Gilera gsm 50 problem

hi there i have a gilera gsm 50 (50cc 2 stroke)

bike was perfect when i bought it, running perfect but the only problem was that it was spitting alot of black oil out the exhaust. So i looked up how to resolve this and found out that doing a top end was my best bet. So i did this fitted the piston perfectly, now the problem

when i close up the top end with the gasket and tighten the bike will start first kick but when i put it into first it moves very slowly and is in no way ridable

so this got me thinking it was a compression issue but if that was the case it would have a hard time starting and not start first kick

side note: replacing the piston did not solve the oil issue.

help would be appreciated thanks

While it was spitting black oil was the performance OK?

It may be a fuel or oil problem.

Is the oil pre-mix (i.e add to petrol) or does it have a oil pump.

If pre-mix make sure you are using good 2-stroke oil and the correct ratio.

If its a pump check it is adjusted correctly.

Check the carb is set up correctly, rick mixture will mean un-burnt fuel which will be black. Can you smell petrol while it is running.

Check the exhausts are free from carbon build up, remove the baffles and clean all the holes out. Remove the down pipes and main pipe and check for carbon build up. Also check the air filter is not clogged.

Check the ignition timing, as well,

Have a look at he colour of the spark plug, this givesa good guide to what is wrong with the engine.

thanks for the help i will try what you have stated