Hitchcocks Forum

Realise this is an Enfield site but do have a 57 Bullet and it is an excellent source of advice. Just back from a ride on my 72 650 T120 Bonneville. Unusually it cut it (once) on ride while drifting down a hill but started easily and maybe coincidental. Arrived home and could hear and feel bike hunting for spark and after a while of uneven running stopped. I restart it and same. On the ride seemed fine after the restart and seemed to tick over at traffic lights although only short stops but did feel somehow not quite right. Wondering what problem is so advice appreciated. Boyer ignition and tested battery (which is 10 years old) but on test holding 12.8 volts after cooling down. Four cells at minimum other 2 just below maximum. I assume battery OK. Ticking over a little fast but then always done this. It is a standard bike with the usual twin carbs. Petrol about 2 months old but started easily after about a week or so in garage and last time out was fine. Ran easily at start (usual second kick) with regular tick over before driving off to do the 15 miles or so it did today so wondering is it a hot ride related issue. Any advice/help appreciated as to what to check next. The Bullet by the way is great and amazes me with its easy start every time, general reliability and obvious magneto/carb simplicity in contrast to Triumph. Thanks, Hugh

Hugh, it could be the carbs are worn. I'm guessing they are Mk1 Concentrics? You don't say how old they are? These wear quickly, producing what is effectively an air leak between the carb body and the slide, causing the symptoms you describe. This usually shows first on really hot days and progressively gets worse! REgards, Mark

Too pick up on Marks reply if one carb is a little more worn than the other slide wise, it will gradually alter the balance of the two..

One may be far weaker on closed throttle and won't help tickover. Is there any way you could check with a vacuum gauge/or U gauge.. Actual slide height measured mechanically may be ok and balanced but excess air pulled around the slide is not good.. Try to ascertain which cylinder is troubled by pulling off one HT lead and check the sound of the other cylinder..

The carb serving the worst performer could have the pilot screw adjusted to richer as it may help to offset the slide wear and improve running..

You could also change the plug on the rough side or even swap them over to see if it transfers the problem to the other side, same for HT leads and caps..

Thanks both fro useful advice. This is my suspicion but not sure how old carbs are. They were on there in 2004 when acquired bike but been published and this has been sustained so not sure if previous chap replaced them and then polished or what (5000 miles since). I have a local classic workshop about 2 miles away so will get them to test as the technical side of close measurement is a bit beyond me and balancing two carbs never achieved - give me my 350 Bullet here. I will change spark plugs first and trial ride to see what difference if any it makes. Regards, Hugh