Excessive back firing

Hi, I am from Sydney and have a 2015 Norton 961 Commando SF that I bought in 2016 with a thousand kms. I have done all the usual upgrades discussed at length in this forum and apart from the “normal” oil in the air box thing the bike has been running smoothly for the past 9 years. It has the full Motad short exhaust and at the time of installation the bike was tuned. It’s a daily ride and has at present just over 22K kms. In Sydney we have no support from Norton and the bike still has the original factory locked ECU. 2 years ago one of the O2 sensors went bad and I tried to contact Norton to get new ones or a reference so I could try get it on the internet but to no avail. The solution I found was to disconnect both O2 sensors and install resistors on the plug and the bike went back to run smoothly again. 4 weeks ago, out of the blue, the back started to back fire excessively when decelerating and in stand still. The back firing is from both cylinders, you can hear from both exhausts. I checked for air gaps but all exhaust connections where tight. The injectors where dirty but where not the cause of the issue. The air filter is clean and the spark plugs look a bit dark but seemed OK. I have the bike on my mechanic but because the ECU is locked they can’t run a diagnostic on the bike to trace the issue. We are just burning time and money trying to figure out what’s wrong with the bike. Have you come across something like this before? Would you have any idea what could be causing this? On open road the bike runs OK, no loss of power and is responsive, but as soon as I down shift and come to a stand still the backfiring is excessive.

Any advice you may have would be most appreciated.

Regards,

Sergio

P.S. The bike always back fired when downshifting and breaking with the engine but not to this extent and never on stand still. When I say excessive, I mean the quantity and the loudness.

I know nothing about 961s but generally backfires are not loud, they are more like pops. If loud bangs, then it is usually unburnt gas exploding in the exhaust.

Thank you Greg

Check your resistors are still intact. Check for leaks around the throttle body and exhaust system.

Thanks Tony

Is it backfiring when idling, or only when revved? When it backfires, does it come from both exhausts at the same time? Can you post a video on Google drive or similar?

TonyA said:

Check your resistors are still intact. Check for leaks around the throttle body and exhaust system.

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What Tony said ^^^^^^ and once you get the O2 sensor issue solved, and the problem persists, use a can of contact cleaner to spray around everything. If it's backfiring at idle but not at speed, (and its not the O2 sensor then it's most likely a vacuum leak(Air). You don't need the ECU unlocked to troubleshoot anything. The ECU either works or it doesn't. The tables don't mysteriously change.

cliffa said:

Is it backfiring when idling, or only when revved? When it backfires, does it come from both exhausts at the same time? Can you post a video on Google drive or similar?

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It backfires at idle too and from both exhausts. Thanks

City Garage said:

What Tony said ^^^^^^ and once you get the O2 sensor issue solved, and the problem persists, use a can of contact cleaner to spray around everything. If it's backfiring at idle but not at speed, (and it’s not the O2 sensor then it's most likely a vacuum leak(Air). You don't need the ECU unlocked to troubleshoot anything. The ECU either works or it doesn't. The tables don't mysteriously change.

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Thank you City Garage.

SMA69 said:

Thank you City Garage.

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Do you have access to a smoke machine? Or even purchase a cheap one on line.