2015 Triumph Bonneville Misfires

Evening all,

Heads up this is a long thread as I try to give as many details as possible regarding the slight misfire I’ve got on my 2015 Bonneville Newchurch.

I purchased this bike in October 2025 with 13,200 miles on the clock and part service history. From the history and MoT checks, it’s been sat up for a good few years having only covered 23 miles since 2022 and that’s when the last service was undertaken.

Prior to my ownership, the throttle bodies were balanced in 2019 at 6,500 miles and in 2021 the valve clearances were checked at 11,800 miles.

So, I’ve now taken ownership. The tank was pretty dry and the fuel light was on when I first rode it home, so a full tank of Shell V-Power went in and only ever goes in.

For my peace of mind, I got the bike serviced in November where it had all fluids and sparks changed. The mechanic (who also has quite a few Bonnevilles both Hinckley and Meriden) noted that it has a very slight misfire. One spark plug was more sooty than the other but told it wasn’t anything to worry about.

Now it’s had a fresh service and the misfire continued. I thought maybe a fuel system cleaner would help sort things out so it’s had a dose of Redex fuel system cleaner go through the tank. I currently can ride 115 miles (give or take) before the fuel light comes on. This is an improvement compared to pre-Redex.

I know bikes don’t like being stored up and I don’t know how the previous owner kept it between 2022 and 25. I’m trying to ride it as much as possible, even in colder weather. I’ve taken a dive from other rides and given it a good ride and a thrash but it’s still there. I’ve ridden just shy of 1k miles in total since getting the bike.

So, what could be causing this misfire? Any help would be greatly appreciated!

P.S Now I’m not hugely mechanically minded, but I have a good amount of tools and am willing to give something ago if it’s not too difficult.

Ryan