Confirming the specs from Allen G. Your air screw at 1 and a half turns out. I got a brand new 276 and it leaked like a colander so got the 376 which worked immediately with hardly any tuning needed.

Is ES not a long reach? For a cast iron head this is not correct and will bash against the piston. As Ian C suggests you can use an extra spacer. I did  on a previous spark plug and it worked fine.

the timing? There is a small nut on the top of the cylinder head on SV which you can use to measure the distance of the piston before top dead centre on the compression stroke. Should be 11mm on M21 engines. I use part of a spoke bent over so it does fall in. I mark it at TDC then wind the engine back to another mark which is 11mm away from the TDC mark. Alter the timing by removing the timing case cover, extracting the top gear which is the timing one on a taper, then the contact breaker is free to move to the 11mm BTDC using a fag paper to get the breaker as it just starts to open on that mark. Then put back the timing gear carefully so you dont disturb your adjustment. The other problem may be that the carb is worn out or blocked. Remove the main jet from the bottom and clean it. A new 376 amal from Buren in Cheltenham will solve a worn carb and at around £285 not a bad way to eliminate a possible fault.

M21 wet sumping over winter. Tried to start it and got utterly jiggered. Put on new rings, new carb, emptied the sump, new spark plug, new hand made copper, (2mm)- not composite- gasket and brmmmm second kick. Tuned up the carb, new gasket on the valve gear cover, which needed tweaking as did the cylinder head and now starts first kick. Running like a good 'un. Thanks for all the help in the owners club.

much easier on a side valve to get at and fix but adjustment is critical as there needs to be a slight gap like on the tappets otherwise if the exhaust valve is not closing properly all kinds of shenanigans will occur. I suspect the same scenario is true of overhead cams. Once the exhaust lifter cable snapped. The engine will not run properly without this as the lifter jumps about all over the shot.

I had exactly this problem after leaving my M21 standing for winter. Found oil in the combustion chamber when putting on a new gasket. Drained the sump and up and running second kick. I have a valve to fit which is supposed to be a one way valve. ie no stop cock. Is this advisable? Or should I get one i can manually operate?

I took the head off my M21 and is has loads of oil in it plus the carb is blackened. Does this mean the piston rings have failed? I put +0.080" rings in but still have the same problem. Should I fit a larger piston, if so which is best please?

i use high temperature liquid stove sealant. Works a treat.

My M21 has oversize piston rings of 0.080" of an inch. This I think is the limit. Does anyone know what I should do after this? Is it still possible to rebore and what size piston and rings should I fit afterwards?

your leaking somewhere. As others have said up to 14v is what your looking for. Anything less than 12v indicates a duff battery I think. Soldering as you have done is a sure way to get a good connection as I find bullet connections, especially when wet will partially fail. Soldering rectifies this. Only a small amount of corrosion will effect an earth connection. I had exactly the same problem with an old VW camper which seem to be wired in a similar way to British bikes. It turned out to be corrosion on the earth and when I found and corrected it, everything worked fine.

oh yes and my M21 kicks back sometimes and once it caught my leg. My goodness the bruise!! Black and blue from knee to ankle. They really can kick back at you...

interesting. So in the end it was a timing issue? I found with my M21 that using the timing hole in the top of the cylinder head with an old wheel spoke with TDC marked along with a mark just before to set the contact breaker to just opening worked well. It used to need retard to start but for the last year or so this wasn't necessary. Not sure why it would change unless the timing had slipped. On checking it was perfectly set. I did, however, put on a new carb which was the 276 with separate float. Now it suffers from fuel starvation!! When it's runing it is fine and dandy. I had the magneto rewound and set up but still find hot starting a problem. Wait 10 minutes and it's fine. I am interested to know how you set the timing?

yes I had the same problem. Over the years the threads wear away and you can't tighten them. I paid Malc's Motorcycles in Reading to take out the original studs and make new threads. Of course now it doesn't leak. Malcolm has a workshop in Caversham next to Reading and not only does a great job, charges are very fair. He could have retired by now if he wanted to, so I assume he works on British bikes more for fun than to make money. If you can do the retapping of threads yourself then all the better.
In the past I have used plumbers PFTE tape to great effect as an emergency repair for loose nuts and bolts but it reacts to petrol and goes all gooey so get the tape which is petrol resistant.

I read somewhere about boiling in a solution of water with bicarbonate of soda. I did this and completely ruined the carb as when I dried it out and blew air through the tubes, it seemed to get filled with some sort of jelly!! I made a right pigs ear of the job, so I bought a new carb from Amal, a 276 to fit the older iron head and this solved the fuel starvation problem but it's an expensive way to solve a simple problem. Other suggestions in this thread are a better bet so whatever you do, DON'T use bicarb. Also when tightening the bolts NEVER over tighten. I think the torque is 4lbs per square inch so this is not much more than finger tight. That was another mistake i made and the carb bit where the 2 nuts fit onto the barrel warped.

thanks for the photo, that shows exactly what I need to do.