A suspicion abot the twin port head beeing what to blame grew on me after reading an article about the Husqvarna engineers testing hot cams in different JAP singles during the 20's. A twin exhaust JAP lost speed if fitted with a hot cam, and a similar but single exhaust JAP went significantly better if fitted with the same.
I therefore got out the original one port W/NG cylinder head from the parts bin, ground in a couple of new valves, fitted new valve springs, gave it a lick of paint and fitted it. No other things done. Just swapped the heads, and fitted a standard single exhaust and silencer instead of the double fish cans. Now it really flies. No problem of hitting 60-65 in an instance, easily keeping great pace in hills of up to 4-5% gradient, possibly a top speed of around 70-75 if needed - but I don't like to stress it to such high revs.
But the main benefit is that it now flows along at ease in 50-55mph, and instantly squirts up to 60-65 if needed. It also pulls sufficiently strong in 4. gear down to 30mph, and can pull away from as low as 20-25mph without jerking or spitting, so it's very easy to just potter along on it too... I couldn't be more 'appy!
Conclusion; a Franklin P2 cam (HS Mk I '54) will make your W/NG quicker and stronger, but only if you keep it as a single exhaust. Explanation must be that the exhaust flow/speed in a double exhaust is so low, due to the large area of the pipes, that the necessary "suction pulse" from the exhaust pipe becomes to weak to help with cylinder filling, which is what you want in the first place, when fitting a cam with more overlap between exhaust and inlet timing. As well, when this benefit from larger overlap is lost, the only thing the large overlap will do is lowering the actual compression ratio, making it slower than it was originally - exactly the same as the Huskie-team found out almost 100 years ago. So, don't go for the good looks of the twin exhausts if you are fitting a hot cam. Stay single.
