Well, my little B40 has generally been running pretty good, until recently. It started running on, spluttering and generally misfiring and running rough. First thoughts were fuel or electrics... Investigations started with the fuelling.
So the Amal 626 concentric carb was stripped out, which is no small job on these. The saddle, tank and air box all have to come off, then source the jets, needles and gaskets etc. I got talking to Lee Mitchell at BSAWDB40.com, who is the UK guru on these bikes. He pointed me at Hitchcocks Motorcycles in Solihull as the place to go for AMAL parts. Sure enough, they had all the variations of carbs on there with diagrams and all of the spares. I had a couple of questions and between Lee & Hitchcocks, the carb was rebuilt.
Lee suggested I add a spacer between the head & the carb to stop the heat of the engine evaporating the fuel before it could be used. Hitchcocks supplied the 20mm spacer and Trevs Vintage Machine Shop supplied the longer and stainless head bolts required.
After a sonic clean of the carb, which washed out loads of crud, I spent fair bit of time on checking the jet sizing, eventually buying a couple of different ones. The standard main jet size on this carb is 160 and the pilot jet runs at 106. At these, the air screw is turned 1.5 turns out from the stop and the throttle screw the same, (give or take a fraction of a turn). I tried variations on each jet, but eventually settled on the standard ones as the bike ran smoother with that. I also replaced the needle, the spring and retaining clip along with some gaskets and the float needle.
The bike didn't come with an air/carb hose. There is a reduction from 45mm to 38mm between the carb and the air box, so finding the right hose became a question of the Facebook B40 WD group and some very helpful people suggested places. Eventually, eBay via Auto Silicone Hoses supplied the solution. I still haven't found a replacement air filter, but more suggestions from the FB group have provided a possible.
As I rebuilt the carb, I found the cable attached to the choke had been reversed. ie it was open fully to start, then eased back to closed. Without thinking about it, I rebuilt it so it was closed to start, then slowly opened as the engine warmed up. This seemed to suit the bike and be a bit more natural. The slide seems to have some wear on it and discussions have suggested the carb may need a re-sleeve to get it to run smoother, I'll look at that next.
After the carb was back in place, I took the bike for a ride. It ran OK, but still seemed to run on once warm. Eventually, it cut out on me and wouldn't restart for about 30 mins on the side of the road. I got it going again with some fiddling, then needed fuel and same again. So back home and re-think.
Talking to a mate who is a proper mechanic, he wanted to look at it and so I took the bike to his place. He played with it a bit, found some more rubbish in the carb, reset everything and the bike started again easily. Still, on a test run, it wasn't right. running on and misfiring again. So we decided to look at the electrics next.
