It could be the starter motor bush. When it wears the starter motor rotor gets too close to the windings and 'poles'. Rather than generating a rotary motion the rotor is just pulled towards the windings. 
 I had this on a Series 2 some time back, replaced the bush and all worked OK again.
 
 
 Colin
				
			 
			
		 
			
				
			
			
				'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
			
			
		 
	
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