Carbureted Thruxton Stutters/Misfires When Warm
Sorry in advance for the long post, but trying to provide all information I could. I have a friend with a Triumph Thruxton that Iím working on, and itís having some issues once it warms up. Iíll start with the back story, explain the work I did, then explain the current status, which is better but not by much.
Iím a little hazy on the ďbeforeĒ details, but this is my general understanding: The bike is ridden lightly and does not have many miles (maybe 12,000??). The owner moved from elevation of ~1,000ft to 5,000ft but rode the bike without issues for ~1 season at 5,000ft. I believe the bike had some sort of running issues at one point and it was taken to the Triumph dealer where they gave it a full tune up, and if I recall correctly, rebuilt carbs and did the valve adjustment. (I have trust issues with most mechanics and all dealers, so Iím doubting they did any work at all). At any rate the owner says the bike rode great for the remainder of the season (a few rides) and this spring when it was taken out, the bike warmed up and would drive great for about 20 mins before missing and sputtering to a stop. The first time it happened he took it to the same dealer who told him it was just out of gas (insert additional trust issues). Obviously the gas issue was not the problem, and the bike would sputter and die repeatably after 15-25 mins of a cold start. At this point my friend called me to get my opinion and help him out.
The first thing I did was clean the carbs (first thing I always do on a carbed bike with unknown history), but the symptoms remained the same, and I was skeptical since it ran so well before it completely warmed up. Next I pointed my finger at the coil since Iíve has similar issues before, and replaced it with a Nology coil. No change so I turned to the internet for research, which told me that a common issue with these bikes is the pickup coil. First I checked resistance and set the gap with no success. I changed out the pick-up coil and once again had no success. We figured the last resort was to replace the CDI/igniter as research showed that being a potential problem as well, so we purchased a ProCom computer and once again had no success.
At this point I was a little stumped, so I pulled the carbs off again to check the settings. Float height was fine, the jets were the same on both carbs (and it ran great before so Iím not going to re-jet yet). I did notice that one fuel/air mixture screw was all the way out, and one was just ľ turn out. I set them both to be ~1.5 turns out. Re-assembled and while the bike seemed responsive of the change, it still has a horrible misfire and tries to die after about 20 mins. It typically doesnít completely die like before, but I suspect this is just due to the carbs being in better shape now.
At this point I feel like the valves could be out of adjustment, but Iíve never experienced a bike that runs great when cold, and horrible when warm and had it be the valve. Plus they were allegedly done at the service less than 1,000 miles ago. My other thought is that the new pick-up coil is bad.