RB Carb Mod Jetting Report


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Just thought I'd post some feedback about my progress in dialing in the jetting for my RB-modified carb.

Motor: 225cc big bore kit, w/light porting.
Air-box mods
FMF torque pipe
power core silencer
plug: BR7ES
Boyesen power reeds (#607)
Fuel: pump gas (Mobil unleaded premium)
Altitude: 1000ft
Temp: 65-70 deg F

Ron's original recommendations were: 155 main, 45 pilot, needle (AEG) on the fourth clip. Air screw 0.5 to 1 turn out. I started with 3/4 turn. With these settings the bike ran ridiculously rich off idle to around 1/4 throttle. Massive burbling and gurgling. Fiddling with the air screw made a big difference. I opened it up in half-turn increments. At 2.5 turns, things got dramatically better. At that point, I stopped screwing around with the jetting and just went riding.

This last weekend, I dropped the pilot to a 42. The air screw for this pilot size seems happiest about 1.5 turns out (though I'm still experimenting). This pilot/air screw combo brought another big improvement. The bike now has a clean, linear pull right off idle.
I'm really pleased at the way the low end power deliver has improved. :cool:

I'm trying to decide what my next move should be. My main jet run was cursory - just to be sure that I was not too lean. I'm considering dropping it to 152 and check the effect. I can't remember how this works: is the high speed circuit affected by the low speed, or is it the other way around? I thought I remembered a post by MX Tuner that changing the low speed affects high speed, but not vice-versa, so tune the low speed first; but I may have that confused.

The other thing I might try is to lean the needle once clip to see what happens, but I'm on the fence about that because the 1/4 to 3/4 range feels pretty good right now.

Where I'm at right now:
Jet Needle: "AEG", 4th position
Slide cut: #7
Air Screw 1.5
Main: 155
Pilot: 42


Seems that jetting for different bikes is considerably different with the mod from RB Designs..what works best is, well, what works best for you.

'Fer instance..I'm surprised you got anything decent out of the AEG. I didn't.
The AEN changed things hugely for the better when I use my KG35 (torque). The AEN being .060mm larger diameter than the AEG, the off idle-to-1/2 is exactly where I needed less gas.

Anyway, I know that the main can effect the lower circuits. Changing the main a couple of sizes will effect my idle speed for example. That's why the jetting sequence STARTS with a WOT chop test.

Using the diameter of the needle effect as an example, changing that will do the same thing as changing the main jet diameter. Whether you increase the main diameter or decrease the needle diameter, you will still see an effect from idle to 1/2 (again, using the needle diameter effect from the needle chart).

'00 200, aribox mod, FMF rev or torque, PC S/A, Boyesen power reeds, B7ES. Alt around 2500, 60-75º

42-150-AEN/4 for torque
45-155-R1173L/4 for rev

Dan is working with some hi-falutin double/triple taper needles. I'm sitting, waiting patiently for someone with the brains and the patience (Hi dan!) to get it figgered out. Meanwhile, my bike runs great!

BTW, my riding buddy was doing some MXing last weekend. He runs pretty much the same setup I do (RB included). He had people asking him WHAT he'd done to his bike, seeing as he could pull every bike he rode with (KX 250/500's included) except a 426!



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I seem to recall acutemp saying they had better luck using the AEG for ported 220 motors, so maybe that's the reason? I'm surprised you had to jump to the AEN. Going leaner on the AEG clip position didn't help? Seems like since mine is on the 4th clip, I have some room to go leaner before worrying about a new needle. I had a chance this weekend to ride a spanking new '01 220 back-to-back with mine. It was interesting. My bike will now tractor with the best of them with what seems like equal-to-better low end grunt, and crisper throttle response down low than the stocker.

jeff joy

was wondering if you could give me the phone# or address to rb,thinking about having it done for my bike.


aeg vs: aen

It wasn't a 'clip' issue, as both needles have the same L1 (38.15). It's the larger diameter of the AEN that made the difference.

I couldn't get the AEG to do ANYthing but blubber (uh..that would be a RICH bog, dan!) .


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BRush, glad to hear that you got your big bore going. I do run the AEG#4-5 on my kdx. I've tried many different setups and this is what works on my bike. It is way to rich for stock ported bikes as CC stated and there is probably a better needle out there I just have to find it. My bike rips with this setup though. I have some more to try and C Dave has some new ideas to, so I will keep you up on anything new. Your jetting is on the safe(rich) side but with all the mods that you were doing at once it is best to start that way. If as it warms up you will more than likely need to lean your jetting out. After you do your break in, do a wfo check to see where you stand on the main. If you seem to be to rich in the 1/8 to 1/4 area you might try a leaner needle.Ron can set you up with this if you need. Here is a setup that a guy in Oregon sent me a couple days ago. He has a FRP ported 220 and has been working on his jetting for awhile. He found that the AEG was to rich for his 220 and this is what he ended up with. 40 pilot,152 main AEK#2. Remember to that these carbs sometimes run best with the air screw in as tight as 1/4 turn from seated. Good luck with your bikes and thanks for the update.

CC, blubering is what I do when I look at this pile of needles in front of me:eek: I think I might need to send some down for you to test:p Say did you ever get thse new knee braces?--Dan
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Mr. dan:

Yep! Got my brandy new asterisks just last week. Haven't ridden with them yet. They sure are cute! With extension adjustment, tendon cuff adjustment and telesoping patella protection...they seem a deal @ $350 a pair.

Don't think the left one will fit quite. That knee's not quite back to 'normal' size, yet.

'We must ride!' is for sure this weekend, though.

They came WITH socks..and a 5/64 ball end driver for adjusting everything.

Too bad they didn't come in kawi green!

I DO have a couple of cool looking decals on my scoot, though!!