Pop on decel... so what's changed?

[adouglas]

I've got popping on deceleration. Weird thing is, it just started happening over the last few hundred miles and I haven't done anything to the bike.

I found this:

http://www.ninjette.org/forums/showthread.php?t=70831

But the advice is to plug the kleen air system and I've already done that.

Bike rides fine otherwise. No serious bogging or anything like that. It just pops when I close the throttle.

The bike is jetted, has a full exhaust, K&N and has had the kleen air system plugged. Here are the full details, from my original post once I'd done the work:

- AreaP QuietCore full exhaust
- K&N panel filter
- Snorkel removed
- Kleen Air system removed and holes plugged
- FactoryPro jet kit set up as follows
- Stock main (#98 Keihin)
- #40 FactoryPro pilot jet (comes in the kit)
- Needles with clip in third groove from the top, plus three washers above
- Idle screws at present are 2 1/4 turns out. Kkim recommended 2 1/2, but the idle was dropping below its set point when the throttle got blipped. It still does it a bit, so I'll probably screw them in another quarter turn... but the bike runs just great so this is low priority.

HP and torque results, obtained on a DynoJet dynamometer (the dyno guy said his dyno is a bit "tight," so the real numbers may be marginally higher than these):

HP: 28.91
Max torque 14.37

[Alex]

Check for an exhaust leak.

[adouglas]

Originally Posted by Alex

Check for an exhaust leak.

Now that you mention it, I did notice something louder than normal in the vicinity of the engine a while back. I did feel around and didn't notice anything at the time, but I'll check again.

Any other possibilities? Could one of my adjustments have gone amiss?

[Alex]

Maybe. The decel popping isn't a big deal in and of itself, but if it wasn't doing it at all, and now is, clearly something changed.

[adouglas]

Resurrecting the thread now that I've pulled the bike out of storage...

The past couple of days have been very warm here in the Northeast US, so it was time to take the bike out of the workshop and fire it up (no problems, thanks to proper storage. I heart green Sta-Bil and Battery Tenders).

Rode to work and took the opportunity at lunch to have a good, long look. Sure enough, it looks like the muffler has shifted back enough to allow a small leak.

What clued me in was examination of the exhaust pipe where it meets up with the muffler slip pipe. I could see marks where the slip pipe had been tightened onto the exaust pipe and they were well forward of the muffler's current location... like two inches. I don't know if that's where I had it or where it was originally... I bought the exhaust used.

The slip pipe has slots cut in it to allow clamping. Closer examination showed a small hole at the end of one of these slots, with a little bit of soot around it. Bingo. Leak.

Will remount the can Saturday and report back. Thanks, Alex!

[Alex]

Cool!

[adouglas]

Just rode to work for the first time after remounting the can. Yep... that was it. I misspoke earlier. It was displaced only an inch or so... just enough to open up that little gap.

Now that I'm more attuned to it, I can hear a little pop inside the muffler on deceleration. Some soot in the end of the exhaust pipe too. I'm thinking the bike is probably running a tad rich. I should probably have a look at the plugs.

Ah, screw it. It runs fine so I'll mess with things only if I've got nothing better to do this summer.

[Alex]

Originally Posted by adouglas

Ah, screw it. It runs fine so I'll mess with things only if I've got nothing better to do this summer.

Sounds about right.

[LoD575]

Originally Posted by adouglas

Ah, screw it. It runs fine so I'll mess with things only if I've got nothing better to do this summer.

I would rather run slightly rich than lean.

[headshrink]

I pretty much have the same set-up as you, (Area P QC, no snorkel but stock filter, electronically advanced timing, and the FP jet kit, but find that combination of jetting isn't even ballpark (lean). I'm currently at #40 pilot, 3.5 turns, 3rd clip+2 washers (=4th), and #105 mains. I'm feel I'm still a tad lean on the low end.... I was just really surprised to see you with the stock #98 main. I know FP has the idea the main should be smaller on our bikes, but this doesn't seems to jive with my trials, or others in the jetting db. Are you certain your main is right?

[alex.s]

IIRC @Nemesis agrees 105 is too big.

[headshrink]

Originally Posted by alex.s

IIRC @Nemesis agrees 105 is too big.

Hmmm... I still am working on getting that low end surge out around 1/4 throttle. It isn't bad now, but I detect it occasionally, and it doesn't exactly jump off the line. I would have to check my notes, but IIRC the 105 mains seems stronger than the 2 sizes below, although I have to say this is mostly felt 1/2-3/4 throttle, because I don't usually twist it all the way (maybe because I haven't gotten it right yet). Top end feels good, but could be better. I would drop the main down again, just to try, but I'm afraid I would loose too much in the middle since my needle is raised to high.

[Nemesis]

Originally Posted by headshrink

Hmmm... I still am working on getting that low end surge out around 1/4 throttle. It isn't bad now, but I detect it occasionally, and it doesn't exactly jump off the line. I would have to check my notes, but IIRC the 105 mains seems stronger than the 2 sizes below, although I have to say this is mostly felt 1/2-3/4 throttle, because I don't usually twist it all the way (maybe because I haven't gotten it right yet). Top end feels good, but could be better. I would drop the main down again, just to try, but I'm afraid I would loose too much in the middle since my needle is raised to high.

Got butt dyno?

[headshrink]

Originally Posted by Nemesis

Got butt dyno?

Yup, that's how I do it. AKA "poor man's dyno."

[Nemesis]

Originally Posted by headshrink

Yup, that's how I do it. AKA "poor man's dyno."

[adouglas]

Originally Posted by headshrink

I pretty much have the same set-up as you, (Area P QC, no snorkel but stock filter, electronically advanced timing, and the FP jet kit, but find that combination of jetting isn't even ballpark (lean). I'm currently at #40 pilot, 3.5 turns, 3rd clip+2 washers (=4th), and #105 mains. I'm feel I'm still a tad lean on the low end.... I was just really surprised to see you with the stock #98 main. I know FP has the idea the main should be smaller on our bikes, but this doesn't seems to jive with my trials, or others in the jetting db. Are you certain your main is right?

Yep, sure on the sizes. I immortalized everything here right after I did it so that it wouldn't get lost in my rattletrap memory.

Could your altered timing have anything to do with the way it's running for you?

[headshrink]

Originally Posted by adouglas

....Could your altered timing have anything to do with the way it's running for you?

Yes, it did affect it somewhat, but even before I added the BRT-TIS, I was using the #105 main. In fact, it is only after adding the BRT that I needed to move from the #38 to the #40 pilot... go figure.

[adouglas]

Okay, so I just had one of those "geez, I'm dumb" moments.

I've been reading up a bit on all the various things that go into making the carb run properly (main jet, pilot jet, needle position, mixture screws).

http://www.gadgetjq.com/keihin_carb.htm

I had a small epiphany. I've always seen the word "idle" and automatically associated it with the engine idling... ticking over at low RPM with no load... without giving it much thought. I'm rethinking that based on what I've read.

Have a look at the attached diagram. According to the chart , at low throttle settings the pilot jet and pilot screw (idle mixture screw) are both a big part of what governs the fuel flow.

Note the way the horizontal axis is labeled. It's about throttle position, not RPM. So the idle mixture screw and pilot jets affect mixture regardless of whether the engine is spinning fast or not, right? Now I realize that "idle" in this context means a closed throttle, not low rpm.

Where I get the popping is when I close the throttle to decelerate or shift... i.e., at idle or low THROTTLE, not idle or low ENGINE SPEED. Different things.

So, leaning out (screwing in, IIRC) the idle mixture screws should reduce the popping. Right?

(Fact is, I'm just fishing for a way to not hassle with swapping the jets if I can avoid it. )

[headshrink]

Random thoughts to rule out as well: I know you looked into an exhaust leak, and although I'm sure you probably looked for vacuum leaks too, it is always worth another look. Also, since you took the kleen-air junk off, double-check that the valve cover is in fact sealed where you disconnected the kleen-air. Carb boots too.... OK, that was just a rule-out exercise, but what I was really thinking about is what are the condition of your slides, and do they move freely. I may be reaching here, but it seems when we can't find the answer it is often that random thing we didn't think would be an issue. Is your state switching fuel formulas right about now? That could be an unknown too. I have the sense this stuff may not be that helpful, but thought I would let my brain vomit in case that one stray thought led to something. Carbs suck!

[adouglas]

Since the only thing it does that isn't absolutely great is go POP - pop - pop on closed-throttle deceleration (and even that's not really a problem per se), I'm inclined to believe that everything's just fine elsewhere.

[ninja250r81]

go for a ride for about 20 mins.

for lean idle.

as its still idling adjust the idle screw until it slightly raises the engine speed. then come back 1/8th of a turn.

for rich.

as its still idling adjust the idle screw until it slightly lowers the engine speed. then come back 1/8th of a turn.

[DaBlue1]

@ adouglas, your thinking, I believe is correct. This chart shows even better how much each circuit has an affect.



From Motorcycle Carburetor Theory 101

[headshrink]

What us the throttle valve cutaway; is it the same as the Float valve?

[DaBlue1]

Originally Posted by headshrink

What is the throttle valve cutaway; is it the same as the Float valve?

Nope. That's the carb slides or the long bottom part of the diaphragm.

Link to original page on YouTube.