My car has just failed its MOT test on emissions.

it is running a simple open loop gas kit with a mixer before the carb.

apprently it was

at 2500rpm - CO 0.07%, HC 3380ppm
at idle, HC 8000ppm+

Any suggestions as to how i should go about sorting this?

I have changed the air filter (which was 1 year old and dodnt look too
bad).
Before the MOT i set the points (which smoothed the idle out)

Can the timing being off affect it too? 

I moved the dissy when doing the points and I cant recall what we set
it to when the converters first set it up - so i set it by guesswork
and how much go it had, and it felt fine to me just before its MOT.

The HBOL says 17btdc@idle - which is presumably for 4* since it is a
1980 car and an old haynes book
Assuming i should advance it a couple of degrees of advance for LPG,
this would put it at about 19btdc@idle

When i tested it just now it was at 30btdc@idle. would this affect my
HC levels?