Large degrees of valve 'overlap' rely on higher gas speeds to function correctly..In other words they are more suited to higher revving engines with high gas speeds in the ports etc., and work best in those as the revs rise.
Greater cam lift, duration and overlap potentially provide for better cylinder filling and increased power but the limitations in other areas of the M21s design pretty much cancel that out and you will get some 'blowback' in the carb. 65-2448 (in) and 65-2450 (ex) would be a more suitable combination and I used these to some advantage in one of my oversize M20s which also had moderate crank lightening and porting carried out...
I found the scrambles cams were too much for a standard spec. sidevalve....The good thing though is you now have an excuse to go out and buy a B33 which will work very well on them...Ian 
email (option): ian@wright52.plus.com
Got that one as well....sad innit!? One day I'll get a life
email (option): petercomley@web.de
Hi Peter..That chart shows 'standard' and 'touring' cams...Ian
email (option): ian@wright52.plus.com
Or should I say, it doesn't, if you use the link..
..Ian
email (option): ian@wright52.plus.com
Hi Pete..No, it's just me having a senior moment, not reading the posts correctly, and getting my 'wires crossed'...All your posts are as you intended. They are good charts actually, very easy to make the comparisons...Ian
email (option): ian@wright52.plus.com
Why not try the scrambles cams and find out for sure?
You have them, its easy to change back if you can't live with it.
I reckon the induction gas speed of an M21 may be under-rated.
In my experience, cylinder filling is cylinder filling.
The SV Harleys ran much more overlap than any BSA.
It probably won't beat a DBD34, but you just might be surprised.
Neale,
I am only a smart as the last thing I read and easily influenced .... As you point out; it's an easy swap; the engine is currently on the bench with the cams installed, easy enough to change now, or later when the engine is back in the bike. I still have the cylinder and valves to sort, so plenty of time to consider a course of action.
I do have the M21 induction tube and the larger carb which I will match to the cylinder block, and I have the aluminum head. I will use a copper head gasket in as thin a gauge as practical (in light of potential crank flexing/piston clearance) to up the compression just a tiddle, so we'll see if any of this make a positive difference.
I am not fooling myself with the idea that I am building any kind of hot rod, but being able to pull a bit higher gearing would be nice.
Thanks for the moral support on the idea of a little empirical experiment....
email (option): teladelujo@ msn.com
I'm all for experimentation...Though I think it's fair to say 'cylinder filling is cylinder filling' is at the very least a rash statement. Direct comparisons between one feature of different engines also has to be qualified by the other features of each type, as no single feature determines the engines spec. or performance....Over the years I've tried all sorts of things with these BSA singles. I would recommend you don't go below .030" for the head gasket material..any less and contact is likely. If that happens you won't miss it...it's noisy...Final compression ratio shouldn't exceed 7-1 to avoid pre ignition under load. That is the commonly accepted maximum figure and I've proved that to be correct by extensive on the road testing.
To get to that you would have to skim the head though..a .010" reduction in the head gasket thickness won't make it go that high.(but will improve the 'squish' effect)
Lengthening the inlet tract is useful for better mid range performance and combined with some mild porting will improve the breathing..(though not the gas speed)
By all means try the cams...As Neale says they are an easy swap so there's nothing to lose. I'd be interested to see what you think of them..Ian
email (option): ian@wright52.plus.com
'Once you have it up and running, there is scope for further low cost and simple experimentation with reed valves instead of the long carb spacer.'
'Properly rigged reed valves should be a complete game changer re overlap.'
Has there ever been a successful four stroke engine fitted with reed valves..?...I know there have been a few failed/short lived attempts by Honda, GM and others.....Ian
email (option): ian@wright52.plus.com
Iv'e never heard of reed valves in a four stroke. How on earth would that work?
What would be the advantage?? I can understand in a two stroke where its difficult to alter the valve timing (porting) but thats no prob on a four stroke.
On large capacity singles with large ports and lumpy cams the fuel/air mixture has a tendency to be forced back out of the inlet port at low to medium engine speeds. This is called 'reversion'. As the carb will function with air flowing in either direction this enrichens the fuel/air mixture as it is forced back through the carb and this over rich mixture is then sucked in yet again...Over rich and over weak mixtures both cause the engine to lose power. The fitting of a reed valve, which in effect is a one way pressure operated valve, was an attempt to prevent this happening.
However, having the reed valve present itself interrupts air/fuel flow in the port so costs power anyway...It seems the late 70s early to mid 80s was the time when this was experimented with..Improvements in engine and carb design and the ultimately introduction of fuel injection made the idea redundant...
There is a Swedish engine which still uses reed valves but it is not particularly successful. In that case the object (I think) is an attempt to use crankcase pressure generated by the downward stroke of the piston to provide forced (pressurised) induction...Ian
email (option): ian@wright52.plus.com