I finally got round to tackling this job over Friday and Sunday (Saturday I had to go and get deafened by some crusty old rockers in a polo field in Surrey… Procol Harem, Jeff Beck, The Who… it's a hard life sometimes).
To start, since I'd be working on the garage floor, I ran the bike up onto some bricks to give a couple of extra inches space underneath so I could get my head underneath.
The gear selector arm didn't appear to have anything wrong with it that I could see, no broken, bent or worn bits were obvious anyway. I figured a detent and return spring change wasn't going to help, but I'd already bought them so didn't have anything to lose by changing them.
The drum was pretty baulky when turning by hand so selectors were next in line for attention. Disassembly went pretty much as Deefer
described aside from the upper bolt on the rear fork pivot retainer which doubles as the pin for the gear lever return spring: this was a bastard to remove as it's a low profile, pre-rounded hex head… your conventional 12-point socket just isn't up to the job of shifting it.
In the end I had to grind the tip and sides of my hex 12mm socket to get it to seat properly over the bolt head.
The other thing Deefer omitted was that the oil strainer/pickup is (obviously) totally in the way so has to come out: the cover levers off OK, but the two screws holding the main casting were very tight… fortunately they're Allen heads and undid after an alarming crack.
At this point I wondered whether his description was just a massive wind-up because the selector forks seem such a long way up inside, but despite appearances they are indeed accessible. The old ones were quite badly damaged and the drum had some signs of wear too… fortunately I'd picked up a complete gear assembly from a local breaker for £45 on Friday so had the drum and selectors ready. Getting them in looked like it would need two hands holding the forks inside the engine where there's barely room for three fingers plus another hand to feed the drum in. I fed some 2.5mm electrical cable in to hold the forks in, but out of the way while pushing the drum in.
Locating the forks is surprisingly easy, just slide then back and forth until they find the groove on the drum (doesn't actually matter what the drum position is).
The drum action was now noticeably smoother, I was able to clock it round from 1st to 6th and back with no problem, so with high hopes put everything back together, clicked it into neutral, rolled the bike off my poor-man's workbench and popped it up on the centre stand for a final check of clutch and gear selection before refilling with oil.
And that's where my self congratulation as an ace spanner monkey ended… no way would it select 1st or 2nd. Exactly the same foccing problem as when I started.
So it must be the gear selector escapement mechanism that's at fault, despite looking OK. Unfortunately that wasn't a part included in my bargain bundle of gearbox bits or I'd have had another to compare it with.
Bollox… another few days commuting by car.