VFR800 V-TEC ... Backfire on overrun

BanditsHigh · Worse than a woman Joined: 21 Mar 2005 Karma :

Evening all, issue with mate's bike ... 03' VFR ... couple of months back he said it'd started mis-firing.

I fitted new plugs for him about a month ago and he says it's still mis-firing ... so he dropped it off so I could have a look at it.

Just went for a quick jaunt and engine running perfectly, sweet as can be ... no mis-fires at all ... only when I tried some slow speed riding did I come across what I think he meant, a backfire.

So, up to about 5-6k rpm, back off the throttle and on the overrun between 3-4k rpm it backfires nearly every time.

I'm sure I can hear a slight exhaust leak (possibly rear header), so will be checking that out.

Question is ... would this slight leak cause a backfire on the overrun (due to air being drawn in at the header)?

All the best ... Barry

serlant · Brolly Dolly Joined: 06 Jul 2009 Karma :

The answer is yes, when my exhaust bracket on the firestorm snapped due to rust and i chopped the throttle closed it was like someone firing a machine gun behind me as I came down the street, hilarious but not good for neighbourhood relations Smile

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04 CBR1000RR, 1995 Husqvarna TE410, 1998 VTR1000(Sold), 1990 XJ600(sold), 1996 KH125(dead and sold)

BanditsHigh · Worse than a woman Joined: 21 Mar 2005 Karma :

Good ... proves I'm not as senile as I thought I was ... well not yet anyway Laughing

All the best ... Barry

Vincent · Banned Joined: 16 Oct 2006 Karma :

Something worth looking into:-

https://johnny.chadda.se/pair-and-flapper-mod-with-bonus-snorkel-mod-for-vfr/

Does your mates bike have the standard exhaust?
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Space Is Deep

BanditsHigh · Worse than a woman Joined: 21 Mar 2005 Karma :

Fully standard as far as I know.

He's not a fast rider and he's happy with the bike as is (apart from the annoying backfire) ... would be fun to go ahead and do all the bits mentioned in your link before handing it back to him Twisted Evil

All the best ... Barry

Vincent · Banned Joined: 16 Oct 2006 Karma :

I'd do a bit of research first, some people still aren't keen on removing the pair system, esp on the VTEC........'one of the first things I did to mine.
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Space Is Deep

ADSrox0r · World Chat Champion Joined: 23 Oct 2012 Karma :

Tends to get mixed reviews, some people swear by pair removal to help with low end throttle snatch (esp on first model 6th gen VTECs '02 to '06) Others switch back because they felt it ran worse with it disabled.

I ripped mine out mainly to ditch the unnecessary lump of weight and fitted blanking plates. As it turns out my reed valves were shagged anyway so it was the best thing for it.

Could also be running a bit rich if there's excess fuel, does she smell a bit whiffy at the rear.....so to speak?
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Current bikes: '08 VFR 800 VTEC(yo) , '07 ZZR1400 Winter hack: '95 Aprilia 650 Pegaso Currently lusting after: RC30