I got the triumph tool to compress the spring, but had to get help from a mate to undo the nut while I compressed the spring with the tool. PITA. But it's not a difficult job to do.
Check your coil wire connections for that problem. They may require cleaning and re-crimping after which you should coat them with a bit of dielectric grease.
Did you install Progressive's springs up front or just change the oil and seals?
Disconnect your batteries hot lead, reconnect and run the 12 minute tune (start ignition without touching the throttle at all and let it run untouched for at least 12 minutes, also untouched, until the radiator fan comes on). That should "force adapt" the ECU to factory settings and just might possibly clear up your backfiring issue. If not, you'll have to troubleshoot it for other sources, which the more intelligent guys here can and will guide you on should you need or want any assistance.