The battery on my Gold Star has been as dead as a doornail for ages, but stupidly I hadn’t bothered to take it off until last week, when I decided to fit a new one. You know how it is – it was one of those jobs I was going to get round to, but other things always seemed to be more pressing. Anyway, as I lifted the seat I got quite a shock – no, not that kind of shock – I was struck by the incredibly vivid colour of the terminal corrosion and the clearly defined way it was presented, almost a textbook illustration. The positive terminal bolt is bright green, its nut orange with rust; while on the negative post, it’s the nut that’s green and the bolt that’s rusted. This is all a bit beyond me, as anyone who is really familiar with this subject is about to realise. As I understand it, all metals have an electrical ‘potential’, with each one having a different value, and when two dissimilar metals are put together, the disparity of the two potentials causes an electrical current to flow from the one with lower potential to the higher one, in a positive-to negative path. This is, broadly speaking, how dry cell batteries work.
Eventually, the low-potential anode is completely used up, at which point the battery has ended its useful life. At the point where the two metals join, ‘galvanic’ corrosion will occur (unplated steel bolts in aluminium are a good example). So how does this relate to my battery? Moisture and salt in the air has created an electrolyte, depositing copper from the wire terminal to the post fastenings – but I can only assume it’s to the nut on one and the bolt on the other because of the polarity of the battery, dead though it is; but I’d be interested to know more. This seems to be a perfect picture to illustrate an electrical phenomenon, although I’m just not sure exactly what that phenomenon is!
The big fix
Cam together: Fitting a gold star cam to a B31 isn’t a fast-track to more power, as one reader has found
Mr JB Wheatcroft writes from Tyneside to ask for my advice timing his 1955 BSA B31. He’s bought a pair of Gold Star cams to give more power, but was disappointed to find that, set to the standard timing marks, they made no difference. Advancing the inlet cam pinion by one tooth produced a considerable improvement, however, and he asks: “Please can you tell me the best way to set up the exhaust cam?” BSA provided a wide range of cams for Gold Stars, designed for applications ranging from trials to road racing. Mr Wheatcroft doesn’t say which profile he’s using, but it seems to me that since a BSA camshaft pinion has 36 teeth, moving it by one tooth alters the timing by a whopping 20° and that shouldn’t be the secret to making the bike go properly (unless the timing mark has been stamped on the wrong tooth).
The best way to find out is to attach a degree disc to the crank, so that the timing can be compared to data in the manual rather than simply lining up the dots. Most cams have ‘quietening ramps’ at the beginning and end of the profile, which are not part of the actual valve ‘event’ – they just get things moving immediately prior to valve-lift to lessen the load. To exclude these ramps from timing measurements, tappets must be set to a specified wide clearance – 20 thou in the case of the Gold Star – instead of the usual four and six thou. The vagaries of cam grinding make it unlikely that opening/closing points will precisely match factory figures, but they should certainly be close enough to work out the right tooth mesh. It’s important to bear in mind that engine design and tuning is a holistic process. Many special components need to work in complete harmony with each other to deliver big power increases. Bearing that in mind, fitting a hot cam without making suitable alterations to porting, carburation or any other aspect of the cylinder filling process may lead to power loss rather than gain. If it was easy, everybody would be doing it!
Top tips: Engine roping and dog-assisted wheel-building
Bruce Cox got in touch to remind me that another way of locking the engine to undo a crankshaft nut or similar is to feed enough length of nylon rope down the plug hole to fill the cylinder and jam the piston as it rises. Then you just pull it out again. He says new nylon rope is best, to avoid leaving debris inside the engine; oh, and make sure it’s on the compression stroke or you’ll bend a valve. A mate used the same trick to change a valve spring without lifting the head. Thanks Bruce. Meanwhile, Kevin Ellis (and Harry the Workshop Terrier – see pic below) succeeded in the BSA wheel I advised on in December Fixes. Kevin laced up the wheel by laying the hub on a garden table with the wheel spindle going through the hole in the middle and the rim propped on blocks, whose height matched the rim offset. Then he knocked up the handy wooden jig seen in the picture below to adjust the truing. Well done Kevin (and Harry, of course).
What a drag
Richard Rae is suffering from clutch drag on his BSA A65. It’s a familiar and immensely tedious problem: you pull up at some lights, bike won’t go into neutral, clutch gets hot, bike starts creeping forward, clutch gets hotter, no way it’ll go into neutral now unless you cut the engine. Cut the engine, at which point the lights go green... The key thing with all British clutches is to ensure the pressure plate lifts squarely. In Japanese clutches there is often a large, flat mushroom lifter sometimes acting on a radial (flat) needle roller bearing. British clutches usually have pinpoint lift from the end of a thin pushrod – so to achieve square lift, the spring pressure needs to be balanced by adjustment. Daft idea? On the face of it, perhaps, but if you know what you’re doing it does allow you to equalise spring pressure. The flat lifter could disguise a weak spring – and Japanese springs are not generally adjustable anyway. So it’s important to check the clutch pressure plate is lifting squarely all round. Another possibility is that the plain plates aren’t flat. Typically, owners only replace the friction plates, but abuse and slipping will overheat the plain plates interspersing them, sometimes making them warp until they act like diaphragm springs, with all the clutch lift being used as they flex. Another big problem is mismatched components. BSA/Triumph clutches are very similar and parts like springs, adjusters and cups are all similar but different lengths. Given that the 750 Bonneville was in production up until the mid-’80s, parts from these were often used – even though they were incorrect, they did fit, making them better than nothing. Similarly, on some four-spring clutches the first plate in is a friction, on others plain. This relates to the type of clutch centre fitted and a muddle here will make a nasty clutch. My first Triumph was built up from bits and the clutch was as horrible as Triumph clutches were fabled to be, but when I built my 1968 Trophy, I had to totally renew the clutch and it’s worked brilliantly ever since. Parts interchangeability is not always a good thing!
How to test and repair a Lucas ignition switch
Reader Doug Jones’ Bonneville ignition problem may be due to a dodgy switch. Here’s how to get it fully functional
- The Lucas ignition switch is simple enough to repair. The first step is to drill out the two rivets that hold it together. Inside is a spring and some simple brass contacts.
- The spring may have become lazy with use. Careful stretching of the coils will revive it, or fit another spring of similar dimensions to put the click back in the switch.
- The brass contacts are easily scraped clean with a pocket knife or similar. Then the switch can be reassembled – this time using 2BA nuts and bolts to facilitate future repairs.
- Spot on: perfect contact in the ‘On’ position. Remember to check conductivity between the switch terminals and the spade connection; riveted together, they can come loose.
Jon Cooper got in touch to say he’s struggling to find the correct wire for his Honda CB750 instruments. The standard-gauge wire supplied by Vehicle Wiring Products (01159 305454) is too thick to fit into the space. I asked Ferret the rewire king (07765 832420) for his advice and he suggested Jon goes back to VWP for some thin-walled (1mm sq) 16amp cable. It seems thin-walled cable has become the standard for vehicle wiring and, rated the same as the old 2mm, Ferret says it is available in an even wider range of colours – and VWP list the brown/white and green that Jon needs. It will be a bit thinner than Jon’s OE cable but at least it will fit the space better. In his quest for authenticity on his rewires, Ferret has occasionally commissioned the manufacture of unobtainable colours – but as the minimum order is 5000 metres, it’s not an option for home restorers.
Stick to it
Another clutch problem, this time on Ken Adam’s BSA C15. Ken says the clutch works fine, with no drag or slip, but if left standing for more than a few hours it sticks drastically and takes a lot of freeing off before trying to put the bike in gear. “It’s like the plates have welded themselves together!” he says; he’s using “bog standard Halfords 20/50 oil”. Sounds like the problem I used to have with my café racer 650 Tribsa – particularly when I had, ah, been performing wheelie displays... Most clutches contain a transmission shock absorber – on Japanese bikes it’s usually springs in the outer basket, but on most British bikes you have rubber buffer blocks contained within the clutch centre. When I stripped the Tribsa clutch, I found the plates were stuck together with a kind of nasty black sticky stuff. I washed it off, reassembled, changed the oil and then next wheelie session the clutch stuck again. Same problem. The rubber had become unstable, due to age and oil attack. They were reverting to the original gum and squeezing out onto the plates whenever the clutch was hammered. New rubbers (and less showing off) cured it. Whether Ken pulls many wheelies on his C15, I don’t know, but hopefully replacing the rubbers will be the answer.