Maoraigh1 Or a bad connection
a good and logical diagnosis, especially as he's had a new "bendix" recently (starter off)
Check battery cranking-volts! A £5 multimeter is OK....12V battery should be 13-14V dropping to~10V cranking....24V battery, ~26 resting, ~20 cranking....any lower and suspect a dying battery or see Maoraigh's post.
If it's pre-engaged....check it's the correct pinion, right number of teeth (YES, REALLY !) and properly engaged in the operating fork. you can check on the ground with a pair of jump-leads and battery....connect to the bug terminal and the casing...bridge the small terminal to the big battery terminal and the pinion should slide out with a smart CLUNK. the other big terminal is connected to a short-stout lead that goes into the carcase....connect the other big terminal (or the jump-lead direct to this and the starter should spin merrily......if you can blow out with an airline, it's good to get rid of all the carbon-copper from brush-wear.
Inertia starters DO use a Bendix...this is a quick-thread sleeve on the armature-shaft that engages a female component fixed in the pinion....a bit like a conjourer pulling out the table-cloth, stsrter spins and pinion slides along the quick-thread until it reaches the end, they then both rotate as a whole.
Cracked sleeve or "star" (female part) or dirt/oil/grease can stop it working. again count the teeth! easy to fit wrong one and it will bind on the ring-gear.
DID THE PROBLEM ONLY START AFTER REPLACING THE PINION?BENDIX?
Due to computer crashing, above was somewhat shortened.
having re-read some of the thread, it seems the starter has been sluggish a while. Is it Pre Engaged?.....that is, it has an integrated solenoid with a direct feed from the battery...there is a small terminal, which , when energised, pulls in the solenoid....this moves a forked-arm to push the pinion into mesh with the engine's ring-gear (flywheel) It also has a heavy contact which joins the 2 big contacts on the solenoid end-cap one is the battery, the other is the starter windings. these internal contacts wear but can be replaced usual symptoms, erratic starting...sometimes nothing, sometimes normal, sometimes slow.
If you are on permit, AIUI it's OK to DIY...check the brushes are long-enough....too worn and the pressure springs or tail-wires hit stops and the brush cannot press firmly against the rotating Commutator (copper segments on the armature)...broken or jammed brushes, same effect...worn bearing bushes will allow pole-shoes and armature to foul each other....very rare! excessive carbon-dust from brush-wear can track down a lot of current to earth..always worth blasting innards with an airline. A field-winding that's tracking to earth...tricky to find, high current consumption is a giveaway but you'll need a meter for ~200 amps! (excess battery-drop whilst cranking is another giveaway and the motor will get hot.
Thrown solder on the commutator segments....caused by using a starter too long, especially with a "low" battery...bad or open windings on the armature + low speed and overheating.
All can cause low cranking speed.
many modern starters have permanent-magnet motors, they're smaller, lighter and run at high speeds,so incorporate (usually) epicyclic reduction gears.
Overheating these motors ,weakens the magnets...starter gets sluggish AND overheats....self destructing,if abused.....the old field-coil starters are much more robust in this sense, but see "thrown solder" , "partially -shorted field windings".....yer don't get owt for nowt!