Backfire/cough back through carbs, i think..

[Mikey3]

I've asked before but i got spammed by a friend
My SV when cold, sometimes coughs back up through the carb, or it sounds like it at least, or misfires, I don't know.
It makes a cough sound, which sounds like its coming back up through the carb or airbox, of which i'm not too sure either.
revs drop, and pick up again next cycle.
It does this once or twice.
I'd like to know what it is, it isn't a bother, i'd just like to know.
Thanks in advance
Mike
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'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[J8Macbeth]

My friend's XT-350 does this too, and my TTR-125 has done it before. I have recently had to clean out the main jet on mine, as it wasn't running right, and since then I haven't had that problem. My friend's bike does the same, and the valve is definitely open at the wrong time, because we've seen a flame come back up the air intake when the cover has been off. I think the problem could be dirt in the main jet, or somewhere in the carburetter.
Hope this helps,
Josh

[Mikey3]

thanks mate,
I had a suspicion it might have been something to do with valve overlap, too much of course, or something like that.
My cam chains are slightly slack and can sometimes rattle a little, so that might have something to do with it.
Its fine once its warmed up, and it is only on occasion
Thanks
Mike
____________________
'04 Yamaha WR450F
'99 SV650 Streetfighter Project - mid build
'Motorcycling is about what you do; not about what you own, or how much you spend on it'

[MickC]

Check valve clearances and timing. Also make sure the down pipes are bolted firmly to the head. Could also be a float sticking, allowing to much fuel through.

But do that rattly cam chain, you really don't want that going or jumping, it can make a really nasty mess - the sort of nasty mess you used to see in the early 80's slasher/gore movies!!!!!.
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[Mikey3]