Hello All -- hoping someone can point me down the right path! My 2008 carbureted Thruxton (7200 miles) is hesitating at WOT, high rpms (4k to 5.5k). It just stops firing for as long as 1-2 seconds, then it'll surge for a moment and cut out again. When I roll off the throttle, it'll start behaving properly again. This happens most significantly when riding up the canyons right outside Salt Lake City, Utah - elevations range from 1300m to 3000m above sea level (4500 to 10k ft). I do believe the carbs are adjusted properly for around-town riding, in the valley at least (1300m). Where do I start?? Spark plugs? Or is this clearly mis-adjusted carbs? Fuel economy: 13 km/L=32 mpg. I'm a decent shade tree mechanic but carburetors are new (to me!) and a little intimidating. How badly can I screw things up? Sure would rather learn than just shell money out to someone. I've had the bike for ~600 miles / 6 weeks and this problem has been there the whole time. Appreciate your thoughts.
Fuel consumption seems low unless you are caning it in lower gears - even allowing for US gallons. Yeah they’re smaller! The Gill electrics in these bikes are the single weak link. The most likely culprit for your problem is maybe not fuel but the pick up coil (crank position sensor OEM spare only) or the main ignition coil - replacement being PVL (sold as Nology in the States). Both have fine windings prone to breaks and going open circuit when hot.....intermittently when starting to die. With 7200 miles the bike has obviously not had heavy use. Have you cleaned the carbs and the gauzes inline fuel filter adjacent to the T in the fuel line? It could be starvation or crud. Next time it stutters check the tachometer. If it flickers you have electrical gremlins.
To add to Callumitys comments if electrical s are eliminated i would kill the engine where the hesitation comes in...preferably with some relatively new plugs...at that elevation it could just be starting to run a little lean when throttle is cranked open.
I think Tricky is referring to doing a plug chop as in checking the plug colo(u)r ..... although it appears he was distracted typing his post
LOL! just a bit what with the pain killers etc but yeah a plug chop at the hesitation point should give some indication if it the altitude that's causing the issue.
Plug chop I can do! It'll take me a little while -- need to order a spark plug socket that will fit in there (recommendations welcomed) ...and review service history to see if they've ever been replaced. To your point, Callumity, I let her drink only ethanol free petrol. I have no clue when/if carbs have been cleaned. Crossing my fingers that'll do it. After weeks of chasing an electrical problem on my Chevy Silverado (problem of my own making while installing power windows) I'm not eager to dive back into that world I'll respond to more of all your thoughts after taking the bike out into the lab for some "tests". Thanks all! By the way: I'm an engineer and I LOVE THE IMPERIAL SYSTEM! ...said no one since 1778
The Imperial system is fine.......or it was a system until it grew. I think the volume difference is down to leaky wooden barrels sailing from Liverpool to Baltimore and what was left when they arrived (that hadn’t been stolen in Liverpool docks!) On the engineering side I recommend a slow walk through the Gettysburg Museum. The variations in weaponry and calibre are horrendous. The Confederacy bought anything they could get while the Union was relatively homogeneous. Great to see Whitworth guns! We still struggle with our ballistic subs that are metric either end with Yankee Imperial stuff in the middle! Anyway, modern fuel leaves a much less pronounced brown ring on plugs......kind of nothing or soot! Just like tails pipes no longer have the silvery dust of lead salts!
Had a similar issue with my carburettor BSA ,turned out to be a badly blocked fuel filter, fuel would trickle through slowly, so ol at low throttle , but not enough at higher throttle openings. Cleaned filter, washed out tank....sorted.
My thoughts are an aged bike that may have been sat some time with fuel evaporating in the tank, filter and carb bowls Turn off the fuel tap, clean/replace fuel filter, drain the float bowls, put a concentrated amount of fuel additive in the tank, Sea Foam for example and let the fuel flow into the carburettors. Run the bike a bit until the fuel additive is in the bowls and let it sit there overnight to clean the system of any old fuel residue This would hopefully clear the jets and any residue on the float needles/seats Worth a go for a few bucks
Thank God almighty! -- The tach is stable when it stumbles. Guess I'll be cleaning the fuel filters and carbs this weekend. The lazy way sure does sound tempting, Red Thunder.
