Cubster mantenance 2019

The gearbox k46 has always been a little growly and hesitant when cold. Not just winter cold, but even in high summer before a period of use. I've previously topped up the oil, which seemed to improve things.
I decided to change the oil. Tuff Torq recommend this at 50 hours and 200 thereafter if it gets tough use. There is no drain plug, so you need to remove both linkages behind the lh wheel, remove breather and steel plate, remove the roll pin on the neutral lever if possible with the fan in place, remove fan if possible (m6 bolt on pulley is normal thread, glued in) remove stay bar on tow hitch,  axle bolts onto chassis, and lower on jack. Takes about an hour.
Tip over into oil tray and leave overnight, although it comes out pretty quick. This was grey/black. Refill with 5w-50 fully synthetic from Carrefour. About 2.2 litres.
A bit fiddly to put back, but about 2 hours including filling and checking.
Seems the manufacturer's data on checking levels through the vent don't correlate with this box. Purged with a drill clockwise in neutral, the forward/ reverse. Oil level unchanged. Then refit and do the same with the engine. Check levels (increased wtf) fit wheels, repeat as above then go for a ride. 
Still growly, but goes quiet in less than 5 minutes on hills.
About 25 hrs on gyro.

Sharpened blades. Cleaned air filter. Oil not due.
Cleaned out a ton of grass as I'd not inserted the grass chute correctly last time and the deck belt sliced the end off. There is no space to replace with a metal strap. A replacement is special order and about 80€. Plastic welding was called for. I couldn't find any suitable polypropylene , so robbed a piece off the rh deck nanny guards, about 25 x 4cm. Hot air max speed, 320C. V prep. Looks a bit ugly, but seems strong. Generally followed
https://www.eastwood.com/steinel-heat-gun-how-to-plastic-weld-plastic-welding-tool

Also need to put a washer under rh deck blade pulley to stop the belt rubbing the deck near the chute clamp.

at 30h11, put new belts on both drive & deck. Deck belt is readily available. Drive belt is listed oddly in ipl, but should read 4L106 (14€). The price for the official belt 754-04304 is between 99 & 153€ + delivery except in Poland where it's ~20€, but I couldn't find anyone prepared to send one. I can't help feeling the belt is a tad long, and running the old one along the floor, it's closer to a 4L105 (~10mm longer) than a 106 (I measured 105 + 106 from a shop). However, the outside will wear down making it shorter. The top to bottom of the old one was 0.8mm larger than the new one. The sides and top about the same. The sides were pretty smooth on the old one. Plenty of play in the mechanism to take up the slack.
However, the hesitation continues, but is less at high revs and after ~5 mins of use. I can slacken or tighten the belt whilst riding, and this makes no difference. Pump on its way out?