If you prefer the sort of riding that the 668 is designed for then you've probably found the 668 is a touch over geared. There are a number of ways of approaching this, but all come back to changing one or other (or both) of the final drive sprockets. Standard fitments are Z16 front, Z42 rear, for a final ratio of 2.625:1. This is at variance with the homolugated rear, of Z40, but apparently this was really only fitted to the first bikes as a way of meeting strict noise level measurement requirements. Most later bikes, and certainly from the 668 onwards, should have come standard with the Z42 rear. If you still have the Z40 rear, then all the following is even more likely to give you a marked improvement.
With the Z16/Z42 combination and with no other modifications done, sixth gear is very much an overdrive and very rarely employed, except at license loosing speeds. With this in mind, you can either change the front sprocket down a tooth, for a final ratio of 2.8:1 or around 7% change, or make a smaller change by replacing the rear sprocket with a larger one. Laverda lists a number of different final sprockets in their parts catalogues, both for the 668 and the 750. These are interchangeable between models. Note that if you change the rear sprocket you are likely to need a longer chain than the standard 110 link 520 O-ring chain. It is possible, given there is still some adjustment left on the chain tensioners, to fit a replacement front sprocket and adjust the wheel back a bit to take up the slack.
I'd recommend getting the genuine Laverda front sprocket as it is offset to clear the rear wheel and as such pretty unusual. I've not managed to find a suitable substitute part after many hours of searching, although the BMW Funduro F650 sprocket came closest. The rear sprockets are flat, mounted on a carrier to the Marcheni wheel. Once again not direct substitute was found after much searching, but a close enough match from a Suzuki trail bike was found that required no more than a few millimeters of machining to enlarge the hub hole. There are other rear sprockets available as well - for instance, Renthal SPK25052042 is a 42 tooth rear sprocket that reportedly fits. If anyone has information on other motorcycles that come fitted standard with the same Marcheni rear wheel and associated sprocket mounting system then I'd love to hear from you.
Sprocket Specialists have rear sprockets that fit under code # 2671 + number of teeth. From Mr F Segura:- "It is Sprockets Specialists, not Sprocket Specialties. The latter said they would custom cut a sprocket for me if I sent them an example of the size. Sprocket Specialists actually have them under the 2671 code. They fit perfectly. (I bought 2) Just make sure to loosen the allens from the nut side, as they contain thread lock which will destroy the allen bolt if torqued."
Another modification worth considering, especially if you have a wish to wring every last ounce of performance from the bike, is to use a non-O'ring chain. Long time racers swear that the performance gains of using a well maintained conventional chain are measurable. Fitting an automatic chain oiling system such as a Scottoiler would probably be a good idea, though, as it would increase the service intervals. Boiling up chains on the kitchen stove could once again become a part of the ownership experience...
© Steve Carr, 2001-2007
Site created February, 2000. Last modified Friday, May 15, 2009
30 May 2005