how far could i take the engine tuning wise? i just want it to have a bit more poke is all, it does about 73mph as it is but acceleration is a little sluggish, a lad i go to college with has the br-s version, the first one to have cdi ignition, hes got a new exhaust and a cone filter, he says its made it much faster but im not convinced, i was thinking about having the ports polished (if its even possible) just to make it run smoother
Note: None of the following is first-hand knowledge - it's all things I've read from different forums, the internet in general - but the information should nevertheless be fairly accurate.
I hate to break it to you, but there isn't a single generation of the CG that is particularly tuneable. The engine simply isn't designed to be tuned, it's designed to be unfailingly reliable, last in harsh conditions, and last even though in some areas it might not get serviced that often. Plus, pushrod engines aren't as tuneable as the OHC engine in a CB might be anyway.
You're right not to be convinced about the guy you know at college - I'd be very surprised if he's found a single horsepower with those modifications - though I'm sure it'll sound nicer.
For one thing - a new filter and exhaust will improve the breathing on the CG. Without changing the fuelling it'll probably now be running lean (the CG runs lean anyway). Of course, a CG's engine isn't highly stressed so it's probably unlikely to get any detonation from running a little too lean (though you never know) but it won't be doing it any good either.
I've heard that some people swap with an earlier engine to make a little more power as they were less constrained by modern regulations, but then chances are an older engine is also a bit knackered by now in terms of making it's quoted power, so not really worth it.
If you're looking at port polishing, it
is possible, but be careful. On the inlet side, polishing out lumps and deformaties is fine,
polishing out the textured surface is not - the turbulence caused by the texture helps mix the fuel and air more effectively. The exhaust port however
can be perfectly smooth - in this case, it helps prevent carbon build-up on the walls of the port.
You can do some things to improve your speed a little though - if you find that at your top speed you're at lowish revs, then theoretically it's over-geared. You can fit a larger front or smaller rear sprocket to shorten the gearing. It should improve your acceleration, and if it allows you to use more revs in top gear then the top speed might be higher too - though it'll decrease your cruising fuel economy and you'll be revving the nuts off it everywhere at top speed
You could try and find a thinner head gasket to increase compression... but not something I'd recommend from an engine life point of view, and valve clearances would need to be changed.
You might gain 1mph from losing some weight
Both from yourself, and the bike
Basically the CG is not the bike to ride if you want performance. Your 73mph is pretty good (though I assume you're talking indicated speed - it's probably a true 65ish). It's only really going to be worth tuning at all if you're planning on keeping it for years and not moving up once you've passed your full test. For that reason, I'd recommend saving your money for something bigger later.