Hello I have a 54 Norton 16h, a while ago it started backfiring through the exhaust when the throttle is rolled off, it’s very annoying and even more annoying to pedestrians, I tried resealing the header pipe and it was no different, the timing is correct and the bike runs perfect and starts perfect, the spark plug looks a good brownish color. Any advice is more than welcome. Thank you
Hi Kelvin, In my experience, an air leak on the inlet side is usually the culprit, eg. between carb and barrel in your case.
Try spraying the joint with WD40 or similar with the engine idling and if the revs drop there is a leak there.
Otherwise, worn throttle slides can also be a cause of overrun backfiring.
Hope that this helps,
Have you tried opening up the valve clearences by about .002 or so to see if the backfiring goes away?
Hi thank you for the replies I will give them jobs a go, the throttle slide was new less than 4000 miles ago as the carb was bored out to suit, so hopefully it’s not that.
I had a mystery issue with loss of valve clearance last year. It turned out to a valve guide coming unscrewed from the case. I had to modify a spanner to get in there to avoid stripping the top off the engine.
Engine timing is out, valve open slightly or my guess us its running too lean. Either sucking in air or carb mixture screw has moved. Check for air leaks etc then run down hill in 3rd gear throttle shut. If mixture weak it will backfire cough and fart. Adjust mixture till it comes down the hill without the backfire. Used to set Morris Minor carbs down hill works a treat.
In my youth, my first motorcycle was an old (1930) Harley-Davidson. Right hand throttle, left hand ignition timing. It made lovely backfiring when rolling back both handles at the same time. So it could be ignition timing as well as the more probable air leaks.
P.S After three years I saw the light and bought my first Norton.