My understanding of this is being lean on the idle circuit. Well I am stumped. I ride at 5000ft and have done my own jetting gymnastics (see below). I have about 5hrs on a new machine so I am still breaking in. It runs great (have not run it wide open yet), but the low rpm decelleration popping is annoying.

Case 1 - Stock with stock jetting (#4 needle, 42 pilot, 158 main, and 1.5turns out on the idle screw) out of the crate it had popping and a bit rough of an idle.

Case 2 - Intalled Pro Flow with Foam Filter and pulled the aibox lid. Went to #5 on stock needle, 2 3/4 turns out, 45 pilot and 170 main. This turned out to be too rich at 0 - 1/4 throttle per some minor sputtering and a black electrode. Still had popping, idle was smoother.

Case 3 - Put the 42 pilot back in and a 165 main. Left the idle screw and needle position the same. Sputtering was gone, but the plug was still black, and the popping was still present. Bike was running great otherwise.

Case 4 - Dropped the needle to #4 and set the idle screw to 2 turns out and left the 165 in. Popping still present, plug lighened up a bit, and the bike is running great. The idle is a little rough though.

I am wondering if I should go back to the 45 pilot and turn in the idle screw to about 1.5 where it was stock or perhaps it is some kind of timing issue (hope not). The bike runs great but this low rpm popping is annoying and I know it can be fixed. It sounds like this has been a common problem with the YFZ's. Has anyone had a similar problem? If so what was the solution? Thanks for the feedback.

 

Case 5: 45 pilot, 1.5 turns out on the screw and no popping, everything else the same. Evidentally the air screw is the modulator in the idle circuit.

 

so you think its just the air/fuel screw and it was out to far or what, i'm hearing a popping on deceleration on my yfz you'll se my jetting specs in sig. and low rpms it'll sputter kinda or pop dunno what you would consider it... lmk what you think

 

With the yoshi pipe on it, you might noot be able to rid it completely of the pop.
Youre correct that a lean idle circuit can cause popping. But if the idle circuit was too lean, youd have hesitation and maybe an intake pop,when getting on the throttle.
You might just have to live with it.
Some other things to check on...intake leaks, exhaust leaks.
Its posible that the next size up, on the pilot jet, might help out, and then readjust the mixture screw. The pilot jet is the coarse adjustment, and the mixture screw is the fine adjustment to the jet.
Im not sure how posible this next idea is, but is the throttle position sensor, disconnected or out of adjustment?
Heres a link to the mikuni carb website, and they have this little article on exhaust popping.

mikuni

Heres a copy of what it says.

4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
5) Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

 

I have a similar problem with my 05 yfz 450. I put a motoworks pipe on it. it made around a 15 hp gain though along with air box lid off and quickshot acccelerator pump, and exhaust cam retarded 1 tooth. However i got it dynoed and jetted down in oklahoma at the dunes. seemed to run good there. now back home in iowa..(very similar elevation) and it seems to be poppin on the decel. no real power loss that i can tell. does idle a lil rougher then i recall though. pipe is glowin red at nite which i am used to by now. i did alot of the jetting myself..then they tuned it on a dyno but i belive im runnin a 165 main. and stock pilot. any suggestions or should i just live with the emberessing pops and crackles..still has a nice sound when im gettin in it. maybe i should just never let off the throttle?

 

I don't know about you guys but mine seems real temperamental. I'm running a 165 main also with k&N and no lid with graydon proline exhaust. Some days I'll have more pop on deceleration than others. I think a little is expected.