Compression

Big imbalance there on compressions have you done a wet test ??

 

I would use few pumps of engine oil can

 

I would say so,

 

Kcroker, just a guess but I'm thinking you might need to regrind the valves and seats, or maybe the valve springs are past their best as it's been sat for a 'few' years.

 

Have you done a leak test on valve seats using petrol ?

 

Did you try turning the engine when you got the beast? and if so did it turn over ok. The reason I ask is my daughters partner got hold of a 1960's Bonnie and before he even tried to turn the engine he gave the bores a real 'generous' soaking in oil and left it for a few weeks. When we did turn it over by hand is was real smooth. Before we tried to start it he did a comp test, can't recall the numbers but it was quite high on both sides. When it did fire up it ran sort of ok but a bit rough.
It's possible that the bores may have been dry and when you turned it over you might have scored the barrels which could cause a drop in compression. I hope it hasn't as you could be in for a fair amount of work.
My daughters chap has the bike in a million bits and has the engine on a bench while the frame is away for repair and re-coating. His slowly going through the engine and checking every single bit knowing him he'll be replacing anything that ain't bang on spec because he is a pedantic Scotsman and fixing mechanical things is a real passion.

 

Try setting the engine to just before/after top dead centre and then put the same quantity of engine oil into each cylinder. Check the level in each with a dipstick of some sort (pencil - blunt end! - or ballpoint pen body) and mark the starting level. Leave it for a short while and then check the levels again. If the tide has gone out on one more than the other then the rings in that cylinder may well be suspect.
At least lifting the head on a Meriden twin is relatively easy, even for a mechanical incompetent such as I was at the tender age of 17 when confronted with a delinquent 5TA Speed Twin all those years ago!

Does this make sense?? You'll get the gist of it I'm sure!

 

It could be timing but also the outlet valves not seating properly. Have you checked the valve springs are not worn?

 

That's impressive but sure doesn't sound good. I agree with Crispey it could be firing on open exhaust valves which is either timing, weak valve spring or even some thing stopping the valves from sealng, ie, some debris. Best of luck.

 

You have low compression on one cylinder is it back firing on that cylinder if it is you need to check why is compression low on that cylinder the cylinder is igniting fuel vapour right down the exhaust hence back fire,(looks like valve or head problem) you have had head off but have not tested seating of valves or for a sticking valve some new valve springs would also be a well worth addition to a head overhaul, if a job is worth doing its worth doing well.