Long story short my m20 has been in boxes for years and hasn't run since 1980 is. I've tried to lightly restore as much as possible to try and keep most of it original.
The bike won't fire I've also tried with the carb off spraying starter fluid directly in the intake. I've double check cam and mag timing, the points gap. I've not done much with the mag other than a new set of points and cleaned all the contacts and earth points. The piston rings are new and the bike can hold all 16 stone of me on the kick-start but I can only get around 70psi using a conventional compression gauge. What should be good psi?
The next big question is the mag, it has a spark but doesn't look very strong, is there anything I can do simple checks on before I head down the route of refurbishing the mag?
Or am I missing other possible causes out completely?
At risk of stating the obvious, have you checked the spark plug? A non-resistor type is best. Where have you set the timing lever? About 7/8 open should be good to start.
As for starter fluid, you don't need to take the carb off for that. Just spray through the carb mouth with the throttle open.
An engine which is properly set up in terms of timing will fire when starting fluid is sprayed into the carb.
Sometimes it's a matter of making sure the engine is spinning fast enough to generate a decent spark.
If you suspect the mag (and if it sparks it should be fine) then you can run a lead from the battery to the points, and from there to a 6v coil (VW beetle coils are cheap) and from there use the HT lead to the spark plug. This is a dodge that people used to use when their magnetos got weak, and once the engine is running you can turn it off. I think I've remembered that correctly!! I have it in my notes... somewhere...
I pulled the mag out and while it sparks across a 5mm gap its a weak thin blue spark, when the plugs on the ht lead and kicked over against the engine its nothing like what i expect it to be. I have taken the points side off and inside the mag all the windings are mouldy on the wrappings(the bike has been in storage since 1975ish, some of it in a damp garage). Decided to take it for a rebuild at priory mags so i should recieve it back by the 8th April will know more then.
