Ok, let me explain my carb situation, my conclusions, and then you can all tell me if I'm sniffing blue mud.
Recently checked the size of my main jet. Turned out to be #70, but the workshop manual showed nothing over #60. I think the dealer did this, and this is why I figure they did it.
After I got the bike, I ran it till its' first service. At the first service, it was derestricted. Ran great for a week or so, then the air filter cover gets stolen. Bike is laid up for a few days, while I try to get a cover. Filters (sponge) are removed and put in a plastic shopping bag.
I get a new cover, and reinstall the filters. Notice a flat-spot from around 1/4 to mid-range throttle. I contact the dealer, and arrange to bring it in some time soon.
Now, I'm travelling to work on the week it's due to go back, and I begin to lose power. Machine just gets slower and slower... Happens twice on the way in, and I arrange to bring it in that afternoon. On the way into the shop, it does this once again.
Bring it to the shop, and they check the filter cover (which they suspected of being incorrect and causing the flat spot and power loss) and determine it is the correct part. They tell me they think the cold is affecting it (November at this stage, quite nippy out, with v. cold mornings). I go off, have some food with my wife, and return to the shop to be told they've fixed it, and that the problem was with the "carb freezing". They install a carb heater. Not knowing anything, I accept this explanation, and sure enough, the machine runs better.
Over time, I notice the flat spot isn't entirely gone, and it takes a little time and throttle fiddling to get it to full speed. Throttle usually does nothing from halfway point up to full, except drink fuel. Meanwhile, I'm being left behind by other 50cc machines from a standing start. It also seems heavier on fuel, giving around 70-78mpg when I'm told these machines give 90+. I'm suspicious.
After learning a few things (many from this forum, thanks!), I recently experimented with the jet needle. Tried raising it one notch from original middle position (richened). Bike bogged. Lowered the needle one notch from original position. Bike runs much better than it has since the problems started.
My deduction is this;
Original problem with the air filter led to dirt in the carb jet(s). This led to the flat-spot, and the eventual "power loss" problem. Given what I've read here, it appears as though my power loss problem was in fact an engine "soft seize", as it fits the symptoms. This would correlate with blocked jets of course (over-lean mixture). Plug also indicated rich running.
I bring it into the dealer. They see that the air filter cover is correct. Most likely, they realize the engine seized (they never mention it to me, they're pretty sick of me by now). So they clean the carb, and at this point fit a #70 main jet, perhaps just to make sure. I'd called on several occasions about what I know now are quite trivial things, but it's clear they dread me calling. Hardly my fault, as I don't even know where the carb is at this stage (first bike, complete novice). I'm quite sure they'd rather never hear from me again. Nice customer service, eh?
Anyway, they fit the larger jet and carb heater and blame the cold, giving me back a bike which runs ok enough that I don't notice the flat spot is still there, only lessened. I didn't have it in its pre-theft state for very long, and haven't gotten used to the machine, so I only have gut-feeling to go on.
As a result of my suspicions, and general reading and learning about these machines, I then proceed to check the carb and needle, which leads me to now.
Apologies for the length, but does this seem like a plausible deduction, or is there something glaringly obvious I'm missing? Which link in my chain of thought is the weak one?
I'm currently getting a few jet sizes, from 68 down to 60 (hopefully) so I'll be able to determine whether I can fix this or not pretty soon.
Any comments would be appreciated.