Chapter 6. Compression, Fuel and Ignition

Like most of us I learnt most of the practical side of motorbike and car maintenance from my father. His usual comment if one of my cars or bikes didn’t start was “have you checked you’ve got compression, fuel and a spark.” It was good advice but I’m not sure he practised it to well himself as I can remember several occasions when the car he was working on caught fire – I guess too much fuel and a spark at exactly the wrong moment.

My BSA B31-2 came with good compression, but not much else. I always plug all the orifices in an engine when I’m working on it, to stop ingress of any dirt and importantly that small nut that pings off in a random direction and can surprisingly easily go down the spark plug hole. We’ve had that happen, thank goodness for magnetic screwdrivers. As this engine didn’t have pushrods it was very easy to turn over my hand and that alone would blow the plug out of the spark plug hole – a good sign from the outset.

The carburettor for this model should be an Amal 276 most likely 7/8” choke. I’m no expert on pre-war carbs and believe it should have been one of the brass bodied ones? My note of the strip down inspection says; ”Inlet port 7/8", 1/4" flange studs on 1-11/16" centres. Flange face is vertical. Originally Amal Type 5, brass (7/8") Possible to use 1" Amal 276C/1B, as fitted to an M20?” Perhaps one of the experts in our team can put me right on all counts.

Of course, my bike came without a carb so I decided to go for one of the later 276 models – they are a common fitment on Royal Enfields for example and there is a good supply via eBay. I am not looking for originality, so I also decided to go for a 1” model. I’ll hear you all saying that you don’t want to over-choke the carb and I would agree 100%. Most of my learning curve – after learning how to put engine fires out – was on cars, and like most of my contemporaries I ended up with an old Mini. I was on an apprenticeship scheme at Hawker Siddeley in Kingston at the time, which meant I didn’t earn too much but had access to some workshop facilities and the help of skilled machinists, also “scrap” materials. 

With my colleagues we experimented with different manifolds, carb sizes, skimmed heads and one guy even tried steam injection – which worked very well in fact. However, we concluded that usually the standard size carb with an extractor type of exhaust was the best solution on our budget. 

I’ll find out if I have the correct unit in a week or two when I try and get the bike running although I am going to cheat a bit and transplant the Chinese Kehin copy I’ve been using on my BSA C15T for the initial start attempt. (By the way, this cost me just over £20 delivered and works perfectly with just the needle moved up one groove. The original Monobloc is stored safely.) To fit the 1” 276 Amal I made up a couple of offset studs and an alloy flange adaptor which is blended to the inlet port and carb flange.

              

   

I have to admit that I have never owned a bike with a Magneto, and being a Mechanical Engineer pre-disposes me not to understand anything electrical, so I have been happy to work with the standard contact breakers, coils and the like, or in the case of my Ducati, a black box under the seat. On my Commando, and even the C15, I have fitted electronic ignitions, both a Boyer system. I’ve have only had one fail in that time and I can remember the occasion – the “original” royal wedding 29^th July 1981 when we had taken the opportunity to nip over to France from the long weekend. On the way home up the A23 Jane leant over my shoulder and shouted “can’t you go any faster”. I don’t understand sarcasm and opened up the Norton, after a few miles at just under the listed top speed she managed to get her helmet out of the slipstream and screamed “I was only joking”. Clearly that sustained run, two-up, tank bag, panniers and a rucksack full of cheap supermarket wine plus the high ambient had a been a little too much for the Boyer as the next morning the bike went from idle to max rpm and back all on its own. They changed the black box free of charge and its still working all these years later.

The BSA B31-2 came without a mag, and although I had the aforementioned idler gears, no magneto pinion. My period parts manual lists many options so I started a search on-line and at Kempton. In parallel I also started looking at an electronic conversion. My research showed that most second-hand units at a sensible price were likely to be of dubious quality. There are plenty of refirb specialists about, but probably the biggest disincentive was not particularly the cost of the work, but the likelihood that two years later I would still be waiting for the refurbished unit. I had bought a café racer seat from a guy down in Sussex who also produced parts for a magneto conversion – worth visiting by the way just for a look as his “shop” which is a gigantic circular storage tank on a semi-derelict industrial estate. I also came across an on-line article from a someone in Australia, with a BSA similar to mine, who had developed a conversion, that allowed him to complete a 300 km run on a total loss battery. He used a Bosch ZE1, which allowed him to fit the electronics, and even a coil within the original magneto case. I was outbid on eBay for a similar Bosch unit, but managed to pick up an ML Type K1 magneto, which was one of the options listed in the Parts book. This was essentially only the case and only part of the rotor assembly.

In the meantime I had sourced an electronic ignition pick-up and rotor from a company recommended by Chris Spain, a volunteer at Brooklands whom I believe some of you know, and also a rather expensive Dynatek type ignition coil from the USA; these are often used on Harleys. They are not cheap but have a reputation for being good quality and are very compact.

I tested the electronics using my lathe and got a good spark down to 80 r.p.m., although I had to make an alloy rotor with one magnet, as the purchased unit was for a 4 cylinder engine. Retaining only the ML casing, and the manual advance ring I made up some inserts, removing the remains of the original bearing races and fitting modern, sealed low profile roller bearings. The new shaft was particularly tricky, to get the right level of fit in the bearing inner races and also the end float right when completely assembled. I turned up an alloy breaker housing to fit the original case and an alloy cap. The electronic pick-up fits inside this but there are only fractions of a millimetre to spare within the various diameters.

I got lucky with the magneto pinion as it has the same tooth profile and number of teeth as one of the change gears for a Boxford lathe, and the guy who supplied these parts for my lathe had the right one in stock, although it is brass, but that should be OK with negligible load.

I have tested the complete unit on my lathe again, and all looks good. Assembly onto the bike required making up some dowels to locate the casing on the platform and the rear strap as only the front one came with the bike. I’ve made up most of the advance mechanism, the spring coming from a brake master cylinder. The coil fits very snugly under the tank and I will install a total loss battery in a housing, yet to be fabricated, mounted on the seat tube, where the original would have had either a battery or an acetylene container.

  

Well, it will be great if it works, and I feel that I have used tried and tested parts in the assembly. If it doesn’t its going to be a seller’s market for any of you that have a decent magneto going spare. That elusive date when I can try starting the bike feels tantalisingly close, I just hope that in my excitement I remember to top up the oil and not set fire to Marge.

All the best and stay safe

Gareth

Postscript to last week’s blog. On further examination of Jane’s father’s bike in the photo I determined it was not his Norton, but with the help of my Motorcycle Team volunteer friends, an AJS Silver Streak, either a 250 or 350. I’m not sure I’ve ever looked that smart on a bike.