I'm having a similar problem which I brought up in a previous thread and I have a similar setup as you (Dynojet 2152 but with a KLX300 exhaust and cylinder)

https://www.kawasakiforums.com/forum...arb-acv-38821/

In my case I'm talking only about popping/backfiring at high RPMs, throttle fully closed, going downhill. I'm now thinking this is due to a lean condition - but only in that one condition. It could still be rich at other throttle openings or even sitting at idle. I got it to stop (mostly) by putting the 300 exhaust final baffle back in. This would tend to make things run richer, but restricting the exhaust undoes my whole reason for replacing the stock exhaust so it's not a final fix for me.

My point in the other thread is that the KLX300R, the DRZ400S, and a bunch of other bikes use CV carbs with air cutoff valves (coast enricher). I believe the ACVs make the mix richer in that one throttle closed condition and they're designed in for a reason so it must be a lean problem. (Also, I'm wondering if drilling the slide - as suggested by Sharan -would make the slide sit higher for the same RPMS making the needle let more gas in, again richening up the mixture. I havent worked that one out in my head though.) I'm trying to get my hands on a 300R carb with the ACV and see if that fixes my problem. I assume non-CV carbs dont have this problem either.

 

I sometimes wonder how effective the coating enricher actually is. I had 4 KLR600/650's with aftermarket pipes, DJ kits, an opened airboxes. They all had some decel popping. It seemed that all the mods to produce more power yielded some decel popping even though all these KLRs had the coasting enricher. I had 3 different Kawasaki Z1 900-1015cc models with 4-into-1 pipes, and they all had a little decel popping. There must be something akin to a scavenging effect or something along those lines that isn't present in a stock bike setup.

 

that's pretty much how mine ran...only had popping when downshifting/engine braking hard..or, coasting down a steep hill in gear....wasn't bad with the fmf q4, but after switching to the hmf, it was like machine gun fire popping on downhills and cannonballs being launched during engine braking...minor adjustment and it was better, but the popping with still loud with the hmf....spark plugs were normal and the klx ran great for 10,000+ miles, so i figured some popping was normal...later, added a fmf megabomb and it eliminated almost all my popping...and, megabomb makes idle and warm up whisper quiet even with the hmf exhaust....i'm sure the fmf q4 and megabomb would be the ultimate quiet full exhaust set up...

 

Now that the weather is getting a little cooler, I have noticed that mine pops a lot less on cool mornings than it does in the afternoons when it's warmer. Still pops like crazy though, but only when decellerating pretty hard (down-shifting or slamming the throttle closed fast). If I keep the throttle cracked a tiny bit, it seems to stop the pop.

 

^^^
Yep, that's exactly what mine does - pops and gurgles on abrupt throttle close (downshifting or coasting down the hill at speed but in lower gear to engine brake). Only tiny crack of throttle and it all disappears. With snorkel removed, the throttle had to be opened a little bit more to have the same effect.

Somehow I think the problem lies to the air intake area. Popping coasting down the hill I understand, my previous bike did it and I can do it with my manual transmission car, but popping like mad when changing gears is annoying.

I live in the area where a lot of dirt bikers congregate to hit the trails, some of them thumpers are so loud that I can hear them for miles - never heard any of them bikes popping like that.

Here's my jetting setup which basically follows the setup couple of Aussie inmates posted here and on ADV:

Keihin #138 main, #38 pilot, N1RX stock needle raised 1.5mm, pilot screw 2.5 turns out, Uni O2Rush air filter, snorkel out, Staintune slip-on. Slide hole - I don't know the diameter (if any).

 

Cool, thanks for all the responses. I had a feeling it was normal to have little popping especially coasting downhill. I will pull my plug and see how she's running probably this weekend. Hopefully I get the jetting dialed in.