Ha Success!
After giving this some thought you can infact work out all the balance masses etc according to the correct method (( reciprocating mass + small end mass * % balance [ low 50's for low RPM, high 50's for high RPM]) + big end mass.
However, I worked out how to do it without needing to split the crank!!!
This is based of the info in the first post - little end mass is 35 grams.
To save everyone all the maths (might post it up later - much easier if I had a scanner) Ill just tell you how to do it. Now my method assumes that the con rod on all the engines is the same (which it is).
Okay. To balance the engine, dismantle it and remove the crankshaft. Set it up on two metal rods which are level and parallel (like a "railway" for the crankshaft to freely move on). Now you can do this 2 ways.
Either let the conrod hang freely and attach a 25 gram mass to the centre of the crank pin (or on the end on the con rod) and then drill the counter weights till they balance OR do what valerdejose did and hold the conrod horizontally and attach a 60 gram (he used 55 grams, not quite right thats why he had vibrations in the pedals - but anyway its was still a huge improvement over the stock balance). Once like this you drill until its balanced (no matter angle you turn the crankshaft, its stays put and wont roll to a more favorable side).
This is why I prefer my method (use the conrod as a mass and only add 25 grams) because when it comes to test for balance you can freely turn it with out worrying how level the conrod is like valerdejose did because he used the big end mass as a part of the balance mass.
So, anyone with their motor stripped down, or who wants it to be smoother - give this ago if you want to have your motor balanced.
PS - I you have the tools, true up the crank in the lathe and finally take a light cut over the counter weights (no doubt they are out of round). Careful when assembling, you may (highly likely) tweak the crank when assembling the 2 case half's so you would then have to re true the crank again ("slight" tap with a hammer over the big end/crank pin part of the counter weights) as the crank would probably get squashed in when you torque up the case bolts. Also, for gods sake don't beat the crankshaft and bearings into the case, support the case with a pipe around the back of the case where the bearing is and use the crankshafts momentum to drive home the bearing (use a wooden temporary spacer to in between the crank halves) to minimize distortion. I probally sound excessive, but a 3k RPM gain over stock is the best upgrade you can do - its like putting gears on your bike! At 10k RPM your stock geared HT will be screaming at 70 Km/h (43 MPH) and more if you port/polish, new carb, expansion chamber, etc.
If your smart about it, modifiy the engine so that it breathes easier AND does not lose torque (hard but possible). That way, you can go fast AND still climb hills. Using a 36T sprocket is a pathetic cover up for lack of revs (Tell me, what engine under 100cc's cant rev over 7K?...besides a HT)
