Rough estimate
 

I intend travelling by motorcycle from Sth Africa north along the west coast ending up in Morocco eventually. Can someone estimate how much of the route ( Namibia,Angola, Congo, Gabon, around via Nigeria, Ivory coast,Senegal and finishing in Morocco) is tarred? 20 percent, 50 percent? Bad knees and biggish bike (devilish combination, I know) prevent too many difficult dirt roads, therfore a reasonably safe route and more tar than dirt is the plan.

Certainly the roads from Nouakchott are tarmac (well ... 98%) right through to the Mediterranean. There was 50km of gravel getting up to Dakhla Junction but that was being tarmacked (November 2023). I believe the road from St Louis to Diamer border is clay and is sticky when wet, but that is second hand knowledge.

Use https://classic-maps.openrouteservice.org/ with you bike spec and you can check the road types for your chosen route and its quite accurate.

Bear in mind that although a given route may be "tarred" it may not be in good condition. The new road north from Congo to Cameroon is apparently excellent, the old coast road north from Pointe Noire (when I lived there) was "tarred" but there were more potholes than tar.

I'm currently traveling North to South, so can only tell you from Morocco to Togo so far, but don't let the thought of this worry you. You could do it almost exclusively on tarmac, with the vast majority being good to excellent. You can then pick and chose when you want to ride some dirt. Plenty of new Chinese tarmac is being laid throughout. I'll try to summerise as best I can:

Morocco - No issues.
Mauritania - No issues. As someone mentioned there is about 20km of slow, potholed dirt to the Diama border on the Mauritanian side but that is far better than crossing at Rosso (so I've heard). Tarmac to Atar is great if you want to go inland.
Senegal - No issues, just endless speed bumps.
Gambia - No issues.
Guinea-Bissau - Pretty rubbish, the main roads are still the original tarmac from before the Portuguese left in the 60s/70s. Lots of potholes but take it steady and you'll navigate them fine.
Guinea - Generally the most difficult you'll encounter, plenty of tarmac but north of Labe in the Fouta Djallon region there is a pretty rough 30km dirt section that takes longer than expected and some other sections of better unpaved roads. There was work being done at the end of it though, so maybe it'll be sorted when your trip comes about. It is however, stunning throughout Guinea and is very much worth it.
Sierra Leone - No issues. Lots of new tarmac.
Liberia - The most perfect tarmac from Monrovia along the northern route to cross to Man (which is a stunning area) in Ivory Coast. The southern route isn't paved and is notoriously bad when wet but I imagine is fine in the dry, as this is the route public transport takes between the countries.
Ivory Coast - Short, well packed dirt from Man to the border but otherwise great tarmac throughout, motorbikes are free on the toll roads.
Ghana - Good throughout the Volta, quite a few potholes coming into/out of Accra and along the coastal border with IC, still very much rideable though.
Togo - No issues.

Rough estimate
 

Thanks to folks for the advice. Much appreciated.