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Airbox backfire

6K views 16 replies 6 participants last post by  Batan 
#1 · (Edited)
I have noticed some firing through the airbox during the last 2-3 rides. It happens off idle while revving the bike a bit at a stop light or in more rare cases right off idle when pulling from a stop.
I had a short ride today to pick something up, 10-15min there and the same way back. The bike sat for about 15 minutes before I got on it to come back. On the way back it didn't do it a single time but on the way there it happened 3-4 times. I could almost get it to do it on demand by repeatedly giving it a bit of gas thus bouncing the revs(but not revving high, just getting it off idle).

I've read others have posted about this, but it seems that people live with it. Today seemed a bit much though. It could probably cause one to drop the bike if it happened at the wrong moment as the throttle tends to "fall through". Any ideas? First that came to my mind is coolant temperature sensor(if Vs have them) if something is indeed not quite right.

The temperature today is 15C/59F and the bike has about 8500 miles on it.
 
#4 ·
I assume you haven't adjusted your main throttle sensor to prevent your lean miss and backfire, particularly near 2800 rpm at low throttle opening variations... Rotating your main throttle sensor counterclockwise by about 1/32" at its green reference mark to have its voltage output within spec at wide open throttle and slightly over spec at idle position adds sufficient fuel at low throttle opening levels, which also greatly improves driveability.

Is it all stock, exhaust and all? What mileage are you at? Are you still on original spark plugs? Have you checked your throttle body vacuum synchronization and your valve clearances?
 
#5 · (Edited)
The bike is stock. Haven't played with any of it yet but I do want
to do the throttle mod, I hate the low speed herk-jerk! Lol 8500 miles.

I should mention that the idles at ~1100rpm. Maybe I should try bumping it up to 1300. The definitely takes off nicer when the fast idle is still in effect.
 
#8 · (Edited)
You're also due for new spark plugs and a throttle body vacuum synchronization. You may also find that some valve clearances are below spec range, particularly the exhaust valves...

Look at the main (grey) throttle sensor's green alignment mark on its front from near your front left flasher, and note how it ligns up. Loosen sensor screw with T25 Torx tool. Rotate sensor counterclockwise by almost a millimeter (~1/32") as seen on pic. Adjust, then tighten Torx screw.

Main throttle sensor's new position:



Main throttle sensor Torx screw:



Fast idle may then be higher. You can rotate subthrottle sensor (black, just above main throttle sensor) also counterclockwise with T25 Torx screw to reduce cold fast-idle rpm. I rotated mine to its maximum counterclockwise position, to which it was already near. Subthrottle sensor's new position:

 
#9 · (Edited)
Excellent instructions, thanks! Does the tank have to be off to adjust the sensors? Or just the fairing?
I will have to see if I have any info on what was serviced on the bike to see if the throttle synch has been done and spark plugs. For some reason I think they were.

EDIT: Just checked the papers, no sign of the vacuum synch nor spark plugs so yes, it's due dammit. Maybe it's time to buy a vacuum gauge.
 
#13 ·
So, I did the throttle sensor mod tonight. It was pouring out so I didn't really feel like going for a ride so a real test is yet to be conducted. I did take it out the garage and as far as I can tell, I noticed no difference in cold or warm idle. I quickly and repeatedly snapped the throttle very slightly open and let it go to return to the original position to see if I could get it to pop through the airbox and it did do it once out of some 15 times I did that. Anyway, we will see if I notice any low speed riding improvement with the sensor mod. It would be nice!
Original primary sensor position:


New position. I think the angle might make it look like I moved it more then I did:


The secondary sensor is already maxed out counterclockwise.
 
#14 ·
Original primary sensor position:


New position. I think the angle might make it look like I moved it more then I did:


The secondary sensor is already maxed out counterclockwise.
Your "Original primary sensor position" photo is my own photo of my main throttle sensor's position AFTER I adjusted it... It does look like you may have rotated yours a bit more than optimal.
Your secondary throttle sensor was originally maxed out counterclockwise?
 
#17 ·
Thanks for asking! Been busy but finally took it out today for a proper test ride after the TPS adjustment.
Now, when it comes to that mod, I knew by reading from others said that it would be better. But it is soooo much smoother in the lower RPM range no! The shifting up is better, the downshifting is much nicer then before and especially getting off and on the gas at low speeds(such as turning a street corner), no jerking anymore, smooth as butter! And rev-matching is so nice now on a downshift because the throttle responds quicker when being "tapped". I was jealous of my wife's SV for its fueling and me being able to downshift to first by matching the revs so nicely, but now the V seems to do it just fine as well.

When it comes to that particular issue , it did it only once today in couple of hours of riding when I was rolling around the corner to pull into the garage. But it is much more faint now and it feels like a slight skip of a beat or a spit, rather then a pop like before.
 
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