Hi All,
Here is a long overdue progress report on the Doublefly!
Since showing the bike at the 2008 Bristol Show, I have put about 350 miles on the clock - not a large mileage, but it has shown up a couple of weaknesses. The most noticeable is the temperamental carburation at low engine speeds. The 'H' configuration of the stacked flat twin engine means that the firing sequence either goes round in a square or a figure of 8 (depending on the phasing of the cranks). This means the engine fires twice on one side, then twice on the other, and also that each carburettor gets 2 sucks then a pause, giving rise to rich mixture on the lower cylinders and weak on the top. I tried running the engine with a balance pipe across the manifolds, but the almost constant suction just pulled the slides against the back of the carbs and they wouldn't close. Even without the balance pipe, above 1500 RPM, the engine is very smooth and torquey and the carburation seems to be OK. The easiest solution to the problem will be to fit carbs with butterfly throttles, and run with the balance pipe reinstated. The carbs I have in mind are the Amal 398's that were fitted to the BSA B40 WD model - they have the same flange fitting as the Concentric and the same fuel banjo fitting - just the cables may need altering. If anyone knows the whereabouts of any of these carbs, I would be most interested.
The second problem that reared it's ugly head is that of rocker gear lubrication. As the engine is a 'four', there is no change of crankcase volume as the pistons rise and fall, so there is no pumping action to push oil mist up the pushrod tunnels to lubricate the valve gear. This first started to show up after about 200 miles - when the engine suddenly became very squeaky. Removing the rocker covers revealed that the tops of the valve stems were dry, and the rubbing action of the rockers had left a brick red, sort of rusty deposit! (like you get from chain links when they dry up). A couple of shots from the oilcan effected a temporary cure, but a more permanent solution would have to be found. The end result is that I have made an auxiliary oil pump that fits inside the timing cover to supply oil direct to the rockers (see attached photo). This pump, driven from one of the camshafts, draws oil from the front of the sump and, via internal piping, feeds it to either side of the timing chest. Fittings through the side of the timing chest then feed it to external pipwork to the cylinder heads. Internal pipes (not shown in photo) then drip oil onto the rockers. Fitting of this pump will have to wait until later as it entails quite a lot of dismantling - exhausts, carbs, heads, timing cover and petrol tank and headlight assy!
Whilst this year's mileage has thrown up a couple of problems, it has also given me chance to evaluate other aspects of the bike. Most of the mileage has been done in short bursts, so I haven't had chance to accurately check the fuel consumption, but it doesn't appear to be much thirstier than the standard Dragonfly. When under way, the extra weight is not really noticeable - much on a par with the standard 'Fly, but when coming to standstill, the higher centre of gravity does become apparent. At first, I felt the brakes were adequate - but as they bedded in, I became more familiar with the bike (and was riding it harder) - so the brakes remained adequate. It wasn't until I got back on my standard 'Fly, that I realised just how good those discs were! The Girling shocks that are fitted have better damping than the early generation units originally fitted to the 'Fly, and seem to cope well with the extra weight. When riding, the most noticeable difference between the Doublefly and standard Dragonfly is the rate at which the revs increase and decrease - necessitating a totally different approach if clean gear changes are to be obtained! At the moment, the bog standard clutch seems to be handling the extra power without any problem. With a top gear ratio of 4.7:1, 70 mph cruising is a pleasure - with the engine not sounding at all busy!
That's all for now , folks,
Have a Happy New Year,
Regards,
Eddie.
