Backfire on slight throttle after decell

Hello everyone!

My first post here, hope to get in contact with fellow vfr enthousiasts.

I own a vfr750 from '92 for which I have recently done some maintance work and it now has a single pop, backfire, when I engage the throttle slightly after a decell. It especially happens in the higher rpm-range. This is a new 'feature' after the maintenance work I've done. The state of the machine before the work was fine, allthough therw where some vibrations in the higher rpm-range. I suspected this was due to the valve clearances hadn't been checked for a long time. Those vibrations are gone but the backfire is new.

The work done is:

1. Valve clearance adjustments
2. Full carb revision, all the rubbers, new jets, ultrasonic cleaning, the lot
3. Carbs are synced using vacuum gauges
4. Air mixture screws are tuned using the following method: all screws set lean 1,5 turns out, then increase individually to the point where the rpm's don't increase anymore

The machine runs incredibly well. Idle is without any burps or rumbles steady at 1200. She is rev happy all the way to red line. The only thing is the backfire. It's a single pop after: I have given her some throttle, let go (decell), and then slightly go back on throttle: Pop (or bark). Always a single one.

What could be the culprit?

Any help or tips are welcome!

Cheers!
Olav

 

Oh also, when the air mixture screws where on the lean side, I also got some 100km's in. The backfire was less, but idle was a little rough.

Suppose this is important info.

 

check that plugs look good and are gapped right. did you do your tuning with a clean air filter in place ?
you could go back into mixture screw adjustments.

 

Plugs are iridium and about a year old. Colour was good but did not check the gaps. Air filter also a year old and looked very clean so did not replace it.

 

I would change the plugs, I had a similar issue when the bike would start, slight pop out the pipe. New plug solved it, old plugs had 8,000 mles? maybe. Could also go for a more stock plug, what ever manual says.

 

Haha okay so I found the culprit: when taking a look at the carbs I noticed I had left a hose adapter in the 4th cylinder intake boot from when I was syncing... I also counted the number of turns I'd set each carb mixture screw to. Turns out I had set the 4th the leanest, because it was also getting air through the vacuum gage hose adapter thingy!

Thanks for your help guys, consider this problem solved. There's no more backfiring