Backfiring…??

[SpankMe]

My SV650s let out a huge bang once when starting it for the first time after I left it for a month while in Thailand.

My SV1000s started constantly poping loudly while decelerating after a service once until I took it back to Colemans who fiddled with the fuel injection and now it’s at a more acceptable level.

Now my GB600 has for the third time made everyone within earshot jump when it backfired while starting. And it wasn't the first start of the day, but after it had been ridden for awhile. It seems to happen when it doesn’t kick over first time (thumb start, not kick start) and I have to give it a couple of goes.

What causes backfiring and does it damage the bike?

[scumdog]

The back-firing when you are attemting to start the bike is very likely caused by an over-rich mixture not getting fired in the cylinder then finding its way into the exhaust where it subsequently ignites.
Could be bad ignition that is not firing when it should i.e. weak spark/bad timing.

H-Ds are bad for it 'cos the ignition 'sparks' each cylinder at the same time - and obviously one is on the exhaust stroke, add a rich unfired mixture to the exhaust and viola', one shotgun blast back-fire.

The other sort of backfiring? meh, more than this kid can tell you, 'specially if it involves fuel injection.

[onearmedbandit]

Unburnt fuel making it into the exhaust and exploding, well at least that's what was happening with my carb'ed gixxer when it was flooded. Doesn't do any harm to the bike as such, however may damage the baffles in the exhaust. If it's backfiring while riding, probably related to valve timing. But I'd be going on excessive fuel making it through to the exhaust, where hot gases from the motor cause the backfire.

[mikey]

were you spraying degreaser an crc around carbs? certainly caused mine to back fire!

[vifferman]

Just to turn things on their ear - popping on deceleration and backfiring are often caused by a lean mixture. This can be caused by fitting a less restrictive airfilter or zorst, or leaks in either the intake manifold (more air getting in) or zorst system (changes the back-pressure in the pipe).

"How's that?!?" you say. (Wotchootalkingabout, Vifff?!?)
Well, some fuel mixture always finds its way into the zorst (more so, if the valve timing is out, or the zorst valves leaking). However, if the bike is running a bit lean, this can cause the exhaust valves and/or port to get rather hot, and when the mixture finds its way into the exhaust port on valve overlap, it ignites there.
Furthermore, if the mixture is overly lean it will make it harder for the bike to start when cold, so more unburnt mixture finds its way there before it finally fires up.
Don't believe this crazy old coot? Try googling "motorcycle carb tuning backfiring" or summat.

[Silage]

I think I have commented before that my GB400 backfires (rifle shot more than shotgun) if I completely throttle off coming toward a stop or corner. It only hapens once the bike is hot. If I leave a tiny amount of throttle on, it is OK. It appears to me that when throttled right off, the mix is too lean to fire on compression and so unburnt fuel gets into the muffler (which is poorly named in this case) and the report is enough for me to almost loose it. I haven't hung around to see the reacton of others.

I have been expecting to see the gutz of the muffler spread around the neighbourhood, but not so far. Not bad after 82,000 km. Had the bike retuned etc but had no effect.

[inlinefour]

that back firing is not good for valves on the head of an engine. I don't know this for a fact, but as everyone has stated above, a tune up should get it running sweet.

[sels1]

Just to turn things on their ear - popping on deceleration and backfiring are often caused by a lean mixture

Yeah, my bike backfires on deceleation sometimes, normally when going down long hill bits - so I just tweak the choke on a bit to thicken the brew - seems to work

[thehollowmen]

Question:

BP 98?
I'm begining to wonder because mine did it too for a while when I switched from 91 to 98. It should sort itself out once it re-maps all the fuel stuff... shouldn't it?

[scumdog]

Question:

BP 98?
I'm begining to wonder because mine did it too for a while when I switched from 91 to 98. It should sort itself out once it re-maps all the fuel stuff... shouldn't it?

I dunno, but anyway you're lucky to have the coice, - down here in hicksville they have no intentions to have 98!!
And it's what my F100 needs

[vifferman]

Question:

BP 98?
I'm begining to wonder because mine did it too for a while when I switched from 91 to 98. It should sort itself out once it re-maps all the fuel stuff... shouldn't it?

You got EFI?
I don't notice any difference in performance between 91 and 98, except 98 takes more cranking to get started, and 91 pinks more when the engine is very hot. Even though the VifFerraRi runs extremely (ridiculously) lean at low revs, it doesn't backfire or pop, probably because it's got O2 sensors in the zorsts. If it had knock sensors as well, I'd just run it on 91 all the time, but that's about the only sensor it doesn't have.

[Ixion]

K series BMWs backfire ferociously all the time. 'Tis a trademark as it were. Apparently does no harm, except to the hearts of pedestrians. I suspect there is a secret "sturm und drang" cell in the factory, myself.

[Motu]

There are 2 ways to get backfiring in the exhaust - from a lean mixture,a lean mixture burns slower and is still burning when the exhaust valve opens,this is where the snap crackle and pop on over run comes from.Or from a rich mixture and exhaust leak,the air leak supplies oxygen to the partialy burnt fuel,there being not enough at the time of combustion,so you get a bang of uncontrolled combustion.A backfire on starting will be rich mixture in the exhaust,on starting there will be some oxygen pumped in on crank,some heat,maybe supplied by late burning lean mixture and we get a bang.Check the idle mixture.

[Storm]

Also, how do the neighbours feel about your wakeup calls? Might be just the thing if they've been having loud party's and getting on your wick. You might be able to hire it out for pissed off folks

[skidMark]

Just to turn things on their ear - popping on deceleration and backfiring are often caused by a lean mixture. This can be caused by fitting a less restrictive airfilter or zorst, or leaks in either the intake manifold (more air getting in) or zorst system (changes the back-pressure in the pipe).

that may just explain the large blue flames that come out of my mini as i decelerate and it pops very cool sounding little backfires...strangely only started happening since i fitted my new performance zorst and performance filter