I'm a newbie with the TR1, I want to post my findings with the TR1 starter and see if anyone agrees.
In my case the clutch and pre-engage mechanism are fine, but the starter appeared weak. I tried my Odyssey gel battery (these gel batteries really are the business) from my R80GS and it was better, but still weak. The starter motor looked sad and tired from the outside and had a cracked lug so I bought a new one from an American outfit on Ebay.
I bought a Yuasa YTZ14S gel battery because its really small and cute (the bike's got no side panels) with claimed cold cranking power of 230A which would be more than a normal-size regular lead acid battery. Result: waste of money. The motor turned over once or twice then all it did was make the solenoid chatter. I reasoned that the battery is not up to it and the voltage is dropping so low under load that the solenoid drops out (the conductors and earthing etc are good). I proved that by bodging-up the R80 battery and the starter works ok, but still it gets to some point on the engine cycle where it just can't turn the engine, and here's the point of my tale (which seasoned TR1 owners no doubt decided long ago): I've come to the conclusion that the starter system is just not up to the job and is never going to be any good. Sure, you can start the bike if you think of it as an electric foot that gives you half a revolution or so, but when you need a good few revs to get it going, you're stuffed.
Shortly after getting back on the road, I went onto reserve just as I rolled up to some traffic lights and it stalled. Now I've enough experience with other bikes to know that that's a bad thing to allow to happen. With a steaming hot engine you can get air locks in the fuel pipes and it can seem like an eternity before it will run again. Does the 'prime' position tap into reserve? What you need is a starter motor that can give it a good bit of welly. I was stuffed. Ended up failing to bump start it, so I just left it for 5 minutes then it started on the button. How am I gonna start it after a winter lay-up?
So maybe Yamaha could get away with shit like this in 1982, but modern big twins (or old BMWs) are not like this. What have they got that the TR1 doesn't have? Don't get me wrong though, I still love it to bits.

Thanks for that, Lutze. Now that I know they can be made to work properly, I'll keep digging.
I need to correct myself; the high technology used by those Odyssey batteries and now other manufacturers is called AGM (Absorbed Glass Mat), I believe gel batteries are supposed to perform more like ordinary wet electrolyte batteries.