Hey guys....

      I just got home from a long ride (took the long-LONG way home from work  ), and noticed my bike do something strange (It's an '01 s4 running an open airbox...) ...I was downshifting from 4th to 2nd, and was in that "in-between" neutral throttle position in 2nd; ie: not accelerating and not engine-braking, when i heard what sounded like a "sneeze +pop" that came from, I'm almost positive, the velocity stack(s).. It was accompanied by a split-second loss of power...

     The loss of power/sneeze  lasted all of 2 milliseconds, and the bike was running beautifully otherwise!! Could it be ECU or injector related I wonder?? It almost sounded like a backfire (but not as loud) accompanied by a split-second hiss....I am fairly positive it came from the airbox area, and to be honest I'm not entirely worried about it as the bike is fine otherwise, but it would be nice to know...

    Any opinions are greatly appreciated!!  (that means you, Speeddog!!      c'mon, you're an S4 guy!!!  )

If anybody nails down the cause of this, I'd love to know.  I've had this issue on and off since my bike was new.  I've heard various theories, but careful valve adjustments and a custom map didn't eliminate the problem for me.  It doesn't happen often, but it's always between 3-4k with the throttle barely open.

Annoying, isn't it? 

My bike does it occasionally as well.
Never consistently enough to diagnose.

It is indeed a backfire, up through the intake.
Hence the weird click/pop/cough sound from the vicinity of the airbox.
Some folks feel the throttle go slack, as the backfire is opening the butterfly in the throttle body.

My original ECU developed this symptom to a very drastic level, along with running very crappy at low throttle.
Eventually it got so bad that it would idle OK when it was cold, but very shortly it would start the backfiring at idle and stall.
I swapped sparkplugs between cylinders.
I swapped coils.
I swapped injectors.
I even swapped the leads going to the injectors ( It would run that way, but sounded a bit sick, never rode it that way).
Through all that, it always malfunctioned on the same cylinder, vertical IIRC.
My conclusion was that the ignition circuit for the vertical cylinder in the ECU was wounded.
I couldn't swap the leads to the coils, as then the ignition events would be totally mistimed and it for sure wouldn't run.

I replaced it with an FIM U59, and the bike ran lots better in general.
But still it does it occasionally.

Perhaps it's a software glitch, brought on by a particular set of readings from all the sensors.

I wish I knew. 

Sounds like a fuel map issue? You can't do much if you don't have a power commander, but they are fun, so go buy one anyway!

My bike used to do that after I altered some exhaust/intake stuff. I made a mental note of my RPM and throttle position and adjusted my map at that location on the grid. It really works. Obviously my map is not professional, but it is "custom!" It's to the point now where I have zero problems with backfiring or poor idle, but I have a nagging suspicion that overall a professional tune could improve it a lot.

im with speeddog and calling it a variety of conditions working together to cause this symptom. 

mine has always done this, and i've done various things to the bike over time to reduce the effect.  now it just makes the sound but i don't feel anything.

what i did to resolve:
PCiii with custom map
tweaks (personal) to map
TB sync and tune
more tweaks to map
cam timing re-set (was off by four degrees between the two cylinders!!)
more tweaks to map

i plan on doing a final custom map to get to a better point now that the engine is set up better.

Most of what you said was just fuel map stuff, except for this:

Quote from: Ash on May 07, 2008, 05:25:50 PM
cam timing re-set (was off by four degrees between the two cylinders!!)

What? Cam timing can get off all by itself?  Really? Now I'm scared!

Quote from: Alex on May 07, 2008, 05:27:26 PM
---------snip-------------

What? Cam timing can get off all by itself?  Really? Now I'm scared!

No, it's just not always set accurately at the factory.

it can be improperly set from the factory.  i believe that's one of the factors why some folks report no problems with their bikes since new while others have nothing but problems.  

Quote from: Speeddog on May 07, 2008, 05:35:58 PM
No, it's just not always set accurately at the factory.

there ya go

Quote from: Alex on May 07, 2008, 05:12:49 PM
I made a mental note of my RPM and throttle position and adjusted my map at that location on the grid.

I tried the same thing and never have been able to eliminate the problem.  Like Speeddog mentioned, it's hard to diagnose because the problem is intermittent.  Did you end up adding or removing fuel?

Quote from: Ash on May 07, 2008, 05:25:50 PM
TB sync and tune

Did you do this yourself?  What does "tune" mean in this case?

"Tune " can be setting the CO with a gas analizer and software to adjust the ecu. It can be setting the TPS to factory specifications or a setting that works better for your bike.
adjusting the air bleed screws.
It could also involve checking cam timing with "factory" tools or even setting cam timing by lobe centers. Many levels of tune, not everyone needs the full cam degreeing but if there is a problem it can be something to look at.

a backfire in the airbox is a lean condition, at the throttle settings you are describing you might try a higher tps setting. if you have a power comander get a custom map done after getting your fuel injection dialed in. there should not be a intake backfire. I have seen a 998 melt its injectors doing so.

Quote from: Speeddog on May 07, 2008, 04:51:18 PM
Annoying, isn't it? 

My bike does it occasionally as well.
Never consistently enough to diagnose.

Wow, OK....So I'm not alone, which I guess in some twisted way is a good thing....

Like I said, I'm really not overly concerned about it, the bike runs great otherwise....Thinking back, it was in the 3-3500'ish range w/cracked throttle, not during throttle on or off....

Must just be "one of those things"...My father used to work on motorcycles (albeit old carbys) but he said it may just be exactly what I described....and engine "burp" and nothing more....I guess I'll just hope it doesn't start doing it constantly and just live with it for now...

**edit** I really am considering throwing it on a Dyno soon though, just to get a visual idea of my engine's habits @ differnt RPM's...

-Thanks for the feedback!!
wt:
-john

Glad we were able to lure ducvet into the discussion. 

Ducvet, by 'higher TPS setting', you mean butterflies open further, or TPS body re-indexed to mimic the butterflies open further?

Speeddog
option B
the tps will richen the mixture and it will be most noticeable at low throttle settings. you do not want to go too high as it will also start to effect the timing.
Small increases can make a big difference. also watch out for the sync to change your tps settings so always set sync first.

Quote from: ducvet on May 07, 2008, 06:48:53 PM
Speeddog
option B
the tps will richen the mixture and it will be most noticeable at low throttle settings. you do not want to go too high as it will also start to effect the timing.
Small increases can make a big difference. also watch out for the sync to change your tps settings so always set sync first.

Thanks, that's clear.

The first time I synced my TB's, the sync on the vacuum was good, and the idle speed was good, but I had the butterflies nearly closed and the air bleeds open a mile.
It ran like crap that way.
Got ahold of the equipment necessary to read the TPS voltage, and realized my error.
Ended up with the butterflies open a good bit further, and air bleeds nearly closed.
Ran *much* better that way.
but I never moved the TPS itself.

Any benefit in having more air coming past the butterfly than the bleeds, or vice-versa, given a constant TPS voltage?

Or should we take this to PMs to avoid giving away all our secrets?  ;:|