What you describe is the engine providing less resistance to the momentum of the car. As George mentioned this can happen if the idle is high, but I assume you would have mentioned a fast idle problem. If you have access to a tester you could check the idle. I think it should be around 800 or lower. If it is 1000 or more that is definitely too fast.

More likely something has happened to change the compression pattern of the engine. Since it happened suddenly it is not a wear problem. If it is caused by the valves being adjusted to a tighter setting, as was suggested, you would probably notice a difference in engine noise. With tighter valves you have less "clacking" sound when the engine runs. Having the valves adjusted properly is a good thing. Having them too tight is a bad thing. You need to check with your mechanic and see if the valves were adjusted. If so, be sure they are in spec.

The timing suggestion sounds more reasonable to me. The timing would have to have been adjusted following work on the carbs. The only issue is whether it is set properly now or not. If the car runs well and has good power I wouldn't make a change. Just use your brakes to slow the car! If you want to check it out spring for a timing light (about $25) and test it. A timing light is a good thing for a 122 owner to have anyway.