So ive removed what i can ,of what im sure is the spark arrester ,then made up my own silencer with wadding,sounds okay,but it still hasnt gained me any more real power.From the feel of the engine,its torque thats lacking,theres nothing until five then a surge.Ive just turned out the air screw and the revs have increased,meaning a smaller pilot jet(it has stock 45)but im uncertain this will actually make it perform better.

The pilot jet only affects up to 1/4 throttle, if you have no mid throttle (1/4 to 3/4) then it's your needle.  I just went through an education in tuning a carb, a member gave me this link which helped me.  As he said it's for a bike with dual carbs but the basics are the same.  Also your plug will tell you a lot on how well the carb is tuned.

Mikuni carb tuning
 

The built in sparky thing is non removable unless someones got in there & broken the spot welds. You say you remove what you can of it?. But then again if it was restricted I'd expect reduced power all the way to its limit & Not a surge at 5 grand so maybe I'm way off the mark harping on about exhaust. So that opens more options but I can't think what at the moment. Did you say the cylinder was modified. if so, then that could be the key to it all if overdone. I don't know & only guessing. Video of reasonable length would be good to hear. Right side seal popped out would make it go all hollow powered but gotta smoke terribly if that. Left side would do similar but no smoke. Be a reluctant starter & idler though so doubt it's that. Broken reed petal if its totaly missing would do similar, or piston back to front but think we've been there. I'll go back & read.

One more maybe silly idea but has it had the crank rebuilt as there's a 5 or 6mm difference in rod length, think 6mm in rods of different models. But think it was just going from earlier piston port motors RT1 to RT2 etc & not sure which one is shorter. If it's something like that going on you'd be very very underpowered & piston skirt would be close to the flywheels at bdc. Reminds me I'm using a short rod in one of my 360's just making a mess lol & shortening the top of the cylinder as a poor man's way of raising the exhaust port--& transfers go along with it. So not sure what rod I used but I'm pretty it was a 311 number. Only coming up with this thought as I think you said in an early post that something is very very wrong.

But wait--there's more haha. Has someone mistakenly fitted a 250 crank. I've done that on purpose experimenting so end up with 6mm shorter stroke--the up & the down of piston travel. In my case I took 6mm off the top of the 360 cylinder & worked well @ 280cc but she'd sure be underpowered if it came 6mm short of the top. Even more underpowered than a 360 crank running a short rod. All that woffling on might be hard to understand sorry.

Yesterday i took down the top end and fitted a new standard piston,with rings that had already been bed in, and took it for a spin.A marked difference in power,seems as though my suspicsions were right all along.Without pictures its hard to describe the original piston,but the ports on it have clearly been played with,thing is so has the barrel to match the piston,so does any body know of a standard barrel here on these uk shores?.

When the photo hosting part of the site is back to normal it'd real interesting to see what was modified, piston & cylinder. Or link your pics to some hosting thing. Too much for me lol, don't know how it works. In saying that i have heaps on Photobucket from a few years ago but can't link them unless i pay--which could be what i need to do. In theory--my theory--it shouldn't matter what was modded on the inlet skirt being reed controlled but has the piston crown been modded to advance the transfers & or exhaust. Be impossible to do with the L shaped top ring. sorry to 'go on' but just curious.

Now the photo hosting is back,i can show you my barrel/piston arrangement.You can see below ,a new original piston(thats now fitted),and the original one which has had the ports in the skirt enlargened to suit the barrel .What i need to know is,has the barrel been tuned,the transfer port changed/enlarged to suite the old piston,as you can see with the new piston fitted,the transfers are much bigger and dont line up at all