popping on deceleration

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debtman7 · #1 Unread post by **debtman7** » Sun Mar 29, 2009 10:35 am

Well, after tearing down the carbs and cleaning them out (admittedly, mostly focused on the jets and bowls, didn't do a full tear down), my 01 duacti monster runs much much better. No more stuttering, hestiation, stalling. Idles great, smooth throttle transitions, etc.

Except... I'm getting some popping/mild backfiring on deceleration. It's especially prevalent while coasting down a steep hill. The bike has an aftermarket exhaust, but it's had that the entire time I've owned it and I don't recall this happening before. Other than that it runs perfectly.

I tried turning the mixture screws out a half turn but that made no noticeable difference. Did I likely miss something in my carb cleaning? Perhaps something a bit gummed up still? Or maybe something else is wrong?

#2 Unread post by **Wrider** » Sun Mar 29, 2009 10:52 am

Eh it just means you're running a little rich. Nothing to be concerned about honestly. What it's doing is taking the excess fuel that isn't getting combusted in the chamber and as it goes into the exhaust pipe and finds fresh air, it's getting hot enough to combust, making that popping sound.

debtman7 · #3 Unread post by **debtman7** » Sun Mar 29, 2009 10:58 am

Wrider wrote:Eh it just means you're running a little rich. Nothing to be concerned about honestly. What it's doing is taking the excess fuel that isn't getting combusted in the chamber and as it goes into the exhaust pipe and finds fresh air, it's getting hot enough to combust, making that popping sound.

Kind of what I was thinking, although most things I read say it's more a sign of it running lean. I'll play with the mixture a bit. I'm just a little concerned because I don't recall it doing this much (although it's always been a bit burbly on deceleration). That may be due entirely to the fact that previously I lived in Ohio and almost never went down a hill, especially a steep one, and I'm now in an area of steep hills.

But it idles great unlike before the carb cleaning so I wouldn't think the idle jets are still clogged up. I'm guessing that going down a steep hill (we're talking like 40 degrees here) is just causing a strong vacuum pull and the idle jet doesn't supply enough fuel for proper combustion, combined with the less restrictive exhaust that's probably it.

debtman7 · #4 Unread post by **debtman7** » Sun Mar 29, 2009 1:16 pm

Well, after examining the bike, I'm kind of thinking that perhaps the 1/8" gap between the exhaust header and engine, covered with black/blue soot might be the problem... Hmmm, think so?

Look at this baby:

Yeah, I'm betting that's an exhaust leak right there, how the heck did I miss that?

So, um, can I just tighten that down? Or is there a gasket or something in there I need to replace? And those bolts are a bit rusty, should they be replaced?

#5 Unread post by **Wrider** » Sun Mar 29, 2009 2:15 pm

Well honestly that can mean lean or rich, depending on how it's popping like that.
And from that picture I wouldn't say that's an exhaust leak, looks more like just plain heat to me.
How much out did you turn your air/fuel mix screws?
Most should be set at around 2.5 turns out from a light seating.
Oh and where are you at? 40 degrees is really steep!

debtman7 · #6 Unread post by **debtman7** » Sun Mar 29, 2009 2:32 pm

I dunno, the dirty plus the gap between the flange and engine seems like a good sign of a leak to me, I'll have to fire it up and see if I can feel it but I'm betting that's it.

Idle mix was at 3.5 turns out, I moved it out to 4 and didn't really notice any difference. I'm going to take those bolts out and make sure they're not stripped, probably get some replacements since they look rusty, and tighten that pipe down. I need to pull the plugs too and see how they look. One thing I didn't think of was to put the choke on while coasting down a hill and see if that would fix it, if it is indeed running lean that would be a good indicator.

debtman7 · #7 Unread post by **debtman7** » Mon Mar 30, 2009 4:56 pm

Well I think that's right, I can't find any leaks around there, I would have thought that should be tight but what do I know

I did however find a pretty good leak where the aftermarket exhaust pipe fits to the stock pipes. I got my squirt bottle out and sprayed all over and at that joint it spit water back and scalded my hand. I also noticed some exhaust fumes coming out of there. That would do it...

Can I just try and tighten down the clamps on that? Or should I pull it and put a bead of high temp copper RTV on there?

#8 Unread post by **Wrider** » Mon Mar 30, 2009 7:16 pm

Ah that could be doing it!
Yeah tightening down the clamps ought to do it. If it still doesn't work you might want to take some heat wrap and use it as kind of a teflon tape for the fit.

ofblong · #9 Unread post by **ofblong** » Mon Mar 30, 2009 10:44 pm

good job on fingering it out

. Yeah that gap is there on just about every bike exhaust I know of. I know mine has it.

jonnythan · Tue Mar 31, 2009 7:31 am

debtman7 wrote: I'm guessing that going down a steep hill (we're talking like 40 degrees here)

People always greatly, greatly, greatly overestimate how steep slopes are!

40 degrees? I don't think so