Jetting vs. Low End Power

[Tom L]

Since leaner jetting gives you more top end power, does richer jetting give you more low end power??? I would like to get a bit more low end out of my 220, and was wondering if it would be better to stay a little on the richer side.

FMF Torque (Woods) pipe, RB Carb Mod., Carbon Tech LT Reeds, No Airbox Lid, Stock Silencer. 148 main, cek needle clip in 2nd groove from top, 42 pilot, air screw 1 7/8.

 

[Rhodester]

I have a 200 so your 220 may have slightly different results from my suggestions.... I found that if I went a step richer on the main that it did have the effect of strengthening the bottom end. I also found that putting the air box lid back on fixed my low end jetting problems of hesitations and flat spots (I had to go way rich on the main and needle clip to counteract the problem until I put the air box lid back on, minus snorkel). A really good change that I made was to go with a DEK needle (I run it in #3 clip position, with a #7 slide -- like yours). That really brought the the low end snap out of the engine. The CEK had a softer bottom end than the DEK...definately a noticeable difference. The only problem that I have now is that I go through back tires faster. I currently run 152, DEK/3, 42, air screw 1 1/4 (stock carb), Air box lid on, no snorkel, FMF -35 (woods), 'Q' silencer, and RAD valve. Eric Gorr told me to advance my timing passed the most advanced timing mark to pick up more bottom end snap (which I did). He said that the KDX engines run so little compression that overheating and detonation won't really be a concern (which is right). The next step is to raise the compression to enhance the bottom end. I had EG mill the cylinder base and relieve the head to raise compression and slightly lower port height to further help bottom end response.

 

[Jim Crenca]

I disagree. I think bikes need to be jetted for proper air / fuel mix and that the only jetting adjustment that makes sense is the air screw, which, IMO, detunes acceleration for climbing muddy, nasty hills, but allows a quick change back for peak performance. In my experience, the CEK needle had the hardest accleration so a needle change for tractability makes sense. Also, I've been very happy with the addition of a Stealhy flywheel weight to calm down acceleration and get the power to the ground.

 

[Rhodester]

Well, I did extensive testing (which I logged the results) over a year and a half to come up with the best combination that I could jetting wise. I'm sure that each bike has minor differences, but I did enough testing to develop observations about certain tendencies that occur by richening and leaning certain carb circuits. I tested 13 needles (CEK and DEK among them) with 9 different main jets and 5 pilots along with 3 different slides, 2 different pipes (-35 and -30) and 2 silencers (TC 1 and Q) in many different configurations. I also run a flywheel weight. I say this so that you know that my opinions are not completely baseless, but I admittedly am still learning and certainly don't know all there is to know about these bikes. Good luck experimenting.

 

[Jim Crenca]

Rhodester,
I have to agree that documenting results with experimentation is the way to go, and ,if richer jetting works for you than that is the right decision for sure.
Is going through back tires due to wheel spin from increased torque or due to quicker throttle response?

 

[Jim Crenca]

Rhodester,
A friend just bought a 2000, 200 KDX that has a PC pipe but is otherwise stock and has been poorly jetted by the previous owner. All my jetting notes are with an RB carb. Where is a good place to start with jetting and needles assuming pump gas, 400' to 1,000' elevation, and 70 degrees. We're looking for response and torque and not an MX hit.

 

[Rhodester]

Jim,
My back tire wear is due to throttle response in the bottom end. I'm so impressed by how good the low end is now that I can't keep from nailing it all of the time.
A safe starting point would be 155, 45, DEK/3, air screw at 1 1/2, with a #7 slide. If he keeps the stock (#5) slide then I'd pick a DEM needle. He may very well end up with a 152 main and a 42 pilot, but 155, 45 will get him started. I'm at about 2800' BTW and run 91 octane pump gas.
The FMF -35 (woods) is definitely the way to go for strong hitless power throughout the power band. I know that PC now makes a Platinum II pipe for enhanced bottom end, but I have no experience with it. The Platinum (I) pipe, I'm sure, is probably the same type of power as the -30 (rev) pipe (ie. lots of hit).
From what I've seen on this site the RB carb jets pretty similarly to the stock carb. They are only 1mm apart in bore size. One of the big differences is how responsive the RB carb is to air screw adjustments. I've thought seriously about getting an RB Airstriker. I even visited Ron at his shop when I was in Portland a while back...we'll see.

 

[Tom L]

Holy Smokes!!! You guys have some great insight on this. I'm printing this thread out for reference!!! Rhodester, I can't believe you mentioned hesitation and flat spots!!! That's exactly what I have been struggling with also. Ever since I got the R-B Carb Mod, my bike has been faster overall, but much more temperamental with a much greater tendency to hesitate and have flat spots. The hesitation is really, really, really bad when it's cold and takes at least 15 minutes of riding to get rid of it no matter how I set the air screw. I even went a step richer on the pilot with no effect other than loading up the spark plug. Leaner needle clip and main jets seem to make it a little bit worse, but not much. I figured I would just have to live with it. I'll try reducing the air intake a bit per your suggestion along with the dek needle and 152 main.

How would the stock 1173 needle compare to the dek???

Thanks again you guys!!!

 

[Jim Crenca]

I think the RB carb mod is the greatest thing since sliced bread and a bargain as well.
There is a long thread in the archive forum that tells you more than you want to know about the history of this mod and suggested jetting.
Thanks for the jetting advice Rhodester

 

[Rhodester]

Jim, you're welcome. I've also benefited from your advice.
Tom, I didn't have good results with any 'B' taper needle as far as hesitation goes. The longer the straight section (L1) the better my bike runs (ie. 'C' and even better with 'D'). The 'B' needles ran better as the L1 diameter got richer, but not real great. The best I could do was turn the hesitation into a flat spot. The air box lid (back on) was the key to getting my jetting to work. If you have a hesitation the best way to deal with it, at least from my testing perspective, is to leave the pilot where it should be and go a step or so richer on the main. My problem was when I was cruising along slowly under a light load and then snapped the throttle open to WOT...= big hesitation. Remember, jetting deals allot with throttle position not so much RPM range. When you snap it open to WOT you're dealing with the Main jet circuit not the pilot. As the taper angle increases on these needle designs the L1 gets longer (before you get into the tapered section) and the needle tip gets smaller (thus exposing the main jet to a little bit more vacuum and therefore slightly richer at WOT....at least that's my theory). To fix a hesitation such as mine you have to be richer at WOT one way or the other (needle taper, main jet, or a combination of both). Also remember that my porting is different than yours, mine being a 200.

 

[G. Gearloose]

Rhodester,
You sound quite proficient and experienced...but allow me one correction...the L1 diameter is a constant. You can vary its location but its diameter is the same needle-to-needle.

 

[Rhodester]

I stand corrected. The "L1" is a constant. I should be more accurate in my communication. I was trying to say that the straight section diameter can be changed (fatter = leaner, skinnier = richer). The straight section is also longer as you go from 'A' to 'B' to 'C' needles, etc. The steeper the taper angle, the farther down the needle it starts.

 

[Tom L]

Thanks Again Rhodester!!! This is all starting to make sense to me now. It's definitely the same hesitation. I drift along for 2 or 3 seconds just above an idle, then snap to WOT ---> Big hesitation. Sometimes it damn near stalls. Following your experience, I replaced the 148 main with a 150 --> Small improvement. I think I'll leave the main at 150 for now. R-B shipped it to me with a 152 --> The spark plug was looking pretty brownish and the top end power was not nearly as good as with the 150 while the low end seemed about the same. Moved the CEK needle clip from 2nd to 3rd position --> Better yet!!! Hardly any hesitation, but still a little bit occasionally. For the heck of it, I tried the 1173 which was noticably larger in diameter at the tip --> Big mistake!!! Very Bad hesitation. I put the CEK back in with clip in 3rd position. I'm going to see if I can pick up a DEK tomorrow and try it. From what you've said, I'll bet that will really be the ticket. I might try the CEK in #4 position for curiosity's sake. I'll let you all know what happens. Thanks a bundle!!! I feel I'm making some progress now.

 

[G. Gearloose]

started with a 148? hmmm

Doing WOT plug chops to set the main, with the stock airbox, will often net a 145-148 main, but this will be too lean (bog,hesitation) until the rpm's come up and you pull big air, the airbox restriction makes vacuum and artificially richens the mix back up. Airbox must be desnorkled, lid drilled, etc.

ReSet the main with plug chops next before spending too much more time dialing in. Shoot for a 2mm soot ring. Repeat the main when changing tapers, because the different needle tips affects the main jet.

 

[Tom L]

Thanks G!!! I have been running without the airbox lid.

 

[Jim Crenca]

You guys are on it!
I think you can get a much better main jet / WOT test if you can hold it wide open for 10 seconds in at least 4th gear but preferably 5th up a slight hill; and then cut the plug with a lathe or saw to look at soot ring as, IMO, plug color is not as accurate. I've got a beutiful soot ring although the insulator looks a bit lean. The bike runs great, no pinging, and it sure doesn't feel lean.

 

[Rhodester]

Tom, CEK/4 was too rich at part throttle for my bike. CEK/3 worked better, but not as good as DEK/3. The brown on the tip of your plug is most likely fuel additive deposits if you're running pump gas. It's not a very good indicator of jetting mixture. I'd be sure to try DEK/2 and DEK/3 with the air box lid back on (without snorkel) with the 150 and 152 main jets just to make sure you've tried all the possibilities. Have fun.

 

[Tom L]

Well, I tried the CEK/4 with the 150 main. --> Ran great! Better low end, good top end, little to no hesitation. Plug got a little too dirty though. Soot build up on the outside edge. Went back to CEK/3 with 152 main. --> Ran just as good but without the soot buildup on the plug. Ceramic is still medium brown but not a concern. Like you said, Rhodester, could be the pump gas. This seems to be about the best jetting. (It's exactly the jetting RB sent it to me with!!! But, Hey, I had to make sure it was right!!!) It just seems weird that I was able to run a bit leaner with a clean plug with no hesitation before I got the RB Carb mod. Oh well !!! No regrets here. The carb mod still added lots of power!!!

The shops around here never heard of a DEK needle. All they want to do is sell me the stock 1173 needle!!!

What's a good place to order needles from???

Thanks Again for all the help!!!

 

[Rhodester]

sudco.com is the place for any carb parts for dirt bikes. The price is also shockingly lower than the dealer. They also have charts on needle comparison and blow up carb diagrams. You can also call them at 1-323-728-5407.

 

[Rhodester]

Also, remember that your stock 220 carb had a smaller bore than your RB carb. That would necessitate smaller jetting (especially the main jet).

It sounds like your dealer(s) has limited jetting experience if he's not familiar with the Keihin family of needles. I would take their advise with a large grain of salt.

 

[Tom L]

Thanks Again Rhodester!!! I really appreciate your help. I'll order the DEK today. I just advanced the timing last night. Can't wait to test it today!!! I'm going to wait until winter to get the head milled when I swap out the stock piston. I'm a little nervous about puting too much more compression on it, after hearing the grenading piston stories. I estimate I have around 80 - 100 hours on it. I pretty much baby it most of the time though, riding just above an idle just purring around on the trails. That's where the low end power really helps!!! I'll let ya know how that DEK works out. You've been right on the money so far, so I'm sure the DEK will be excellent!!!

 

[Rhodester]

Getting them to run up to their potential is half the fun!