Check that the diaphragm hasn't split in the vacuum advance unit on the side of the distributor. You'll have to get a bit of vacuum hose, attach it to the advance unit and try to suck through it. If you can suck air through it the diaphragm is split and the advance unit needs to be replaced.

I've had a couple like this and replacing them makes a massive difference to the fuel consumption and power too.
Cheers
Mark