I'm not familiar with the internals of your particular engine but I'm wondering if the piston may be fitted 180 degrees out, if it was it would (depending on design of transfer port cut-outs &/or skirt design) mess up induction &/or transfer timing, either of which would have an adverse effect on crankcase compression

Good one Bern, i had not thought about that, too simple to be true...but simple things happen.
Lyn.

Firstly yes fuel spray/mist will be seen with bellows removed,however as it ran better tells me that the carb needle is set too high causing an over rich mixture causing the bogging to occur before the bike can run on the main jet . I would suggest dropping the needle 1 notch at a time and then testing roll ons and a plug chop ,core of plug should be of a rich tea biscuit colour . I would also check the float height and wether the main jet is of the correct size as it has been known for the wrong ones to be fitted .

Thanks for ideas, the piston is fitted as the haynes manual with lettering on the top towards the exhaust port so that is ok. Regarding jet and needle settings I will take out old jet as new carb has a different number on it and will try lowering the needle to the next groove, will report back asap

Oh float height cant be adjusted as it is a needle braised on to the float and cuts off fuel as it fills. The carb has a spring loaded pin which you push down ,as a choke, to, manually push down on the float which floods the carb for cold starting !..........it must be a czech thing

The carb has a spring loaded pin which you push down ,as a choke, to, manually push down on the float which floods the carb for cold starting !..........it must be a czech thing

That would be a good old fashioned Tickler, old Brit bikes used to have Ticklers

Andy

Still having issues, starts fine one day, garaged overnight, wont start next day.
Trying to get back to basics I decided to do a compression test. Plug out , throttle open , kicked until gauge stopped rising 95psi. I squirted a bit of oil into the cylinder and same procedure gave 100 psi.......is this ok ..?

That's not that far off for a small low state of tune engine . If I recall correctly you have new seals in the bottom end ,the top end is all good ,new carb . So clean fuel new plug and it will start ok and run fine one day but not another . Does it cut out when riding ? Does it restart easily ? Do you turn the fuel off when stopped ? Does it smell of fuel ? Have you done a plug chop when the bike is running as this gives a lot of info . Is the plug wet when it won't start the next day before and after you start kicking away .

Thanks for all the feedback and suggestions,
I have played with this bike for too long now and , with no spare engines on ebay etc, have put it up for sale..........its sold
A chap from the czech republic bought it for its " buy now " price, sent a vehicle and courier to collect it , paid cash and its gone,
He sent a message saying he is delighted with the bike so he must have a ready supply of engines and bits and the bike is back where it was born......circle of life etc ......happy ending

ive had the same issues float is not cutting the fuel off you have to lap the float with solvol or valve grinding paste other wise the petrol just keeps dripping into the crankcase bike is over fuelling so it will start first thing in but after stopping the engine leaving it for abit it is flooded and wont start had me beat for a while but cracked it now
polecat