Hi Rahul,
First of all great M20 you have there
I am a collector and an avid rider of old brit bikes(including BSA M20). I will definitely second your opinion that these bikes were not meant for today´s stop and go city traffic conditions.
Regarding the heating problem of these bikes my personal experience says the following:
1- its not just only about the oil you use.
2- It also depends on how open the road is where you are riding and how much air does the engine get when you are riding. At an average city traffic speed of 25 - 35 Kmph and lots of stopping on traffic signal your bike is bound to heat up.
3- These bikes were developed for use in colder european countries. Thus we face this problem in Indian climates
4- check the timing of your engine and the setting of your RETIRE/ADVANCE lever of your bike. For a quicker start adjust the lever to entirely advanced and when you have warmed up the engine in gradually adjust it to a medium setting which is between advance and retire. For longer rides you can further retard the engine using the same lever but be aware that excessive retirement of engine setting will lead to missing.
Running the bike on Advnace mode would overheat the bike and eventually lead to halting.
It is sort of a manual fine tunning of the timing and comes with experience
5- For relatively hassel free running use a new age carburator (from a Bullet 350/Rajdoot) This also helps in reducing overheating to some extent. This is mainly because the old Amal carbs do not really get finely tuned because of their age and non availability of parts this leads to poor idealing and you constantly use the throttle as you feel that the eingine would stop.
6- The most important thing - Most of us restore these bike and also overhaul the engine using sleeves or reboring to new pistons and rings do remember that these are not new age bikes - they need to be ridden to adjust to the new overhauling till the time every single component settles in the engine compartment. The excessive friction which leads to overheating in newly rebored engine will subside after the bike has run approximately 1500 -2000 km. Most of us never get to ride these bikes so much until and unless one is using these bikes as daily commutors or riding long distances atleast I dont !!
So for liesure riders, they might never be able to experience the ride of a truly "runned in bike".
Regarding perpetual leakages drop by drop: It is absolutely correct that all the old brit bikes are not "drippers" - most of my bikes dont leak it all depends on how they have been kept all these years.
1- Heating is a big cuprit for this too - the metal peices do get expanded due to heat and leaves a little space for oil dribbling - this should stop automatically ones the engine has cooled down to surrounding temperature.
2 - Secondly, we do not get those thick oils anymore as in the olden days, the new age oils do get a little thinner when heated and thus slips out a bit. this should also stop ones the oil has cooled down.
3- The Alluminium or for that sake any industrial metal of those days - refer to the world war era - was nothing compared to today´s quality. In those days the metal supplies were so short that anything which was available was used inspite of high prices(several countries had to subsidise the prices of metals to help millitary machine builders of any sort). Metals tend to get weaker and porous with time, these chambers also sometimes have trapped air bubbles inside(i have seen myself)and thats also a reason why a lot of old mechs dont like to "fiddle" much with them.
4- Facing the two parts on a glass pane would also help (if the condition allows you)
5- You can also try 3M liquid gaskets - I always use it instead of hand made packing(of cork or paper)
Normally these bikes(atleast the ones used in war times) were very low maintenance but with the age they do get their share of problem.Some other small tips which I regularly follow are :
- Always start your bikes(if you are not using them regularly) atleast ones a week. This would ensure that the oil doesnt get settled in the chamber below because if it does get settled there and you start the bike it will through out a lot of oil.
- Always keep them in a dust free and dry area and clean them reqularly.
- Before firing the bike always use the kick 3-4 times to rotate the parts inside the engine compartment.
- Never rev the engine on high RPMs when the bike is in a static positions.
- Check the magnets and dynamos regularly - a weak magnet can give you nightmares in the middle of the road!
- Clean the plugs regularly.
- Never leave the gas tap open when you park the bike even for 5 min - old carbs are not so "open petrol friendly". This might lead to over flow!
- Keep all grease nipples well greased - it is vital!! there are a lot of solid metal moving parts.
- If you wish to park your bike for long period of time use both front and the rear stand - this would save you front tyre from getting deshaped. Also always place a bowl below the chamber. Apply slight coat of vaseline to all chrome plated parts this would avoid rust.
-Always use right tools to maintain your bike. In case of a stuck screw/nut/bolt prefer to use zorrik spray or to heat it up(if possible) and then open instead of getting ready with a hammer - you never now the "soft spots" which these bikes have developed over all these years and you might end up damaging something vital.
- Never use a plastic cover for the bikes always use a cotton custom made covers. Plastic covers tend to arrest moisture and dust for a longer period.
- Always lubricate the control cables regulary - the tend to snap if you are not using a new age PVC coated outer for these cables.
- Always listen to sound of your bike when you start it after several days to point out unwanted sounds from the engine.
- At regular intervals check and tighten the screws, nuts and bolts to the right torque.
- If possible put an extra switch between the main wiring and the "Cut Out" this will ensure that whenever you replace the battery the points in the cut out doesnt stay stuck.
Generally keeping these points in mind and practicing them would keep your bike maintained.
I hope I did not sound like a teacher to you as you already own two gems and might have enough knowledge about all these things.