake held slightly for 5 to 7 seconds at a time before killing ignition & pulling in clutch. I do this 5 to 7 second run three times on each plug before cutting it. With a 150 Main And the above plug chop practice, the ring around the base of the porcelain is approximately 4 mm tall (2mm would be ideal for me).

I apologize for the long winded post, and I feel like I might have answered part of the problem with mention of the porting. But if anyone has any recommendations or suggestions, they are appreciated.I believe the next step to try will be a needle or two with varying richer steps from zero to quarter throttle in the straight section? The better pull at low revs I’m reading about everywhere I’m sure would be improved with more fuel from 1/8 throttle to 3/8.

Thanks everyone!

Re: RB carb CEK, lean (not fault of carb)

Posted: 01:46 pm Oct 09 2020
by SS109
That's an unusual plug chop regimen. I've never heard of doing it that way. I do no more than one pull on any plug, never drag the brake, and I'm always on fairly level terrain. By doing multiple pulls while under even heavier load (brake drag/uphill), I think you're skewing the results of your plug chops. As long as it isn't showing a dangerous lean condition, always tune for the best throttle response. Plug chops are just a tool to help get you there but are not the "be all, end all" in tuning your engine. Also, porting doesn't normally affect the jetting on a carb more than just a small amount.

Having to run the needle in the #5 (richest) position indicates you need a larger main jet. I would try a 152 main with the CEK needle in the #3 position and see how it runs regardless of what the plug chops says. Play with the needle positioning if you need to but I think you'll find what you need with the 152 main.

Re: RB carb CEK, lean (not fault of carb)

Posted: 05:55 pm Oct 09 2020
by spencerdiesel
Thanks for the response SS109. The only reason I drag the break and make a pull uphill is to maintain full load on the engine at wide-open throttle. On flat terrain where I ride, the bike is wound out and seeing less load and I have trouble darkening the plug enough to see (also the reason for 2-3 WOT runs per chop, the height of the soot ring doesn’t change, it only darkens with more time at WOT) Dragging the brakes lightly while making a pull up hill Allows me to do a slower speed run while being loaded, rather than making a run down the road tapped out in sixth on knobbies.
If I do a WOT run on the little flat area I have, there’s not enough load or time on the engine/plug to give a good reading. A 152 main results in less power and the top end falling off sooner. The bike makes more power with a 148 than the 150, but then it’s even leaner 1/8 throttle w/ needle in richest position.

I agree the 152 main helps my lean spot on the CEK, but it’s about 2 mains richer than peak WOT power.

Re: RB carb CEK, lean (not fault of carb)

Posted: 09:11 pm Nov 17 2020
by treelimb
So my reply will be against the grain therefore l will state that my next mod will be an RB carb.
I am unorthodox regarding jetting having never done plug chops and admit I've been lucky with my 113 hour top to bottom new engine on my 200. I actually race my bike in the SORCS series and ride often between races averaging 24-27 miles every ride not including the 2 hour races. I just ride it and look at the plug every other ride. Never fouled a plug.
Running 160,45, top
clip, 40-1 motul 800, 100% 93octane and a ngk br7. I played around with the jetting including running a br8 but went back to a br7. Always a tad moist but never fouled out. Ever. I do indeed ride my kdx.
I guess my point is just ride it man. You'll eventually sort it out and inbetween times you will enjoy the bike and the rides and will not stress over an issue that you probably already understand the reason for.
Keep in mind that I am a 59 year old that is a couple months away from being 60 so my logic is entrenched in years of dirt bikes and what a caref for bike can do for the soul and doing my best to adhere to that old cliche- " if it ain't broke, don't fix it". Just get on it and ride. Hours on the bike will reveal more than minutes but I am not a mechanic. I use a 24 year old carb too! Not for much longer though.
Did I just digress?