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So I just bought a new beautiful KDX220 with Gnarly Rev pipe, RB carb and head mods, and 13/47 gearing. I know I’m used to 4 strokes but this thing is a little bit of a monster with a hard powerband hit. I’m looking to tame it down to make it more woods friendly as it just dies when I go slower through tight woods. I’m sure the jetting needs to be adjusted as well I don’t think it’s quite dialed in. Would a steahly 10oz flywheel be a good place to start, or get rid of the rev Gnarly for the woods gnarly? I don’t need to go fast and like riding technical stuff. My bone stock KDX220 in Japan was very pleasant to ride but didn’t have nearly this much pull and power. I know I know stop being a sissy and hit it with my purse

What do you mean by it "dies"? Engine cuts off or simply power drops off? If engine cuts off, then you have an idle problem. Likely caused by improper jetting.

I'll bet your 220 in Japan has OEM carb (33 mm) and gearing at 14/44 or such.

Did your new engine get modified by Ron Black, too? Is the ignition timing advanced in your new engine?

My 220 has same gearing as your new one, gnarly pipe, OEM carb, OEM top end, and stock ignition timing. It pulls like my 4-stroke DRZ 400S that's also geared low. I ride slow technical areas. Lot's of steep inclines, lots of boulders & rock, a little bit of tree forests, and arroys. Long ago, I competed in observed trials. Now that I'm old, I ride my trail / dual sport bikes similar to trials riding. (I'd like to have a trials bike again, but them it wouldn't be possible to legally ride county roads to trail heads.)

Yes, a flywheel weight my tame the power delivery a little bit. But, you need to know what your starting point is.

Here's what I'd do.
1. Verify ignition timing setting
2. Verify whether top end is modified (ports mod, head skimmed)
3. Verify whether carb is bored (throat >33mm) and what jets are installed
4. Once I know what I have, then I'd begin making decisions about what to change.

Changes may include:
A. trading your 220 with another KDX owner who has a stock KDX but wants a hot-rodded KDX
B. If top end is not ported & head skimmed, then install 33mm carb
C. Replacing front sprocket wth 14 teeth or dropping 2 teeth on rear sprocket
D. Installing cylinder & head that meet OEM specs
E. Adjusting ignition timing to stock
F. Jet carb so engine idles

Thank you for the detailed reply I will begin doing that. I dabble in trials as well and like riding slow technical stuff so all these mods are a bit wasted on me. I have a bunch of paperwork with the bike with everything RB did so I will start going down your list

Your kdx220 in Japan would be the SR model which uses the smaller keihin pe28 carb and quite a few other changes which make it so user friendly, I also own a kdx220sr from Japan and my other kdx was the SR model but changed to R model specs and was a complete mess when I bought it
The kdx220sr is such a user friendly bike and superior to the American spec kdx220r in quite a few ways except for outright horsepower

The kdx220 bike in stock form is exactly what you are after so put everything back to stock , first thing is change that pipe and go back to a stock pipe, I sold all my aftermarket pipes and went back to stock, they provide the best idle,off idle and smooth power delivery

You don't need a bigger flywheel, this is just a band aid fix for a poorly set up bike

I used to have an RB carb on my kdx but couldn't get it jetted right but that was in my very early days when I had no jetting experience, I would keep it in your mind to maybe sell the RB carb and replace with the stock OEM 33mm keihin PWK, any second hand carb will be worn out, I recently got shafted in a purchase from Canada, luckily PayPal gave me my money back

Once you have put everything back to stock you need to make sure all the jetting is stock, including the OEM needle, 1174 or 1173, jetting is the key to smooth power delivery

I'm like you and ride slow technical trails so I want a smooth torquey bike, I also ride mine in the city and often meander to some local villages at the top of mountain wearing shorts and flip flops and no helmet so I want the experience to be user friendly
So in order of improvements
1.Get rid of that pipe and replace with stock pipe, do not buy aftermarket of any kind
2. Confirm the airbox lid and snorkel is in, this is critical for smooth low end power
3. The bike will need to be jetted perfectly but you will have to study your ass off and be an expert in jetting to get that RB carb sorted and even then there is a chance the emulsion tube is worn or corroded and you will never get it sorted

I do mostly tight, slow singletrack. My head is RB modded and had the rev pipe when I bought it, swapping to the woods pipe made a very noticeable difference in low end power/torque. I also am geared lower at 13/51. I can't speak to carb settings as I got tired of poor performance when my riding went from 5k to 11+ during a ride, so switched to a lectron. One thing to verify is that your silencer and packing are in good shape.

I would definitely recommend switching to the FMF Gnarly Woods torque pipe uf you want very exceptionally smooth and linear power. It doesn't have the top end rev like the Gnarly Desert, or the massive mid range and great top end power like the Pto Circuit Platinum 2 (which also has almost as much torque as the FMF Gnarly Woods as well as similar top end power or better than the Gnarly Desert (doesn't rev quite as high but makes as much peak potas the Gnarly Desert). You will be very pleased with that setup and proper jetting. Playing with the air screw settings and needle height once the jets are correct can make a MASSIVE difference in efficiency and power delivery. The Gnarly Woods pipe is very smooth, a trait which I miss when running tge Pto Circuit Platinum 2, but the Platinum 2 is the fastest and best all around pipe of the 4 options by far. The Desert pipe really is only good for wide open non technical areas unless there are a lot of other mods to boost low end power, pr else if you are slipping the clutch all the time like riding a 125 2-stroke.

I would ABSOLUTELY keep the RB head and RB carb, no flywheel weight, just add the Gnarly Woods pipe... Go 48T on the rear but I'd be hesitant to go any bigger. Some run a 50-51T, but that should be reserved exclusively for trials type riding. You'll be short shifting always and the bike will feel more like a tractor that a 2-stroke performance machine. Gear down only enough to crawl at above idle at the slowest typical ground speed upu might find yourself crawling through rocks at. The bike will always accelerate faster when the engine is allowed to do more work i.e. pushing a bigger gear stock 13/47 or a 14/47 or 13/45 etc will pull harder than a 13/48.

In summary:

#1 ***FMF GNARLY WOODS TORQUE PIPE***

#2 JETTING DIALED IN

#4 check ignition timing setting. There are 3 marks on the stator plate, one end is max retard for more top end rev at the expense of low end response and torque, the other is maximum advance for more torque but very rapid throttle response doen low, at the expense of top end power. In the middle is typically best, as the more advance for more low end torque setting also gives you explosive throttle response down low! Aftermarket ignition boxes (hand made by two members) like Jaguar's can give you more initial advance at the same middle setting of the stator plate while also giving you more high end timing retard for more top end power. These are great but $200+ and made to order.

#3 13/48 gearing or similar if you need to crawl through boulders etc at a standstill pace