As of late I have been playing around "down there" and my bike is running great. However...I shaved the spacer to 1/2, installed the straight pipe, and rejetted to a 152.5 all with no problem. Then went and drilled holes in the air box cover. ( big mistake ) Man I never heard so many backfires in my life. Took all of about five minuets to pull it off again and tape up the holes. She's a might bit more satisfied now. But...It still backfires rolling off the throttle. Mostly when changing to 2nd, but so far not at shut down.

Here is what I'm thinking.  

1. I should jet higher. Maybe 155, though I am curious why rob is running a 300

2. I should cut the spacer to 1/3 or 1/4. Some others here went that route but I'm not sure who.

3.I could have an exhuast leak in the pipe connection.

And it also seems that my top speed may have dropped. I havent really tried to push it, just sounded like I was at the top of the revs early. But I get there quicker with much more power.  

Any thoughts?   ???

As an example of how little it takes to make things lean up, I have been contemplating sticking in a #55 pilot jet as I still have a touch of backfiring.  Then yesterday, I got to thinking about what all I had done to the bike and decided to try a couple of things before doing the jet.

I had desnorkeled when I put my K&N filter in, so I decided to reinstall the snorkel and see what happened, with the premise being I would be restricting air flow, thus increasing the richness of the mixture.  When I did that it cut the backfiring down considerably to almost nil.  Now, I think that with just some tweaking of the idle mixture when I get the chance, I can cut that all the way out and not have to rejet the pilot circuit.

Had this not been as effective as it appears to currently be, my next step was to desnorkel again and reinstall the OEM air filter, knowing that it is way more restrictive but add a bit more air without the snorkel.  But I don't believe I'll have to do that.

I even have (had) a plan to slowly trim say a 1/4" at a time off the bottom of the snorkel to slowly increase the air to counter things if I got too rich, but that too is probably going to be unnecessary.

So, that said, little things such as drilling holes in the air box can make a big difference in the overall fuel/air mixture.

Stock pilot jet is most likely a #52.5
Go up one size, (increments of 2.5)
I've not read of anyone going any higher than #55
My 05 S40 is suffering from the exact same symptoms.
From what I've read, popping and backfiring are to be expected from a finely tuned Mikuni carb with extremely open exhaust, but sometimes it's a bit much. I'm also suspecting a lean pilot circuit, but when I went to the stealers, they only stocked increments of ten, and wanted to sell me a jet kit. No thanks, just the jet please.
I'll try someplace else tomorrow.....

My local stealership doesn't stock em either. They told me they come in packs of 4 and they could order them if I bought them all. $20.00 for the four of them. I've got $5.00 for anyone who has an extra and I'ld also happily kick in the 40 whatever cents it is to stick it in the mail vs. paying for 3 extras.

Steelwolf,
I have a  55 pilot. Dennis Kirk sells them in two's. I'm running a 883/1200 sporster muff and it doesn't backfire on shutdown at all. It pops and spits a little on hard decel, but I think that is fairly normal.
Let me know if you want it. How bowt' 3 bucks and we'll call it good.

got it, check yours

Not to be childish.

But, YOUR IT! I QUIT!