8v GRiSO - Only firing on one cylinder

Hi Folks,
About 6 weeks ago my 2008 8v GRiSO started misfiring on the left pot at low revs under load. Once speed was up to 40mph plus, it was fine. If I feathered the throttle and gently picked up speed, it was okay.
This steadily became worse though.
I checked the coils (with a coil from another one of my Guzzis) , changed the plugs, caps & leads (both sides), no change.
It then stopped firing on the left pot altogether.
I've checked all the electrical connectors around the engine that I can see and the earths, cleaning & protecting them as I've gone along.
I've removed the engine support bolts and cleaned them.
I've checked the wiring in general and can't find any that are chafed or damaged.
I've also checked the fuel pipes to both injectors, the left was a bit crimped, but I've rerouted it a little. The injectors are working fine.
I haven't done a compression test yet.
I'm becoming a bit stuck and would appreciate any advice you could offer please? Thanks
I did pull one of the lower rubber hoses of the air box and it seemed to be full of the light brown gunk you find on the underside of the rocker cover when changing the oil, but I couldn't see why this might cause a misfire?
I've checked the valve clearances and they seem fine. I installed one of Beetle's maps years ago and it's run great since. It's been rollerised.
It's done over 86k miles (approaching 138k km).
Cheers
Andrew

In the old days ...
Failing spark under load indicated an HT problem

Have you swapped the plug-caps and leads from left to right?

Despite the caps having been replaced this really sounds like dud cap syndrome. Have you had the *New* caps off since installation? If you have how did you remove them?

Have you checked the injector spray pattern and fuel pressure to the LH injector?
Check cam timing is still aligned and compression check on both cylinders to compare.
Had a similar issue and mine was the fuel pressure