The problem you have could come from a number of issues. Please confirm that the primary issue began before you had the carburettor cleaned/set. i am also presuming you have the M115 motor - either a 200 or 230 4 cylinder.
Start with the easier problems. 1) Check your vacuum system carefully. The vacuum advance/retard hoses could be loose, leaking or possibly incorrectly fitted. There is a also a vacuum non-return valve on the primary hose between the Inlet manifold and the Brake booster housing, this could be faulty and contributing to faulty ignition advance/retard.
2) The ignition distributor shaft could be worn around the area of interface with ignition contact points, a visual inspection can show this.
3) I presume you are setting the timing using a strobe/timing light. If indeed you are seeing a significant variation of timing readings, then you probably have a major timing system fault. This can be caused by a sticking timing chain tensioner. This has to be removed and inspected manually, and invariably replaced if it is not operating freely. A faulty tensioner can cause damage to the chain system itself, so avoid over-revving the engine until this is resolved.
4) By removing the top/valve cover, you can inspect the camshaft gear for wear or any damage, but the more plausible suspect would be the intermediate gear which drives the ignition distributor and rotor. This could also be worn and introduce significant play which causes your timing to 'jump'.
I hope this helps,