// The definitive guide to making your C90 go better
Tuning c90 Cubs:
There is no magic single restriction on a cub to tweak for instant power, you need to consider the type of riding you do and the budget, a combination of tweaks will however make a huge difference.
1: Ignition timing, designed to ignite poor quality fuel, 3 degrees advance by either buying a fancy cdi unit at a high price, or welding and dressing a 3mm long blob onto the flywheel, or modify the bracket holding the pickup sensor in place, to shift the position 3mm clockwise.
2: Compression ratio, using an aftermarket piston or a c70 dome top will give a very useful midrange punch. This may be too much for long term reliability, as the c70 piston gives 12:1, nothing found between 10:1 and 11:1. It will run hotter too.
3: Exhaust, restrictive, however replacing it with a home made straight-through unit will make huge noise and throw the fuel mixture out, best left as standard unless racing and everything else is also being changed.
4: Carb, the standard 16mm is too small, using a 20mm carb off the 100cc/110cc Honda/clone models is a cheap way of getting more mixture through. Check that the inlet manifold has the correct bore; the plastic heat spacer may cause a step and disrupt the gas flow. Using a K&N on the standard carb will help a bit if the jetting is suitably altered, the induction noise can be very intrusive though. Correct jetting is essential. I reckon a job for a proper dyno and experienced tuner. You can still run the standard intake and filter. Worth drilling a few holes in the back of the airbox to let a bit more air in.
5: Camshaft. Quite easy to swap for the home mechanic, various profiles available from the monkey bike shops, DONT get a race profile unless racing. It will make the engine unrideable. They make a big difference as the standard is quite "soft".
6: Cylinder head: There are various heads available takegawa etc, ok if you have the budget! It is worthwhile cleaning up the ports with a dremmel, to help the gas flow.
With these mod’s you can achieve an increase from 4.5 BHP at the rear tyre (which is normal); to 6.5 BHP.
A rough price guide:
Advanced ignition, 3mm or degrees using weld @ £0
Honda 20mm carb, orig airbox + holes, @ £20.00
Fast road cam @ £43.00
New gasket kit off eBay @ £ 7.00
Dyno time @ £60.00
Courtesy of www.fuzzibitz.8m.com & members of www.cubc90.co.uk